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CrosslinkWinter 2001/2002 Vol. 3 No. 1

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Departments

2 Headlines

4 ProfileCommunication systems

engineer,

Donald H. Martin

53 Bookmarks

56 The Back PageCommunication satellite

glossary and timeline

A History of U.S. Military Satellite Communication Systems

Donald H. Martin, Senior Engineering Specialist, Architectures and Spectrum Management Office, has been with Aerospace more than 30years. His primary work has focused on requirements definition and communications design for space-system architectures, communicationstechnology development, and spectrum management. He has authored sev-eral books, the latest of which, Communication Satellites, Fourth Edition, was published in 2000. He received The Aerospace Corporation’sPresident’s Award in 1998. He holds an M.S. in engineering from the Uni-versity of California, Los Angeles (donald.h.martin@aero.org).

Spectrum Management

Albert Merrill, Senior Engineering Specialist, Architectures and SpectrumManagement Office, has more than 32 years of experience in space communications. Since 1989, he has specialized in frequency managementand spectrum-use planning. He helped initiate a methodology for applying frequency management to space technology and spearheaded a reallocation of spectrum from civil to military use. He was awarded Meritorious Unit Citations from the Director of Central Intelligence in 1996and 2001 and received The Aerospace Corporation’s President’s Award in1998. He holds a Ph.D. in electrical engineering from the University ofSouthern California and has been with The Aerospace Corporation since1967 (albert.w.merrill@aero.org). Marsha Weiskopf, Senior Project Leader,

Wideband Systems Directorate, has more than 20 years of experience in communications payloads andsystems engineering and has been working on spectrum management for the MILSATCOM Joint ProgramOffice for seven years. She has supported many successful NTIA national allocations and ITU bilateral co-ordinations. She holds an M.S. in communication systems engineering from the University of Southern Cal-ifornia. She joined Aerospace in 1993 (marsha.v.weiskopf@aero.org).

Bandwidth-Efficient Modulation Through Gaussian Minimum Shift Keying

Diana M. Johnson, Director, Communication Systems Engineering Department, leads communication-systems engineering efforts involving ar-chitectures, payload design, waveform definition, secure and protected com-munications, digital signal processing, modulation and coding, mobile/personal communication systems, and networking. During her 12years at Aerospace, she has worked with the MILSATCOM Joint Program Office on the Advanced EHF program and also served as Project Engineersupporting Milstar II. She holds an M.S. in electrical engineering from Stanford University (diana.m.johnson@aero.org). Tien M. Nguyenis Associate Director of the Communication Systems Engineering Depart-ment. His interests include advanced signal processing for wideband appli-

cations, modulation and coding, carrier and timing synchronization, detection theory, and radio-frequencyinterference analysis for wireless systems. He received a Ph.D. in engineering mathematics from the Clare-mont Graduate University (tien.m.nguyen@aero.org).

Forward Error-Correction Coding

Diana M. Johnson (see previous). Dean J. Sklar, Senior Member of the Technical Staff, Communication Systems Engineering Department, sup-ports a variety of programs in the design and evaluation of modulation andchannel-coding techniques, including high-rate turbo coding for widebandcommunications. He is also involved in the analysis and simulation of vari-ous bandwidth-efficient modulation formats. He has published several tech-nical papers and has a patent pending in the area of turbo coding. He holdsan M.S. in electrical engineering from the University of Southern California and has been with Aerospace since 1999 (dean.j.sklar@aero.org). Charles C.Wang manages the Analysis and Networking Sectionof the Communication Systems Engineering Department, leading the ana-

lytical work in modulation, coding, space-time diversity, and other advanced wireless communication tech-nologies as well as system-architecture definition, network study, and protocol development. He has beenstudying turbo codes since he joined Aerospace in 1996. He has published 14 technical papers and hasbeen granted three patents (with another four pending) in the area of turbo coding. He holds a Ph.D. inelectrical engineering from the University of California, Los Angeles (charles.c.wang@aero.org).

On the cover is a photo of a National Oceanic and Atmo-spheric Administration antennain Fairbanks, Alaska. Aero-space supported the procure-ment and installation of threesuch antennas in Fairbanksand two more at Wallops Is-land, Virginia. Used by permis-sion of NOAA.

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Robert B. Dybdal, Senior Engineering Specialist, Electromagnetic Tech-niques Department, has participated in a wide variety of communications,radar, and remote-sensing activities. He holds a Ph.D. in electrical engineering from The Ohio State University and has been with Aerospacesince 1968 (robert.b.dybdal@aero.org). Don J. Hinshilwood, EngineeringSpecialist, Electromagnetic Techniques Department, has specialized inadaptive antennas for the last 10 years. He holds M.S. degrees in physicsand astronomy from the University of Massachusetts, Amherst, and hasbeen with Aerospace since 1985 (don.j.hinshilwood@aero.org).

Ronald G. Nishinaga, Principal Director of the Air Force Satellite ControlNetwork Directorate, has 30 years of experience developing command,control, communications, and information systems for space. He receivedThe Aerospace Corporation’s Program of the Year award in 1994 for his or-ganization’s work on the Consolidated Space Operations Center. He re-ceived a Ph.D. in space systems engineering from the University of Califor-nia, Los Angeles (ronald.g.nishinaga@aero.org). Leonard L. Domenicworks in the Ground Systems Applications Office, Space Support Division,supporting the Spacelift Range Modernization program. He served as man-ager for the Federal Interagency Communications Center (FICC) Improve-ment project throughout the design and development phase and into thefielding of the first mobile elements. He holds an M.S. in engineering from George Washington University.He joined Aerospace in 1999 (leonard.l.domenic@aero.org). Winfred L. Battig, Senior Project Engineerand current manager of the FICC Improvement project, divides his time between new business and theAir Force Satellite Control Network and Spacelift Range programs. He holds an M.S. in systems engi-neering from West Coast University and joined Aerospace in 2000 (winfred.l.battig@aero.org).

Mak King joined Aerospace in 1976 in the Communications Systems Sub-division of the Engineering Technology Group. He has participated in a widevariety of communications-related projects and programs. He transferred tothe Milstar Program Office in 1990 as Senior Project Engineer and becamethe first director of the newly formed Advanced Programs Directorate of theMilsatcom Division in 1991. He was named Distinguished Engineer in 2000.He holds a Ph.D. in communications and information theory from the Uni-versity of California, Los Angeles (maurice.a.king@aero.org). Malina M.Hills, Director, International Space Technology Department, Project WestWing, leads studies of foreign satellite technology and makes regulatoryand technology-transfer recommendations to U.S. government policymak-ers. She holds a Ph.D. in chemical engineering from the California Institute of Technology. She has beenwith Aerospace since 1987 (malina.m.hills@aero.org).

Glen Elfers, Senior Project Engineer in Advanced Programs, MILSATCOMDivision, is engaged in new program initiatives and communications archi-tecture development in support of the MILSATCOM Joint Program Office.Previous Aerospace assignments have included communications architec-ture development in support of two intelligence organizations and the Na-tional Security Space Architect. He was a member of the foundationalteams defining architectures for the Global Broadcast Service, IntegratedBroadcast Service, and Mission Information Management Communica-tions. He holds an M.S. and Degree of Engineer in electrical engineeringfrom the Naval Postgraduate School, Monterey, California, and has beenwith Aerospace since 1994 (glen.e.elfers@aero.org). Stephen B. Miller isSystems Director for the Wideband Gapfiller Satellite Program. He previously worked in the Evolved Ex-pendable Launch Vehicle and Space Test Program offices. He holds an M.S. in electrical engineering fromthe Massachusetts Institute of Technology and recently completed his 20th year at Aerospace(stephen.b.miller@aero.org).

Michael R. Hilton

Advances in satellite technolo-gies have made space-based

communication a funda-mental part of life. Nonetheless, theprimary communication satellitesused by the United States and its allies will soon need replacement.Though challenging, this situationpresents immense opportunities.

New systems could provide long-needed interoperability among U.S.military branches and allied forces.High-capacity satellite crosslinkscould reduce the need for costly and potentially vulnerable ground-relay stations. Powerful computerscould process data onboard satel-lites for more flex-ible routing andpacket switching.

On the otherhand, such highlycapable systemscarry a high pricetag. Although theprocurement com-munity has been exploring stream-lined acquisition methods and cost-sharing initiatives, recent events inthe civil and commercial sectorwarn against excessive reliance onprivate industry.

Aerospace has made major con-tributions to the field of satellitecommunications and continues todevelop cost-effective solutions totomorrow’s problems. Research intomore efficient modulation, more reliable signal coding, and more robust antenna design has shapedthe system architecture of the future.At the same time, collaboration onspectrum-management issues hashelped policymakers understandand protect this vital resource.

This Crosslink provides a briefoverview of The Aerospace Corpo-ration’s important work in the in-creasingly important field of spacecommunications.

David R. HickmanConsulting Editor

From the Editors

Communication Technologies for Remote Regions

The Challenge of Shared Military Communications

Future U.S. Military Satellite Communication Systems

Adaptive Nulling Antennas for Military Communications

Miniature satellites are generallycheaper to build and deploy thanlarger units, but also provide less

capacity for generating power. A recentAerospace invention could overcome thisobstacle while enhancing the thermal per-formance of such small spacecraft.

The invention, called the PowerSphere,is essentially a geodesic globe or “bucky-ball” formed from pentagonal and hexago-nal solar panels. Prior to deployment, thesepanels remain stacked flatly at the twoends of a strut attached to the payload.During deployment, each stack of panels

unfolds, creating two hemispheres thatlock together like a clamshell to encase thesatellite.

The resulting solar-array sphere canconvert a constant amount of solar radia-tion into electrical power regardless of itsattitude relative to the sun, explained EdSimburger, project lead. The design alsoeliminates the excess mass of a standardsolar array along with its requisite attitude-control system. An added advantage, hesaid, is that the PowerSphere provides acontrolled thermal environment for theelectronics and battery it encases, whichare subjected to extreme temperatures inspace.

Simburger’s team has already beengranted a patent for the deployable geo-desic solar-panel array. Another patent hasbeen filed for the deployment method.

The NASA-funded project calls forcompletion of an engineering developmentmodel by June 2003 and an engineeringdevelopment unit by June 2004.

Headlines For more news about Aerospace, visit www.aero.org/news/

New Tools for Testing Antennas

Anew testing facility will enableAerospace to provide more accurate

and secure assessment of large,complex satellite antennas.

The near-field antenna range uses a largeplanar scanner to charac-terize the radiation pat-terns of antennas andscale models. It’s alsoused to investigate newmeasurement techniquesand methodologies. Spe-cialized data-processingroutines enable the Aero-space range to capture anantenna’s entire radiationpattern—an achievementthat would be impossibleusing conventional pla-nar near-field techniques, said facility di-rector Paul Rousseau.

The previous facility, though muchsmaller, made key contributions to a Na-tional Reconnaissance Office program, de-termining, for example, that the far side-lobes (unwanted spikes in the radiation

pattern) of an antenna system were causedby electromagnetic scattering from struts.Mock-up measurements developed at therange also helped validate a numericalanalysis, saving the program an estimated

$5 million, Rousseausaid.

The near-field rangebecame operational inOctober 2001, but ad-ditional enhancementsare planned. New ma-terial for absorbingmicrowaves will beinstalled, with slightlylarger dimensions toallow measurementsat lower frequencies.Researchers will also

install a larger 12-by-12-foot scanner thatwill enable them to analyze antennas up to10 feet in diameter.

Upgrades are also planned for the facil-ity’s compact range, which uses a largeparabolic reflector to simulate the long-range performance of a satellite antenna.

Milstar Block II Satellite LaunchedPowering Small Satellitescan reportedly process data at speeds of1.5 megabits per second. Special antennaswill provide added security for military

users.The pivotal launch

completes the plannedconstellation of fouroperational satellitesevenly spaced aroundthe equator. With thisconfiguration, the sys-tem can provide jam-proof, secure, unbro-ken communicationsaround the globe, witheach unit exchangingsignals directly withcounterparts to theeast and west.

The joint-servicesystem will link com-mand centers with

ships, submarines, aircraft, ground sta-tions, and related military resources.

The successful launch of the Milstar 5communications satellite in January 2002will significantly reduce the time it takesfor U.S. armed forcesaround the world to ex-change critical infor-mation. The AerospaceCorporation has sup-ported the Milstar pro-gram through develop-ment, production, andoperations, said RonThompson, principaldirector of the Milsat-com Systems Engi-neering division. “Ourlaunch and early-orbittest team played a keyrole in the successfullaunch and deploymentof Milstar 5,” he said.

The huge spacecraftis the second Milstar Block II satellite tocarry the medium-data-rate payload, which

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Supporting the Nation’s Needs at Ground Zero

PICOSat develop-ment and a spe-cific launch op-portunity was notidentified until thespacecraft wascomplete, Aero-space developed alaunch-vehicle in-terface specifica-tion to determinethe suitability ofthe potential U.S.launchers. Aerospace also reviewed testplans, provided independent assessments,and generated a finite-element model to an-alyze test load cases.

Similarly, Aerospace analysis helped en-sure the flight-readiness of PCSat. For ex-ample, when an actuator in the separationsystem failed, Aerospace determined thecause, validated the replacement, and veri-fied that other payloads were not at risk. In addition, Aerospace wrote proceduresfor replacing and retesting a defective

When NASA’s original payloadfor an Athena-I launch vehiclewas delayed, the Department of

Defense Space Test Program—with Aero-space assistance—was able to negotiate anattractive joint mission. The agency al-ready had three small satellite payloads al-most ready for launch. These spacecraft—PICOSat, PCSat, and Sapphire—joinedNASA’s Starshine 3 to become the KodiakStar mission.

“To accommodate four spacecraft inplace of one, a new payload upper deckhad to be designed for the launch vehicle,”explained Steve Weis, project leader forAerospace. His team in Albuquerque pro-vided critical support during the integra-tion of this unique deck configuration.

“As for PICOSat,” said Weis, “our sup-port started long before the launch oppor-tunity was identified.” Aerospace providedtechnical and programmatic oversight forspace vehicle acquisition, development,and testing. Because the original launchvehicle became unavailable early in the

Experimental Satellites Achieve Orbit

When the twin towers of NewYork’s World Trade Center collapsed after the terrorist at-

tacks on September 11, 2001, public offi-cials faced the formidable task of siftingthrough the wreckage to find victims andcollect forensic evidence. The AerospaceCorporation immediately offered its assis-tance and resources. As a result, a remote-sensing instrument developed by Aero-space helped generate and analyze data tosupport the investigation.

The instrument, known as the Spec-trally Enhanced Broadband Array Spec-trograph System (SEBASS), is a midwave and long-wave hyperspectral imager optimized for airborne sensing. It wasflown over the site in October to character-ize the distribution of gas and materials.

The information was used to confirm or refute the presence of asbestos in andaround the wreckage area as well as thelandfill where the debris was deposited.The concentration of asbestos was not

high enough for the sensor to detect, butthe distribution of fiberglass (which wouldbe expected to have a distribution pattern

electrical connector on PCSat’s launch-vehicle interface harness at the launch site.

The Kodiak Star mission marked thefirst orbital flight from the launch complexin Kodiak, Alaska, and the first multiple-payload mission for an Athena-I rocket. Inaddition, the three Space Test Programspacecraft were deployed at one altitudeand the NASA spacecraft at another. Theselaunch challenges were successfully com-pleted, thanks in part to the technical workof The Aerospace Corporation.

similar to that of asbestos) was mappedout. SEBASS also detected and mappedthe spread of freon and ammonia gas.

The left image shows a nighttime false-color infrared picture constructed from data gathered in 15passes over lower Manhattan in a Twin Otter aircraft on October 24, 2001. In the right image, the infrared data have been processed to show the debris-fallout pattern, which appears not to cross theEast River into Brooklyn.

Profile

Satellite communications is the most significant application ofspace technology, and the definitive reference for this tech-nology is Communication Satellites by Donald H. Martin.The book has been widely praised and recommended. E. W.

Ashford, head of the Communications Satellites Department of theEuropean Space Agency, described the third edition of Communica-tion Satellites as “the most comprehensive such reference in thefield.” In his review in the international journal Space Communica-tions, Ashford recommended the book as “an excellent source bookfor anyone doing research in satellite communications.”

Martin, senior engineering specialist in The Aerospace Corpora-tion’s Architectures and Spectrum Management Office, has been col-lecting information on satellite communications since 1972, when hismanager offered him a choice of assignments: of the three options, hechose to write a description of communication satellites then in orbit.The assignment grew the next year to include a report describingsatellites being built, and gradually expanded to the first edition ofCommunication Satellites in 1986. The fourth edition, published in2000, is more than twice the length of the first edition and is the firstto use and cite Internet references.

“I had no thought at the beginning of this task that it would last 29years, and no thought of publishing a book,” Martin recalled. “Thebook has gradually evolved, and it’s been a lot of fun.” Martin wroteCommunication Satellites primarily to support the needs of Aero-space customers. One of the earliest customers for the book was theDefense Department’s Net Technical Assessment Office, which usedthe information to support its work in comparing technical capabili-ties and technologies. The third edition was a response to a specificrequest by U.S. Space Command.

Martin said the book has also become a resource for professionalgrowth, enlarging his knowledge in the field. “On the one hand, I wasgathering communication satellite information for the sake of writingor revising the book; on the other hand, I found that same informa-tion kept me up to date on satellite system design, communicationtechnology, and related business issues. This helped me support proj-ects I was working on,” he said. “Reading about communicationsatellite systems developed by U.S. commercial companies and byforeign governments and companies gives a broader context that hasbeen very useful in supporting U.S. government customers.” Much ofthe information for his article in this issue of Crosslink on the historyof military satellite communication comes from his book.

Martin joined the Communications Department in the EngineeringGroup at Aerospace in 1968 after receiving B.S. and M.S. degrees inengineering from the University of California, Los Angeles. He hasstayed at Aerospace these 34 years because he likes the people, theopportunity to work directly with government customers, the varietyof work, the environment, and the chance to interact with and to learnfrom specialists in many engineering disciplines. He especially likesworking on requirements assessment and concept development early

in the life of a program, where Aerospace has a great deal of impacton the evolution of the architecture.

He also likes the diversity of his assignments, having worked withall aspects of communications and communication satellites, both inengineering and in the program offices, providing expertise directlyto government programs. “Although I am a communications engi-neer by training and by focus, I’ve found that I really enjoy thebreadth of the work that I have done,” he said.

Martin describes himself as a generalist who knows about com-munications—a systems engineer interested in the broad-scale look

Donna J. Born

Communication systems engineer, Donald H. Martin

• Born and raised in Los Angeles

• Had amateur radio license in high school

• Received B.S. and M.S. degrees in engineering from UCLA

• Married to wife Karen for 32 years; they have three children

• Has worked at Aerospace in communications since 1968

• Received The Aerospace Corporation’s President’s Award in 1998

Broad SpectrumSystems engineer, communications generalist, historian, and author, Donald H. Martin records the remarkable evolution of communication satellites since their beginnings in 1958.

of Satellite CommunicationsChronicling the

of space systems—rather than a technical specialist who looks at spe-cific technology and design details. He wrote Communication Satel-lites in that context: “The history was done to gather information onsystems, to have that broad information available when we need it todo work for customers and to answer customers’ questions.”

A recent Aerospace effort in which he was involved, for example,relied on such information to estimate the amount of interference between proposed government and commercial satellite systemssharing a frequency band: “We looked historically at communicationsatellite systems in other frequency bands or for other applications—how many were proposed, how many were licensed, how many even-tually got into operation, and how long the first systems took from theinitial proposal, to licensing, to operation. By looking at the historicalpattern, we were able to make projections of what the environmentwill be in 2010 to guide us in our analyses in support of the govern-ment system.”

In another effort, information gathered for the book helped to accomplish a short-term commercial project in 1999, in contrast tothe long-term support to the government that is the usual Aerospacebusiness. Martin was part of an Aerospace team that did a technicaland spectrum-use risk assessment for a group of European banks thatwere considering a loan of several hundred million dollars to a commercial satellite company.

Many factors are involved in formulatingthe best system design to meet governmentrequirements, Martin explained. Develop-ment of a satellite system has to respond togovernment needs, obey the laws of physics,be economically reasonable, and be compat-ible with available technology. “New satel-lites have to communicate with a large in-ventory of Earth terminals, ranging fromunits that can be carried by one soldier toterminals with 60-foot-diameter antennasanchored in concrete. With each satellitesystem, we’re trying to do more. But we alsohave to be compatible with what the govern-ment already owns—what the military serv-ices have already deployed in the field.”

In communication satellite technology,bandwidth and power are the primary meas-ures of capacity. “And we always find thatthe users want more capacity,” he said. “Asthe frequency at which the satellite systemoperates goes up, the available bandwidthgoes up, but our technological ability togenerate the downlink power decreases.Also, as the frequency goes up, the atmo-spheric attenuation goes up.”

Current Aerospace programs to developcommunication technology include wider-bandwidth equipment andhigher-power, more linear amplifiers. Another is bandwidth-efficientmodulation—the ability either to transmit the same amount of infor-mation in a narrower bandwidth or to use the existing bandwidth totransmit more information. “Forty years ago, civilians were happywith simple telephone service, and soldiers in the field were satisfiedwith poor to moderate-quality voice communications,” he said. “Nowthe civilian world is accustomed to mobile communications, Internet

access, hundreds of TV channels, and the military needs the sametechnologies that provide these services to give them the informationsuperiority they need to do their jobs.”

Most recently Martin has been working in the area of spectrummanagement—how multiple systems and diverse users share the fre-quencies that are practically available for use. The communicationsand general electronics explosion in recent years has placed increas-ing demand on the electromagnetic spectrum. Competition for fre-quencies and the greater need to share them makes frequency use asignificant constraint on space-systems architectures.

“Twenty or 30 years ago, we knew certain frequencies would beavailable, and we designed to make use of them. Now availability ofthe spectrum must be looked at in the earliest stages of designing asatellite system,” Martin said. “There is no easy solution. Over thedecades we have used higher and higher frequencies, and we’re get-ting to the point where atmospheric physics limits us, technology limits us.”

Satellite communications has not, however, reached a limit in itsevolution, Martin said, and will certainly grow over the next severaldecades and be applied even more widely. Eventually, if optical fibergoes to every home and every business on Earth, satellites will beused only for mobile communications to aircraft, ships, hikers in the

mountains. Bringing fiber to every site,however, is not necessarily economicallyworthwhile. A satellite is by no means inexpensive to launch, and a satellitelaunched today has a design life of 10 to 15years. But once in orbit, it can serve every-body in a large area, whether a country, acontinent, or a hemisphere.

Martin has received many awards for hiswork, including Aerospace Team and Indi-vidual Achievement Awards and The Aero-space Corporation’s President’s Award, oneof the company’s highest honors. He hasalso been part of two teams that have re-ceived National Foreign Intelligence Com-munity Meritorious Unit Citations.

Work he has done in the past has con-tributed to space systems now in orbit thatare being used to help coordinate actions inthe air and on the ground in the nation’sstruggle against terrorists. “Space systemstake a long time to develop,” he said. “Typi-cally the government will take more thanfive years to prepare for a new system or anew generation for an existing system, thentake five more years to develop the actualsatellites and begin launching them. Systemarchitecture and space technology work that

I’m doing now will see fruition in space systems launched in the 2010to 2020 decade.”

Although a fifth edition of Communication Satellites is not cur-rently being planned, Martin expects there will be one and is lookingfor a coauthor. He continues to collect new information, both for thebook and for use in support of Aerospace customers. “The book isone of the variety of assignments I’ve enjoyed during my career atAerospace. It’s been a very interesting time.”

Communication Satellites, Fourth Edition, is co-published by The Aerospace Press and theAmerican Institute of Aeronautics and Astronau-tics (2000, 602 pages, ISBN 1-8849899-09-8).

A History ofTwenty-five hundred years ago the Chinese general Sun Tzu wrote, “If you know theenemy and know yourself, you need not fear the result of a hundred battles.” But howare U.S. soldiers, operating covertly in unfamiliar and hostile territory, to know wheretheir allies are, where their enemies are, and what each is doing? How are they to receive commands and report status? The answer is satellite communications.

Satellite communication has beena vital part of the United Statesmilitary throughout the spaceage, beginning in 1946, when the

Army achieved radar contact with themoon. In 1954, the Navy began communi-cations experiments using the moon as areflector, and by 1959, it had establishedan operational communication link be-tween Hawaii and Washington, D.C.

As the U.S. space program grew in the1960s, the Department of Defense (DOD)began developing satellite communicationsystems that would address the special re-quirements of military operations. In ad-dition to protection against jamming,these needs included the flexibility to rap-idly extend service to new regions of theglobe and to reallocate system capacity asneeded.

The goal of these systems has been toprovide communications between, and tosupply information to, military units insituations where terrestrial means of com-munication are impossible, unreliable, orunavailable. This goal was partly realizedwith the earliest DOD communicationsatellites, and as satellite and communica-tions technology has improved, the goalhas been realized to a much greater extent.

Early DOD satellite communicationexperiments led to initial operational sys-tems, which evolved to a complete mili-tary satellite communications (milsatcom)architecture encompassing DOD’s uniquerequirements. Within this milsatcom ar-chitecture, different systems were devel-oped for three broad populations of users:wideband, tactical, and protected. Each ischaracterized by its own satellite designs,

Earth terminals, and applications (see mil-satcom timeline, page 56).

The Aerospace Corporation has been akey player throughout this history. Fromthe early days of the space age, the com-pany has taken a significant role in the de-velopment and deployment of militarycommunication satellites. Aerospace par-ticipates in all planning efforts for thesesatellite systems, including studies of re-quirements, surveys of current and pro-jected technologies, and analyses of mul-tiple alternatives to satisfy requirements.

Aerospace assists DOD in the defini-tion of technical requirements for satellitesystems. As satellite hardware is de-signed, built, and tested, Aerospace re-views the designs, analyses, and testplans; observes testing; and studies testresults. It also assists with launch prepara-tions and support of on-orbit operations.

The First Satellite Communication ProgramsThe first U.S. military communicationsatellites were of an experimental natureand used low-altitude orbits. They weredeveloped to provide basic experiencewith satellites and to explore what satellitecommunications could do. Later systemswould see actual military field use.

SCOREThe first artificial communication satel-lite, Project SCORE (Signal Communica-tion by Orbiting Relay Equipment), waslaunched in 1958, primarily to show thatan Atlas missile could be put into orbit.The secondary objective was to demon-strate a communications repeater builtinto the missile. A repeater receives a sig-nal, amplifies it, and then retransmits it.

The Army Signal Research and Devel-opment Laboratory created the repeaterby modifying commercial equipment.Two redundant sets of equipment weremounted in the nose of the SCORE mis-sile. Four antennas were mounted flushwith the missile surface, two for transmis-sion and two for reception.

SCORE’s other equipment includedtwo tape recorders, each with a four-minute capacity. Any of four ground sta-tions in the southern United States couldcommand the satellite into playback modeto transmit the stored message or intorecord mode to receive and store a newmessage. One was a Christmas messagefrom President Dwight D. Eisenhower.

CourierThe objective of the Courier program wasto develop a satellite of higher capacityand longer life than SCORE for use incommunication tests and assessments oftraffic-handling techniques. Courier’s pri-mary operating mode, like SCORE’s, wasstore-and-dump (storing data onboard tobe later “dumped,” transmitted to aground receiving station when one is insight of the satellite) using tape recorders.Unlike SCORE, however, Courier was aself-contained satellite.

The first Courier launch was unsuc-cessful because of a booster failure; thesecond, in October 1960, succeeded.Communication tests were performed byground terminals in New Jersey andPuerto Rico. The satellite performed satis-factorily until 17 days after the launch,when a command-subsystem failurestopped communications.

Donald H. Martin

U.S. Military Satellite Communication Systems

AdventCourier was a relatively simple satellite.Since it was designed for experimental use,the Advanced Research Projects Agencyundertook the Advent program in 1960,concurrent with the Courier program, toprovide an operational military communi-cation satellite. The concept for Adventwas far more sophisticated than the tech-nology available at the time; hence a num-ber of problems occurred in development,and it was canceled in 1962.

West FordWest Ford grew out of a Lincoln Labora-tory study on secure, survivable, reliablecommunications. The West Ford “satellite”consisted of 480 million thin copper wires,each about 1.5 centimeters long. The 19.5kilograms of wires were dispensed from anorbiting container in 1963. During the firstfew weeks after launch, voice and datawere transmitted from Pleasanton, Califor-nia, reflected by the wires, and received atWestford, Massachusetts, the source of the

project name. Four months later, when thewires were further dispersed, they could re-flect only very low-rate data from Pleasan-ton to Westford. Because of this low capac-ity and the growing use of active satellites,no more experiments like West Ford wereattempted.

A few of West Ford’s 480 million copper wires.

The Geostationary Orbit

The German astronomer Johannes Kepler(1571–1630) formulated three laws ofplanetary motion that also apply to the mo-tion of satellites around Earth. According toKepler’s third law, the orbital period of asatellite is proportional to its distance fromEarth. Satellites in low orbits, altitudes of afew hundred to a thousand kilometers,have orbital periods less than two hours; incontrast, the moon, at an altitude of about380,000 kilometers, has an orbital periodof about 27 days.

Between these two extremes is an alti-tude that corresponds to an orbital periodof one day. A satellite in a circular orbit atsuch an altitude revolves around Earth atthe same speed as Earth’s rotation. Thisaltitude is 35,787 kilometers, and the orbitis called a synchronous or geosynchro-nous orbit.

If the orbital plane of a satellite is not co-incident with Earth’s equatorial plane, thenthe orbit is said to be inclined, and the an-gle between the orbital plane and theequatorial plane is known as the orbit’s in-clination. In a geosynchronous orbit, thepoint on the Earth directly below the satel-lite moves north and south in a narrow figure-eight pattern with northern and

southern latitude limits corresponding tothe inclination.

If the inclination of a geosynchronousorbit is zero (or near zero), then the satel-lite remains fixed (or approximately fixed)over one point on the equator. Such an or-bit is known as a geostationary orbit. Anadvantage of the geostationary orbit is thatantennas on the ground, once aimed atthe satellite, need not continue to rotate.Another advantage is that a satellite in thistype of orbit continuously sees about one-third of Earth.

One disadvantage of the geostationaryorbit is that the gravity of the sun andmoon disturb the orbit, causing the inclina-tion to increase. The satellite’s propulsioncan counter this disturbance, but since theamount of fuel a satellite can carry is lim-ited, increased inclination may remain aproblem in some scenarios. The geosta-tionary orbit’s finite capacity is another dis-advantage; satellites using the same fre-quencies must be separated to preventmutual interference.

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Early Operational Satellite Communication ProgramsPolicy debates in the early 1960s addressedthe question of whether military and civil-ian communication satellite systemsshould be separate or combined. Aerospaceparticipated in 1964 congressional hear-ings that resulted in a government policy toestablish and maintain separate militarysatellite communication systems to satisfyunique and vital national-security needsthat commercial systems could not satisfy.The government was still able to use com-mercial satellites if those satellites pro-vided links of the required type and qualityin a timely manner at reasonable cost.

Lincoln Experimental SatellitesAfter the West Ford program, Lincoln Lab-oratory continued its investigation of spacetechnology for application to military com-munications, developing the Lincoln Ex-perimental Satellites (LES) series.

LES-1 through LES-4 carried equip-ment for communication and propagation

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experiments. The primary experiment onLES-1, -2, and -4 was a repeater and aneight-horn electronically switched antenna.Frequencies were in the 7900-to-8400-megahertz range for the uplinks (the Earth-to-satellite transmissions) and the 7250-to-7750-megahertz range for downlinks(satellite-to-Earth transmissions)—a com-bination used by later military communi-cation satellites that is called X-band.

The LES-3 frequency was in the portionof the spectrum that DOD called ultrahighfrequency (UHF), commonly used forDOD tactical communications amongsmall terminals. LES-3 was intended toinvestigate the extension of UHF commu-nications to links with satellites. It trans-mitted a signal to be used in atmosphere-propagation measurements.

The first four LES satellites werelaunched in 1965. Although not all reachedtheir intended orbits, they were used formore than a year. They demonstrated pay-load operations in space, supported propa-gation measurements, and helped improveground equipment for both communica-tions and satellite control.

The LES-5 and -6 satellites had commu-nications equipment that operated in theUHF band. Both had transmitter designswith significant improvements over thoseof prior satellites.

LES-5 was launched in 1967. Airborne,shipborne, and fixed and mobile groundterminals were involved in a large number

of successful tests with LES-5, which oper-ated until 1971. LES-6, used in similartests, was launched in 1968. These satel-lites clearly demonstrated that reliablecommunications could be extended to mil-itary units equipped with small terminals.

Initial Defense Communication Satellite Program

When the Advent program was canceled in1962, a recommendation was made for an-other program that would be operational,not experimental. As a result, the Initial

Defense Communication Satellite Program(IDCSP) was created. Its design principlewas simplicity. Each IDCSP payload had asingle repeater with a capacity of about 10voice circuits or 1 megabit per second ofdata when communicating with large ter-minals on Earth.

Seven IDCSP satellites were launchedin 1966 with additional groups of three toeight satellites launched in 1967 and 1968.Twenty-eight satellites were placed into or-bit, operating for periods ranging from one

Aerospace and Air Force program managerswith a model of an IDCSP satellite in 1966. Thepolyhedral body is covered with solar cells, andthe antennas extend to the right of the body.

Eight IDCSP satellites mounted on a launchdispenser in preparation to be launched fromCape Canaveral, Florida, on a Titan IIIC launchvehicle.

The type of mission a satellite program isdesigned to accomplish has a large influ-ence on the selection of frequency bands ituses. The required bandwidth must be afraction of the operating frequency, so themore bandwidth required, the higher theoperating frequency. The user type and lo-cation (e.g., fixed site vs. ship) determinethe kind and size of equipment to be used.

Spectrum allocations associate portions of the electromagnetic spectrum with variousradio services. A satellite system must selectfrequencies that are within the allocationsthat match its mission. Furthermore, a spacesystem must select frequencies allocated forspace use. The spectrum environment—thenumber of other users and the interferencethey would cause—has an influence on thefrequency selection.

Equipment performance also affects fre-quency selection. The performance of anten-nas is a good example. For an antenna of a

given size, beamwidth is inversely propor-tional to frequency, so the higher the fre-quency, the narrower the beam that can beformed. On the other hand, the accuracyrequired on the antenna’s surface tightenswith increasing frequency. In contrast, satel-lite amplifier efficiency (useful output power divided by input prime power) is relativelyinsensitive to frequencies under 15 giga-hertz, then gradually decreases as fre-quency increases.

The disturbances and attenuationcaused by the atmosphere and ionosphereinfluence frequency selection. Ionosphericdisturbances are high at low frequenciesand drop with increasing frequency; theyare generally insignificant above 2 giga-hertz. Atmospheric attenuation is low at lowfrequencies and increases significantlyabove 10 gigahertz.

Military satellites operate at a variety offrequencies. Those serving mobile users

operate below 1 gigahertz in a band longused for terrestrial military communications.DSCS operates at 7 and 8 gigahertz forseveral reasons: the allocation is appropri-ate, the higher frequencies support thewider bandwidths needed for the mission,and building the equipment was possible inthe 1960s, when the system began.

Milstar uses 20 and 44 gigahertz be-cause, as in the DSCS example, the alloca-tion is appropriate, the bandwidth is wide,and building the equipment is possible. Inaddition, the propagation disturbancecaused by nuclear explosions is lower atthese frequencies than it would be at lowerfrequencies. Atmospheric attenuation, al-though high, is tolerable. Milstar crosslinksuse frequencies near 60 gigahertz becausethe very high atmospheric attenuation be-tween 55 and 65 gigahertz isolates thecrosslinks from terrestrial attacks and be-cause the allocation is appropriate.

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to ten years. The IDCSP satellites driftedin orbits slightly below geostationary alti-tude. In contrast, almost all subsequentDOD communication satellites operated inthe geostationary orbit.

In 1967, increasing military activity inVietnam led to the establishment of an operational communication link that usedIDCSP. In this link, digital data were trans-mitted from Vietnam to Hawaii throughone satellite and on to Washington, D.C.,through another. In 1968, the system was declared operational, and its name was changed to Initial Defense SatelliteCommunication System.

Tactical Communication SatelliteThe IDCSP satellites and the advancedsatellites that followed them were forstrategic communications between large-antenna, fixed or transportable ground sta-tions and large shipborne equipment. TheTactical Communication Satellite (Tacsat),following the Lincoln satellites, was de-signed for a complementary function: op-eration with small land-mobile, airborne,or shipborne tactical terminals.

The Tacsat communication payload wasdesigned with both UHF and X-band capa-bilities to permit operation with a wide va-riety of terminals. The requirement to oper-ate with small terminals called for

the use of high-power transmitters, which necessitated a very large, cylindrical bodyto provide the required solar-cell area. Theneed for the large body in turn required thedevelopment of a new stabilization tech-nique, which was refined and subsequentlyapplied to many commercial communica-tion satellites.

Tacsat was launched in 1969. On-orbittesting was done with a variety of termi-nals, including large ground stations, mo-bile ground stations, aircraft, and ships.Tacsat was used for operational support ofApollo recovery operations; it connectedthe aircraft, the aircraft carrier, and theground stations. Military use, especially ofthe UHF band, was extensive. Operationscontinued until an attitude-control failurein 1972.

Milsatcom ArchitectureBy the early 1970s, DOD had determinedthe need for a milsatcom architecture toguide the development of technology andprograms that would be responsive to mili-tary users’ requirements and realizablewithin the DOD budget. In 1973 the De-fense Communications Agency (DCA),now the Defense Information SystemsAgency, was assigned responsibility fordeveloping this architecture.

The first comprehensive milsatcom ar-chitecture, published in 1976, has been re-fined several times since then. It has threesegments: wideband, mobile and tactical(or narrowband), and protected (or nuclear-capable). Within each segment is a mix of

The Tacsat satellite;the large cylinderwas covered withsolar cells, and thefive helixes were theUHF antenna.

A DSCS heavy terminal with an 18.3-meter-diameter antenna, used at major communication nodes.

users similar enough to be supported by acommon satellite system.

Aerospace participated from the begin-ning in the development and refinements ofthis architecture. The DCA office that di-rected the architecture work was headedfrom inception until 1976 by an engineeron leave from Aerospace. Additionally,Aerospace simulators have been used andrefined over many years for studying theperformance of architectural options invarious military scenarios.

Wideband SystemsUsers of the wideband segment primarilyhave fixed and transportable land-basedterminals; a few have terminals on largeships or aircraft. Their data rates vary frommoderate to high, and their connectivitymay be point-to-point or networked at dis-tances ranging from in-theater to intercon-tinental. The wideband systems are the De-fense Satellite Communication Systems(DSCS) II and III and the Global Broad-cast Service (GBS) payload on the UHFFollow-On (UFO) satellite.

Defense Satellite Communication System II

The IDCSP satellites were the DSCS PhaseI space segment. They demonstrated thatsatellite communications could satisfy cer-tain DOD needs, so in 1968, DOD decidedto proceed with the development of satel-lites for DSCS Phase II.

DSCS II satellites had a command sub-system, attitude control and stationkeeping

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capabilities (the ability, on command fromEarth, to adjust satellite orientation or or-bital position), and multiple communicationchannels with multiple-access capability.IDCSP had none of these features; how-ever, the DSCS II design was compatiblewith modified Phase I ground terminals aswell as new terminals specifically built forPhase II.

The DSCS II communication payloadhad four channels with various combina-tions of bandwidth and antennas. The com-binations provided the flexibility to handlea wide variety of links and to communicatewith many sizes of terminals. Initial termi-nals constructed at major nodes had 18.3-meter-diameter antennas.

The DSCS II program began with sixsatellites launched in pairs, the first in1971. Major technical problems caused the

satellites to fail in 1972 and 1973. Analysesof these problems, with significant Aero-space contributions, provided the basis fordesign modifications for the next satellites.By 1989 a total of 16 satellites werelaunched to establish and maintain an or-bital constellation with at least four activeand two spare satellites. All are now out ofservice and have been moved above thegeosynchronous orbit to prevent interfer-ence with active satellites.

DSCS IIIThe DSCS program was planned for long-distance communications between majormilitary locations. During the system’sevolution, both the number and variety ofterminals increased as more DOD unitssought to benefit from satellite communi-cations. By the 1990s, a majority of DSCSterminals fell into the categories of small,

transportable, or ship-board. Phase III DSCSsatellites were developedto operate in this diverseenvironment.

The primary DSCS IIIcommunication payloadoperates in the X-band andhas eight antennas that canbe connected in variousways to the six transpon-ders (“transmitter/respon-ders”). Each transpondercan be configured to servea specific type of user re-quirement. Besides Earth-coverage antennas, thesatellite has one multi-beam receiving antenna,

The January 2000 launch of a DSCS III satellite on an Atlas IIAlaunch vehicle from Florida.

A ground mobile forces satellite terminal with the 2.4-meter antenna;communications electronics are inside the truck.

which can form a beam of variable size,shape, and location, and two similar multi-beam transmitting antennas.

DSCS III development started in 1977.The first satellite was launched in 1982,and 11 additional satellites have beenlaunched since then. All are operational.Five occupy the prime operating locationsof the DSCS constellation, which arespaced around Earth in geostationary orbit.The others, spares for the five primarysatellites, augment the capacity and cover-age of the constellation.

To satisfy increasing user needs, the lastfour DSCS III satellites were enhanced toimprove their communications capacity by200 percent, with up to a 700-percent in-crease in capacity to tactical users (thosewith small terminals) in certain scenarios.Aerospace played a key role in identifyingand analyzing options for this enhance-ment program—an important activity,since the number of tactical users increasedgreatly over the past two decades. Manyuse truck- or trailer-mounted terminalswith 2.4-meter-diameter antennas. Suchterminals do not operate while in motion,but can be set up at an unprepared site bytwo or three people in less than 30 minutes.

Global Broadcast ServiceThe Global Broadcast Service (GBS) is an-other part of the milsatcom architecture’swideband segment. Its mission is to deliverhigh-rate intelligence, imagery, and mapand video data to tactical forces using small,portable terminals. Phase I of GBS used acommercial satellite and a limited numberof commercial receive terminals. Phase IIuses the GBS payload on UFO satellites 8

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through 10. This payload uses 30-gigahertzuplink and 20-gigahertz downlink frequen-cies, often called Ka-band. Phase III will bedeveloped in the future.

Information to be disseminated throughGBS is assembled into a broadcast streamtransmitted to the satellite and rebroadcastto a large number of users in one of severalspot-beam coverage areas. (Spot-beam an-tennas focus on a limited area of Earth.)Each user, typically a small military unit,has an easily moveable set of receiving anddisplay equipment.

Mobile and Tactical SystemsUsers in the mobile-and-tactical segmentof the architecture are characterized bysmall terminals with relatively low-gainantennas; they are located on ships, air-craft, and land vehicles. Data rates are lowto moderate, and connectivity is typicallyin networks at distances ranging from in-theater to transoceanic. Systems in thissegment are the Fleet Satellite Communi-cations System, the Leasat program, andthe UHF Follow-On (UFO) program.

Fleet Satellite CommunicationsTacsat and LES-5 and -6 were experimen-tal satellites that demonstrated UHF (225-to 400-megahertz) links with mobile termi-nals. These satellites were used for numer-ous tests, and Tacsat and LES-6 provided alimited operational capacity for DOD. TheFleet Satellite Communications (FLTSAT-COM) system was DOD’s first operationalsystem (dedicated to supporting militaryoperations) for tactical users.

FLTSATCOM served Navy surfaceships, submarines, aircraft, and shore sta-tions. The largest antenna, used for UHFtransmissions, was a 5-meter-diameter pa-raboloid that had a solid center section anda deployable outer mesh section. The sepa-rate UHF receiving antenna was a singlehelix deployed to the side of the large pa-raboloid. Most links were in the UHF band.

The first FLTSATCOM was launched in1978, the last in 1989. For this program, asfor others, Aerospace performed a struc-tural dynamics analysis of the satellites assubjected to launch vehicle loads.

One FLTSATCOM satellite was dam-aged during ascent and could not be used.The others operated as expected andgreatly exceeded their five-year designlives. They have been removed from serv-ice and replaced by UFO satellites.

LeasatIn 1976 and 1977 Congress directed DODto increase its use of leased commercialsatellite services, and specifically appliedthis direction to the tactical satellite systemthat would follow FLTSATCOM. The re-sult, the Leasat program, primarily servedthe Navy, plus Air Force and ground forcesmobile users. FLTSATCOM terminalswere used with Leasat. Leasat had fourtypes of communication channels, withcharacteristics very similar to the FLTSAT-COM channels, and four communicationsantennas: two X-band and two UHF; allwere Earth-coverage.

The contract for Leasat development wasawarded in 1978 and called for five years of

communication service to be provided ateach of four orbital locations. The first twolaunches took place in 1984, the last in1990. Leases on satellites 2, 3, and 5 wereextended into 1996. The Leasats have beenremoved from service and replaced by UFOsatellites, with the exception of one now inuse by the Australian Defence Forces.

UFOThe UFO satellites replaced the Navy’sFLTSATCOM and Leasat satellites. TheNavy’s requirements for UHF capacity hadgrown considerably since the first FLT-SATCOM launch in 1978. At that time,four operational satellites plus one orbitingspare were planned. The 1991 constellation

was double that size and included sixFLTSATCOMs and four Leasats.These satellites were still functioningproperly at the end of 1995, but by1999, all had been removed fromservice.

The Navy replaced the older satelliteswith a constellation of eight UFOs, plus onespare. The UFO satellites have more chan-nels than the earlier satellites and are de-signed to be compatible with the more than2000 UHF terminals used with FLTSAT-COM and Leasat. Satellite capability in-creases have led to a reduction of terminalsizes. Some terminals can be carried, set up,and used by one person, thus minimizing

The UFO satellite in orbit; the large structure on the front of the satellite body is the UHF transmitantenna, the small square toward the bottom of the body is the UHF receive antenna, and the equip-ment on the top is the GBS antenna apparatus.

The Army’s Enhanced Manpack UHF Terminal,which is capable of being carried, set up, andused by a single soldier, communicates via theUFO satellites.

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Milstar Block I and IIThe Milstar system is designed to empha-size robustness and flexibility. The term“robustness” here refers to the ability tooperate under adverse conditions, includ-ing jamming and nuclear attack. Aerospaceplayed a key role in assessing Milstar sys-tem performance against a DOD-validatedthreat model. “Flexibility,” in this context,is the ability to provide worldwide un-scheduled access and worldwide connec-tivity to terminals on all types of platforms.

The Milstar program includes two BlockI and four Block II satellites. These blocksare also known as LDR (low data rate) orMilstar I, and MDR (medium data rate) orMilstar II. The block change resulted froma 1990 program restructure in response toglobal political changes.

Relaxation of survivability requirementsand improvements in satellite electronicsallows the MDR satellites to provide ro-bustness and flexibility for 32 MDR chan-nels, for single-user data rates up to 1.5megabits per second, in addition to the 192LDR channels, for single-user data rates upto 2.4 kilobits per second. Aerospace was a

leader in the development of the standardfor the LDR and MDR waveforms.

The Milstar system has three segments:mission control, terminal, and space. Themission control segment plans mission ac-tivities, allocates system resources, testsand controls the satellites, and resolvessatellite anomalies. It includes a fixed siteas well as mobile units. Intersatellitecrosslinks enable monitoring and control ofall Milstar satellites from a single location.

The terminal segment, developed by the Air Force, Navy, and Army, containsmore than 1000 terminals of many types;some are vehicle-transportable or human-portable, while others are located at fixedsites or on airborne command posts orother aircraft, ships, or submarines. An-tenna diameters vary from 14 centimetersfor submarine terminals to 3 meters forfixed command-post terminals.

The space segment consists of the Mil-star satellites. Each has a central bus, twopayload wings, and two solar arrays. At theouter end of each wing is a crosslink an-tenna. Other antennas are mounted on thewings. Features that support the system’s

The Milstar satellite in orbit with the two equipment wings and two longer solar-array wings deployed;various antennas are visible on the equipment wings.

the burden on military users and increasingthe number of military units that can benefitfrom satellite communications.

The UFO contract was awarded in 1988;a total of ten were ordered by 1994. A con-tract for an eleventh satellite was signed in1999. The first UFO was lost as a result ofa launch vehicle problem, but the nextnine, successfully launched between 1993and 1999, are in use. Satellite 11 will belaunched in 2003. Besides performing itsusual systems engineering tasks through-out the UFO program, Aerospace also de-veloped a telemetry analysis workstationand installed it in a satellite control center.

Protected SystemsMobility characterizes users of the pro-tected segment of the milsatcom architec-ture, whether they are on ships, aircraft, orland vehicles. They accept very low tomoderate data rates in exchange for con-siderable protection of their links againstphysical, nuclear, and electronic threats.Systems in the protected segment of themilsatcom architecture are the Milstar sys-tem and the Air Force Satellite Communi-cations (AFSATCOM) and extremely highfrequency (EHF) payloads.

AFSATCOMAFSATCOM served Air Force strategicaircraft, airborne command posts, andground terminals. AFSATCOM satellitepayloads are on the FLTSATCOM satel-lites and on several satellites situated inhigh-inclination orbits to provide coverageof the north polar region, which is not visi-ble from equatorial satellites. AFSATCOMalso uses a single-channel transponder onDSCS III satellites.

AFSATCOM uses a mix of UHF and X-band frequencies with antijamming protec-tion for most of its uplinks by frequency-hopping (rapid switching of frequenciesduring transmission). A portion of the Mil-star communications payload continues theAFSATCOM mission.

FLTSATCOM EHFFLTSATCOM satellites 7 and 8 containedan EHF payload (44-gigahertz uplink and20-gigahertz downlink) called the FEP(FLTSATCOM EHF Package). FEP wasdeveloped to demonstrate operational ca-pabilities of EHF terminals and prove keyfunctions of the Milstar system. This pay-load had both Earth-coverage and spot-beam antennas, and it processed receivedsignals before their downlink transmission.Both links were frequency-hopped.

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robustness include frequency-hopping, ex-tensive onboard processing, and crosslinks.Features that support flexibility includemultiple uplink and downlink channels op-erating at various rates; multiple uplink anddownlink beams, including agile beams;and routing of individual signals amonguplinks, downlinks, and crosslinks.

The Block I satellites were launched in1994 and 1995. Aerospace had a lead rolein the analysis of one launch-vehicle fail-ure and another anomaly just prior to thefirst launch, and provided launch-readinessverification for the Milstar 1 launch vehi-cle. The first Block II satellite was lost be-cause of a launch problem. The second waslaunched in 2001; it and the Block I satel-lites are in operation. The third waslaunched in January, 2002. The last one isscheduled for launch in November, 2002.

UFO and Interim Polar EHFBeginning with the fourth satellite in theUFO series, an EHF communications pay-load compatible with Milstar terminals wasadded to that satellite. An enhancement tothe EHF payload beginning with UFO 7doubled its communication capacity.

The EHF payload accommodates multi-ple uplinks distributed between the Earth-coverage antenna and the deployed steer-able spot-beam antenna. Each uplink istime-shared by multiple users. The down-link (at 20 gigahertz) is a combination ofall the uplinks (at 44 gigahertz). Both linksare frequency-hopped.

The Interim Polar Program adapted theUFO/EHF payload for use on host satellitesin high-inclination orbits. These payloadscommunicate with military forces operat-ing above 65 degrees north latitude, wherevisibility to geostationary-orbit satellites ispoor or impossible. The first launch with aninterim polar payload was in 1997. Twolaunches remain, in 2003 and 2005.

SummaryU.S. military satellite communicationshave improved and expanded greatly overthe past four decades, from SCOREthrough DSCS III, UFO, and Milstar. Ca-pabilities have grown dramatically with thedevelopment of satellite and electronicstechnologies. Higher-power and wider-bandwidth satellites have enabled in-creased information transmission to anever-wider assortment of terminal typesdeployed with an increasing number andvariety of military units.

Throughout this history, and now, Aero-space has been involved in every phase ofdevelopment and deployment of DOD

A Communication Satellite Payload

Every satellite is composed of a bus—thatis, the set of systems that keep it going—and a payload that accomplishes its mis-sion. The bus plays a supporting role; itsfunctions include power generation anddistribution, attitude control, and propul-sion. On a communication satellite, thepayload is the communication subsystem,which carries out the communications mission (receiving and transmitting infor-mation). The payload of a communicationsatellite has one or more antennas, receivers, and transmitters, as well ashardware and software that perform someinformation processing. Redundant (spare)units are included for all equipment exceptthe antennas, with each spare correspon-ding to one or more operating units.

Antennas serve as interfaces betweenuplinks (signals transmitted from Earth)and downlinks (signals transmitted toEarth) and the electronics inside the satel-lite. Earth-coverage antennas receive signals from all points on Earth with approximately equal sensitivity and/ortransmit signals to all points on Earth withapproximately equal power. Spot-beam antennas concentrate their receiving sensi-tivity or transmitting power on a limitedarea on Earth; the location of this area can be fixed or steerable.

Receivers are designed to amplify thevery weak (less than one-millionth watt) received signals while minimizing noise.They also filter the received signal to rejectout-of-band noise and interference fromunwanted signals. Processing can be as lit-tle as a frequency translation from the up-link frequency to the downlink frequency; ifthis were not done, the transmitter’s powerwould prevent all reception on the samefrequency. However, processing often in-cludes additional filtering and routing differ-ent groups of received signals to differenttransmitters, and it can also include de-modulating and decoding the uplink infor-mation, then remodulating and recoding itfor the downlink.

Modern communication subsystems divide received signals into many separategroups for efficient transmission and forrouting to multiple antenna beams. Eachgroup of signals and the associated trans-mitter is called a repeater (or transponder).The core of each transmitter is a high-power (typically 10- to 100-watt output)amplifier. A transmitter can also includepost-amplifier filters, as well as switchesthat route the signals to various antennas.

satellite communication systems, fromconcept development and requirementsdefinition through design and test reviewsto launch preparations and on-orbit testingand operations. Aerospace regularly ap-plies lessons learned in the course of oneprogram to all DOD satellite programs.

As military satellite communication sys-tems improve, they continue to provide in-formation superiority to the U.S. military.This enables our military forces to remaindominant in the increasing speed and di-versity of their actions during times ofpeace as well as times of conflict.

Further ReadingAir Force Link, fact sheets (Space fact sheet list includes DSCS, Milstar, UFO), http://www.af.mil/news/indexpages/fs_index.shtml,accessed January 2, 2002.

F. E. Bond and W. H. Curry, Jr., “The Evolutionof Military Satellite Communications Systems,”Signal, Vol. 30, No. 6 (March 1976).

W. H. Curry, Jr., “The Military Satellite Com-munications Systems Architecture,” Paper 76-268, AIAA/CASI 6th Communications SatelliteSystems Conference (April 1976). Reprinted inSatellite Communications: Future Systems,Progress in Astronautics and Aeronautics, Vol.54, D. Jarett, ed. (1977).

I. S. Haas and A. T. Finney, “The DSCS IIISatellite—A Defense Communication Systemfor the 80’s,” AIAA 7th Communications Satel-lite Systems Conference (April 1978).

P. C. Jain, “Architectural Trends in MilitarySatellite Communications Systems,” Proceed-ings of the IEEE, Vol. 78, No. 7 (July 1990).

D. H. Martin, Communication Satellites, FourthEdition. (The Aerospace Press, El Segundo,CA, and AIAA, Reston, VA, 2000).

P. S. Melancon and R. D. Smith, “Fleet SatelliteCommunications (FLTSATCOM) Program,”Paper 80-0562, AIAA 8th CommunicationsSatellite Systems Conference (April 1980).

MILSATCOM Joint Program Office Web site,http://www.losangeles.af.mil/SMC/MC,accessed October 29, 2001.

D. N. Spires and R. W. Sturdevant, “From Ad-vent to Milstar: The U.S. Air Force and theChallenges of Military Satellite Communica-tions,” Ch. 7 in Beyond the Ionosphere: FiftyYears of Satellite Communication, A. J. Butrica,ed. (NASA History Office, Washington, DC,1997); also Journal of the British Interplane-tary Society, Vol. 50, No. 6 (June 1997).

V. W. Wall, “Satellites for Military Communi-cations,” Paper 74-272, AIAA 10th AnnualMeeting (January 1974).

W. W. Ward and F. Floyd, “Thirty Years of Re-search and Development in Space Communica-tions at Lincoln Laboratory,” The Lincoln Labo-ratory Journal, Vol. 2, No. 1 (Spring 1989).

The last two decades have seenmajor changes in spectrummanagement. The state of af-fairs 20 years ago was one of

widely available bandwidththat could be used simply byfilling out the proper paper-work. This clear channelsparadigm was gradually re-placed by a more contentiousenvironment, wherein fo-cused analytic studies wereneeded to allow users to co-exist without conflict. Fur-ther increase in spectrum de-mand now requires users toshare spectrum in an activelycooperative manner. Thenext decade may see assign-ment of priority based on a system’s valueto national and global interests. All thisplaces a greater burden on the spectrum-management community as well as thespectrum-management process, whichmust anticipate and balance future mili-tary, civil, and commercial interests onboth the national and international level.

The Allocation ProcessThe worldwide communications commu-nity follows an established Table of Allo-cations that identifies how different por-tions of the spectrum may be used. In theUnited States, the National Telecommuni-cations and Information Administration(NTIA) and the Federal CommunicationsCommission (FCC) create additionalrules, regulations, and notes that further

complicate the matter. The NTIA answersto the President and manages spectrumuse by the U.S. government, determiningspectrum-use viability prior to program

approval. The FCC answers to Congressand performs a similar role for U.S. civiland commercial organizations. No other

major nation separates these functionsto the extent that the United States does.

International interactions are managedby the United Nations through the Inter-

national TelecommunicationUnion (ITU). Every countryhas the sovereign right tomanage spectrum use withinits borders. The ITU generatesrules and procedures that ap-ply whenever communicationsignals go beyond one na-tion’s borders, as is generallythe case for space communi-cation. The ITU has no polic-ing powers, but member na-tions generally abide by theregulations, motivated both byinternational treaties and con-

sideration of mutual benefit. Member na-tions can also deviate from the rules inmatters of national defense.

Albert “Buzz” Merrill and Marsha Weiskopf

Competing claims on the available radio-frequency spectrum have placed the Department of Defense in a difficult position. Military satellite communications mustprovide unique warfighter mission support, operational security, and high capacity ondemand. But with commercial interests demanding more of the usable spectrum, thesegoals will require strong leadership and vision to attain.

SpectrumManagement

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100 MHz1 MHz 10 GHz

LF MF HF VHF UHF EHFSHF

The radio-frequency spectrum extends from about 3 kilohertz to 300 gigahertz, but communicationsabove 60 gigahertz are generally not practical because of high power needs and equipment costs.As a result, potential users must compete for a very limited slice of the electromagnetic spectrum.

Given the exponential growth in spectrum

demand, it’s fair to say that after cost and

mission objectives, spectrum availability

is the primary driver in space system

architectural design.

To obtain a measure of formal protectionfor spectrum use, member nations applyfor a “registration” from the ITU. Obtain-ing these registrations is critical becausethey establish the formal and legal rights ofthe recipient. These registrations are dis-pensed on a first-come, first-served basisand can only be obtained if the desired useis consistent with the Table of Allocationsand does not create any unresolvable con-flicts. Once an ITU registration is submit-ted, the registrant must bring the systeminto use within seven years or forfeit theprotection.

Registration is a lengthy process. ITUsubmissions commonly take two years justto be entered into the computer databasefor processing, and on the national level,U.S. registrations often take years to com-plete. The Aerospace Corporation hasbriefed various government advisory

boards on this matter and has suggestedmethods to reduce this backlog, predomi-nantly through greater use of computer au-tomation and human networking.

When Conflicts AriseWith so many parties seeking spectrumregistrations, conflicts are inevitable. Allparties adhering to ITU regulations are ob-ligated to resolve any conflicts in goodfaith, and in practice, some accommoda-tion can be reached. In resolving conflicts,three distinct factors are considered: actualspectrum use, including details of powerand modulation; geometry, including satel-lite and terminal locations and antenna directionality; and time or operational limitations.

Spectrum use and geometry are the pri-mary considerations in conflicts involvinggeosynchronous spacecraft. Systems oper-ating in vastly different spectral bands, for

example, present less risk of interference,as do spacecraft that remain on oppositesides of the globe. Problems arise whenthese systems want to use the same trans-mission bands and when their fields ofview overlap.

Contention for the desired bandwidthgets resolved by detailed analysis of mutualradio-frequency interference and direct ne-gotiations between the affected parties—first in terms of U.S. military interests, andthen in terms of broader government con-cerns. Sometimes, U.S. spectrum-use pol-icy requires favoring one mission over an-other to obtain high-priority concessionsfrom other nations. Conflict resolution cantake several years and numerous face-to-face meetings, and may even become a po-litical issue. The length of time puts greatemphasis on knowing which contendingspacecraft are (or will be) registered and ofthose, which are actually viable.

System designers must adhere to this allocation table in order to receivespectrum management approvals. The complexity of this chart clearly un-derscores that many different types of services must coexist, and can only

do so by following the regulations set up by the ITU, NTIA, and FCC. A farmore legible version of this chart can be accessed and downloaded athttp://www.ntia.gov/osmhome/allochrt.html.

stead of the current fragmented practice ofpursuing individual programs, sometimesat the expense of other programs.

Geosynchronous CrowdingTo avoid interference, geosynchronousspace stations need to maintain an orbitalseparation of about two degrees for Ka-band systems and even greater for lowerfrequencies, with actual separations de-pending on how narrow the field of view isfor the ground-terminal antennas. Two de-grees out of a 360-degree orbital arc wouldseem to allow for an abundant number ofspacecraft, but registrations in this areahave gone from very few to oversaturationduring the past 15 years. In the UnitedStates alone, for example, military systemswill include Milstar, Advanced Extremely

High Frequency, Defense Satellite Com-munications System, UHF Follow-On,Wideband Gapfiller satellites, AdvancedWideband System, Global BroadcastingService, the future Mobile User ObjectiveSystem, and Space-Based Infrared System(SBIRS)-High, all stationed in geosyn-chronous orbits and all making use of themilitary K band (20.2–21.2 gigahertz) forsignal transmission.

Compounding the problem, some geo-synchronous satellites occasionally driftacross the sky from one registered positionto another to fulfill their military missions.The challenge is to coordinate all of theseactions, given that each program considersits missions to be of the highest priority.Moreover, each program may fall under

Security is obviously an issue becauseany users of the spectrum must reveal ba-sic mission information to other users. Onthe other hand, the full exploration of thisinformation (needed to identify any poten-tial or actual radio-frequency interference)can expose system vulnerabilities thatshould not be communicated outside ofclassified channels. For this reason, thespectrum-management community ismoving toward more confidentiality, in-cluding the use of generic or nonidentify-ing names instead of actual programnames for registration submissions. Thisseparation will become more important inthe future because it will facilitate the exe-cution of a coordinated U.S. government-wide policy on frequency management in-

All messages or signals sentwithout wires must modulate afrequency somewhere in theelectromagnetic spectrum. Un-fortunately, only a small portionof the spectrum can be used in apractical manner—no one, afterall, would want to receive a cell-phone call via X ray! This usablerange, known as the radio-fre-quency spectrum, extends fromabout 3 kilohertz to 300 giga-hertz, but even this span is fur-ther limited by physical and prac-tical barriers.

Radio waves are attenuatedby rain, fog, and even the waterand oxygen molecules in the air.This attenuation increases signif-icantly as the frequency in-creases. In fact, the atmospherebecomes almost opaque in theregion right around the oxygen-molecule absorption lines (around60 gigahertz). While communica-tion is possible at frequenciesgreater than 60 gigahertz, thepenalty is extreme in terms ofhigher power needs and equipment costs.

The demand is greatest for spectrum below 3 giga-hertz—the “beachfront property” of the spectrum-allocationworld. Users of this frequency range can get by with smallerantennas and lower-power transmitters. Simpler equipmentis obviously of great value to soldiers in foxholes and in air-craft, where mass and power needs are demanding—butit’s also of great value to commercial cell-phone service

Loss

(de

cibe

ls)

10Frequency (gigahertz)

150 mm/h

25 mm/h

5 mm/h

0.25 mm/h

0.1 g/m3

A

C B

A

A

A

1 100 1000

1000

100

10

1

0.1

0.01

Radio waves are attenuated by fog, rain, and even the water and oxygen molecules in the air. Inthis chart, lines A represent rain (in millimeters per hour), line B represents fog (in grams of waterper cubic meter), and line C represents atmospheric gas.The diagram shows a 15-decibel attenu-ation over a distance of only 1 kilometer at 60 gigahertz, whereas actual link distances—and re-sulting attenuation—are much greater.

“Beachfront Property”

providers. The ability to use smaller and simplerhardware is also important because the cost of put-ting a kilogram of equipment in orbit is tens of thou-sands of dollars.

Most of this prime real estate below 3 gigahertzhas already been taken. Now, the challenge is how toprioritize and share this very limited and highly valu-able resource.

the management of adifferent agency, so rela-tive priority adjudication is nosimple matter. All of this is compli-cated by potential conflict with other U.S.government and international systems.

To this mix, one must also add the lowEarth orbiting SBIRS-Low system, whosespacecraft will pass through the downlinkbeams of these other systems. With non-geosynchronous spacecraft such asSBIRS-Low, some radio-frequency inter-ference is inevitable if spectrum used bythe various parties overlaps. The issue ishow much interference can be expectedand for how long. Moreover, nongeosyn-chronous spacecraft are given lower prior-ity than geosynchronous systems in thespectrum-management world, and theytherefore bear all the burden of preventingor mitigating such interference. To avoidgenerating radio-frequency interference,SBIRS-Low is considering using satellite-to-satellite crosslinks with alternate rout-ing from several different spacecraft to agiven ground station or stations.

Spectrum and AcquisitionGiven the lengthy domestic and interna-tional registration process, it’s clearly im-portant for spectrum-management supportpeople to work closely with the acquisitioncommunity. A thorough consideration ofthe opportunities, limitations, and generalrealities of spectrum use is critical to cost-effective space-system conceptualization,acquisition, and overall operation. The

Department of De-fense (DOD) Instruction

on the Operation of the De-fense Acquisition System already re-

quires program offices to submit registra-tion paperwork before beginning thesystem demonstration and productionphases of the acquisition cycle. This paper-work in turn triggers a formal review bythe applicable military agency and theNTIA. Unfortunately, even at these stages,the system architecture may already be es-tablished in such a way that puts unreason-able demands upon the available spectrum.Aerospace is working with the governmentto require an increased level of attention tospectrum issues well before the system ac-quisition phase so that critical issues suchas sensor-data generation, communication-channel requirements, and realisticallyavailable spectrum can all contribute har-moniously to an overall integrated systemarchitecture.

For example, at the Air Force Space andMissile Systems Center, Aerospace helpeddraft a Commander’s Instruction that requires the various programs to develop aplan addressing spectrum-use viability inthe conceptual stages. This Instruction fur-ther dictates that all programs must iden-tify any deviations from spectrum regula-tions so that they can be addressed early inthe acquisition stages.

IMT-2000It must be emphasized that the military’scommunication needs are fundamentally

distinct from commercial needs. Militaryspectrum requirements are based upon theneed for high-volume communicationsand sensing 100 percent of the time whenfighting a war. Because the United Statesis not fighting a war most of the time,much of the military’s allocated spectrumgoes unused (except for training exercises)for long periods of time. This sporadic useleads to the unfortunate misconceptionthat the military is an inefficient user of spectrum. As a result, the Defense Depart-ment sometimes has a difficult job defend-ing its allocations.

A prime example of this difficulty canbe found in the International Mobile Tele-phone 2000 (IMT-2000) initiative, whichhas been occupying spectrum analystssince October 13, 2000, when PresidentClinton directed the NTIA and charged theFCC to help select the spectrum to be usedby the third-generation mobile wireless(cell-phone) service, known as 3G.

As part of this effort, NTIA studied thepossibility of using a bandwidth of1755–1850 megahertz and FCC looked at2500–2690 megahertz. Most of the1755–1850-megahertz band is utilized bythe Air Force Satellite Control Network(AFSCN) for tracking, telemetry, andcommand and also for other governmentservices such as Air Combat Training. Thisband uses the Space Ground Link Subsys-tem for implementation and is commonlyreferred to as the SGLS band. Most of the2500–2690-megahertz band is used for

Many U.S. military spacecraft use (or will use)the K band (20.2–21.2 gigahertz) for downlink-ing national security information to ground sta-tions. These include Air Force MILSTATCOMand surveillance systems and Navy tacticalsystems. This congestion demonstrates theneed for tighter and more effective communica-tions, both in planning and operations, betweenthe various space-faring branches of the armedforces.

WidebandGapfiller UFO

AdvancedWideband

Milstar

GBS

SBIRS-High

SBIRS-Low

AEHF

educational television, Internet access, andsimilar services. Clearly, the militarywould prefer that the 2500–2690-mega-hertz band be selected for 3G use, but sofar, that has not been the indicated courseof action.

AFSCN ground stations around theworld use large antennas (10–18 meters indiameter) and high power levels(100–10,000 watts) to support about 120expensive and mission-critical spacecraft.To ensure successful communication, par-ticularly during anomalous conditions,AFSCN generally uses high-power trans-missions. Aerospace has worked to analyzethe impact of using the SGLS-band for theproposed 3G implementation, particularlyin terms of possible spectrum sharing andmitigation of potential radio-frequency in-terference. The effort considered in greatdetail and high fidelity the 3G system pa-rameters and the characteristics of theDOD space-system orbits and equipment.The study indicated that the 3G system ofbase stations can seriously disrupt the re-ception of SGLS uplink commands undermany circumstances because all the radio-frequency emissions from 3G users withinthe field of view of a spacecraft antennacombine to cause a significant amount ofinterference. This study was cited in a re-port by the General Accounting Office,which indicated the need for further study.

The other side of this issue is the impactthat the high-power AFSCN ground

stations will have on the 3G systems. Initialanalyses by Aerospace and the DOD’sJoint Spectrum Center (an office charteredto ensure the military’s effective and effi-cient use of the electromagnetic spectrum)have shown disruption to 3G users as faraway as 320 kilometers. Aerospace ana-lyzed the possibility of mitigating these ef-fects. Filtering of the AFSCN output sig-nals was shown to have great benefit for 3Guse within a few megahertz of the frequen-cies used, provided that certain innovativeand potentially costly techniques are used.For overlapping spectral use, Aerospacedevised a novel technique called dynamicreallocation, which would essentially warnthe 3G system of an intended AFSCN radi-ation burst. The 3G system could then ad-just its spectrum use to avoid AFSCN inter-ference. This would reduce the 3G serviceby a moderate amount in the region aroundthe AFSCN station, but only while com-mands are being sent to military spacecraft.This concept has security implications andis just one of the many alternatives beingconsidered by the joint government teamseeking a consensus that would supportU.S. commercial interests while ensuringdefense mission satisfaction.

ConclusionGiven the exponential growth of spectrumdemands during the last 10 years, it wouldnot be an exaggeration to say that, aftercost and mission objectives, spectrumavailability is the primary driver in space

system architectural design. Consequently,organizations such as Aerospace that sup-port DOD must study not only militarymission requirements but also global eco-nomic trends. While it is true that Aero-space is chartered to support national se-curity space and not commercial interestssuch as cell-phone sales, the demand forspectrum will inevitably result in these twoforces colliding. An appreciation of thecommercial world enables the militaryspectrum-management community to an-ticipate and understand the technical issuesassociated with bandwidth sharing andthus support the overall objective of aneconomically strong and secure nation.

National security spacecraft are de-signed for extreme longevity, and their ba-sic spectrum use cannot be changed whilein orbit. Therefore, system designers mustanticipate future needs for spectrum use.Aerospace has thus written long-termspectrum-use plans and white papers forits various customers to facilitate this con-versation. The focus and decision-makingdialogue generated by such an activity val-idates the need for conceptual planning inspectrum use. More work on detailed long-range spectrum-use plans for DOD or forthe government overall is clearly needed.

A national spectrum-management planmust acknowledge the critical need to fos-ter cooperative relationships with interna-tional policymakers. Within the spectrum-management world, relationships are

On the international level, spectrum allocation is managed by the Interna-tional Telecommunication Union (ITU). Requests for registered slots withinthe 7–8-gigahertz range—commonly used by geosynchronous satellites—increased significantly in the last two decades and have remained high forthe past several years.

The United States and NATO have agreed to restrict the 20-gigahertz bandto military purposes, but that has not stopped many commercial partiesfrom joining the fray. Years ago, spectrum analysts thought that programsmight “escape” to this band through the use of advanced technology,thereby avoiding contention for spectrum at lower bandwidths.

1994 1995 1996 1997 2000 2001Time

300

200

100

0

Inte

rnat

iona

l tel

ecom

mun

icat

ions

unio

n fil

ings

(nu

mbe

r)

All other countriesU.S. commercialSoviet Union/CISU.S. government

1986 1992 1995 1996 2000 2001Time

Inte

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(nu

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All other countriesU.S. commercialSoviet Union/CISU.S. government

800

200

100

0

300

700

600

500

400

1989

Growing Demand at 7–8 Gigahertz Growing Demand at 20 Gigahertz

Terrestrial Other International Other Adjacentradio- radio Mobile space/ Space spectrum No

frequency navigation Telephone ground contention terrestrial problems

noise systems 2000 users users yet

perhaps even more important than rulesand regulations because rules are alwayssubject to interpretation by key people.Trusted relationships are also critical be-cause spectrum-use planning requires theexchange of database tools, mission goals,trend evaluations, and various potentiallysensitive mission details.

Finally, emerging technologies such aslasers and new modulation techniques mayfoster more-efficient use of the availablespectrum and improve the ability of usersto share this limited resource. But as al-ways, technology must be leveraged bysound policies, clear foresight, and con-structive engagement within the spectrum-management community. A combinationof vision, leadership, and synergism is crit-ical for achieving harmonious, efficient,and effective use of this increasingly lim-ited resource.

Further ReadingI. Brown, A. Kavetsky, M. J. Riccio, M.Weiskopf, “Spectrum Management and Interna-tional Filing from the Acquisition ProgramManager’s Perspective: Current Process andRecent Changes,” Proceedings of MILCOM2000, Vol.1, pp. 1–7 (October 2000).

“Connecting the Globe—A Regulator’s Guideto Building a Global Information Community,”Federal Communications Commission (June16, 1999).

Defense Information Systems Agency, Office ofSpectrum Analysis and Management, http://www.disa.mil/d3/depdirops/spectrum/, accessedNovember 20, 2001.

Federal Communications Commission, 2003World Radiocommunications Conference,http://www.fcc.gov/wrc-03/, accessed Novem-ber 20, 2001.

Federal Communications Commission, RadioSpectrum Home Page, http://www.fcc.gov/oet/spectrum/, accessed November 20, 2001.

International Telecommunication Union, http://www.itu.int/ITU-R/index.html, accessed No-vember 20, 2001.

Joint Spectrum Center Home Page, http://www.jsc.mil/, accessed November 20, 2001.

B. Z. Kobb, Wireless Spectrum Finder:Telecommunications, Government and Scien-tific Radio Frequency Allocations in the US 30MHz–300 GHz, McGraw-Hill ProfessionalPublishing (March 2001).

Naval Electromagnetic Spectrum Center, http://www.navemscen.navy.mil/, accessed Novem-ber 20, 2001.

NTIA Manual of Regulations & Procedures forFederal Radio Frequency Management, Janu-ary 2000 Edition, May/Sept 2000 Revisions,U.S. Department of Commerce, NationalTelecommunication and Information Adminis-tration, http://www.ntia.doc.gov/osmhome/red-book/redbook.html, accessed November 21,2001.

NTIA Office of Spectrum Management, http://www.ntia.doc.gov/osmhome/osmhome.html,accessed November 20, 2001.

P. C. Roosa Jr., NTIA Special Publication 91-25, Federal Spectrum Management: A Guide tothe NTIA Process, U.S. Department of Com-merce, National Telecommunication and Infor-mation Administration (August 1992).

Radio Regulations, Volumes 1–4, InternationalTelecommunication Union (2001).

Field Manual No. 24-2, Spectrum Management,Department of the Army, Washington, DC (Au-gust 1991).

U.S. Army Spectrum Management and Com-munications, Publications, http://www.army.mil/spectrum/library/regulations.htm, accessedNovember 21, 2001,

D. J. Withers, ed., Radio Spectrum Manage-ment: Management of the Spectrum and Regu-lation of Radio Services (IEEE Telecommuni-cations Series, 45) (January 2000).

Military spacecraft in geosynchronous orbits (GSO), medium Earth orbits(MEO), and low Earth orbits (LEO) will face new challenges in the comingyears. Contention for orbital space is a growing concern, particularly for

geosynchronous spacecraft. An even greater concern is the proposed implementation of IMT-2000, the next-generation international mobile telephone service.

Milstar (GSO)

UHF Follow-on (GSO)

Defense Satellite Communication System (GSO)

Space BasedInfrared System-High (GSO)

Space BasedInfrared System-Low (LEO)

Defense MeteorologicalSatellite Program (LEO)

Global Positioning System (MEO)

Frequency band UHF L S X Kt Ka W

✓ ✓ ✓ ✓ ✓

✓ ✓ ✓ ✓

✓ ✓

✓ ✓ ✓

✓ ✓ ✓

✓ ✓ ✓ ✓

✓ ✓ ✓ ✓ ✓

Program

Issues

The recent proliferation of ter-restrial and space-based com-munication systems has givenrise to an increasingly critical

problem—the lack of available frequencyspectrum. One tool that satellite systemdesigners can use to maximize the use ofavailable spectrum is bandwidth-efficientmodulation. This technique can enhancebandwidth efficiency while retaining rea-sonable power efficiency and implemen-tation complexity. Because of the wideapplicability of bandwidth-efficient mod-ulation to most new satellite systems, TheAerospace Corporation has performed extensive research in this area. One recentapplication can be found in the AdvancedExtremely High Frequency (AEHF) program.

A successor to Milstar I and II, theAEHF program will form the basis of themilitary’s next-generation protected com-munication system. Specifications calledfor a tenfold increase in capacity over thecurrent Milstar system; however, earlystudies clearly indicated that the newdownlink requirements could not be metwithin the existing frequency allocationsimply by extending the Milstar design.The MILSATCOM (Military SatelliteCommunications) Joint Program Office atthe Air Force Space and Missile SystemsCenter asked Aerospace to help investi-gate alternative signaling methods thatwould use the allotted bandwidth more efficiently.

Phase-Shift ModulationAerospace researchers began by charac-terizing traditional binary phase-shift key-ing and quarternary phase-shift keying—

two commonly employed satellite signal-transmission techniques—in light of thenew capacity requirements. Milstar cur-rently uses differential phase-shift keyingfor its downlink. This method is similar tobinary phase-shift keying and exhibits thesame power spectral density, a measure ofthe distribution of signal power versusfrequency.

Systems that transmit multiple signalswithin a given bandwidth have several op-tions for sharing frequency resources.One technique, called frequency-divisionmultiple access, assigns a carrier or chan-nel to each signal, centered at a uniquetransmission frequency. Designers typi-cally want to space these channels asclosely as possible to increase the system

capacity, but as the spacing gets too close,the power spectra start to overlap, andpower from one channel spills into an-other. This phenomenon, known as adja-cent channel interference, increases theprobability of transmission errors, alsoknown as the bit-error rate.

The power spectral density of both bi-nary and quarternary phase-shift keying isfairly broad, and when channels arepacked together too tightly, the adjacentchannel interference can be severe. In thecase of the AEHF program, the channelswould have to be spaced far apart to avoidlarge degradations from such interference.Researchers discovered that they simplycould not fit enough channels within theallocated downlink frequency to meet the

Using bandwidth-efficient modulation, communication satellites can transmit signals through a smaller frequency band. The Aerospace Corporation’s research into one such technique has yielded tangible benefits for the military’s protected communication satellites.

Diana M. Johnson and Tien M. Nguyen

ModulationThrough Gaussian Minimum Shift Keying

Bandwidth-Efficient

Frequency from carrier (megahertz)

Nor

mal

ized

pow

er s

pect

ral d

ensi

ty (

deci

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)

–100

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Differentialphase-shift

keying

Gaussianminimum

phase-shiftkeying

The power spectral density for Gaussian minimum shift keying is much more compact than that ofdifferential phase-shift keying and does not exhibit the same pronounced sidelobes. In this example,the bandwidth–bit-time product is 1/6.

capacity requirement using standard bi-nary, differential, or quarternary phase-shift keying. Other, more advanced modu-lation techniques would have to be found.

Nor

mal

ized

pow

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keyingBit-time

product = 1/2

1/4

1/6

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Gaussian

minimum shiftkeying signal

Data pulses

Continuous phase

Pha

se o

ver

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Time0 2 4 6 8

Gaussianfilter

Frequencymodulator

Filtered pulses

–0.4

–0.2

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Time

Gaussian Minimum Shift KeyingAerospace had been studying a modulationtechnique known as Gaussian minimumshift keying for potential application in the

With Gaussian minimum shift keying, the rectangular pulses representing input bits are converted intoGaussian shaped pulses. The resulting carrier signal is smooth in phase, and therefore requires lessbandwidth to transmit. The configuration shown here uses a bandwidth–bit-time product of 1/5.

Gaussian minimum shift keying waveforms with varying bandwidth–bit-time products are comparedwith binary and differential phase-shift keying. As the bandwidth–bit-time product decreases, thewaveform spectra grows narrower.

Air Force Satellite Control Network andrecognized that it might be a good candi-date for the AEHF program.

Gaussian minimum shift keying is aform of continuous phase modulation, atechnique that achieves smooth phase tran-sitions between signal states, thereby reducing bandwidth requirements. WithGaussian minimum shift keying, input bitswith rectangular (+1, −1) representationare converted to Gaussian (bell-shaped)pulses by a Gaussian filter before furthersmoothing by a frequency modulator. Also,in most cases, the Gaussian pulse is al-lowed to last longer than one bit time—theamount of time a binary 1 is in the “on”position. Consequently, the pulses overlap,giving rise to a phenomenon known as in-tersymbol interference. The extent of thisoverlap is determined by the product of thebandwidth of the Gaussian filter and thedata-bit duration; the smaller the band-width–bit-time product, the more the databits or pulses overlap.

The resulting carrier signal is verysmooth in phase—particularly in compari-son to waveforms generated through stan-dard binary or quarternary phase-shift key-ing. This is important because signals withsmooth phase transitions require less band-width to transmit. On the other hand, thisvery smooth phase makes the receiver’sjob much harder. With Gaussian minimumshift keying, there are no well-definedphase transitions to detect for bit synchro-nization, and the energy from each bit ismixed with the energy from several otherbits. The transmitter output looks nothinglike the data input, and on the receiver side,a special demodulator of increased com-plexity is needed to extract the data bits.For the receiver to achieve a given bit-errorrate, the transmitter must generate morepower to overcome the receiver noise inthe presence of the intersymbol interfer-ence. In other words, the Gaussian mini-mum shift keying waveform is usually lesspower-efficient than more traditionalwaveforms such as binary phase-shift key-ing and requires a more complex receiver,but this potential reduction in power effi-ciency and increase in receiver complexitycould be rewarded with a very significantenhancement of bandwidth efficiency. So,with Gaussian minimum shift keying,there is a trade-off between bandwidth effi-ciency and power efficiency.

their narrow bandwidth occupancy and therapid roll-off of their power spectra. Thesetwo factors strongly influence the ability topack many different channels into a lim-ited amount of bandwidth. The Gaussianminimum shift keying waveform exhibitsa steep power spectrum and therefore co-exists well with adjacent channels in a frequency-division multiple-access system.

All communications are degraded by nat-urally occurring noise in both the environ-ment and hardware. This noise interfereswith the transmitted bit energy, and if it isstrong enough, it may cause the receiverto make an error in deciding which bit (0or 1) was transmitted. Because noise israndom, a digital receiver’s performancecan only be described in terms of theprobability that it will make a bit error,given the transmitted bit energy and thenoise environment.

By plotting the bit-error probabilityagainst the bit-signal-to-noise ratio,

designers can get an idea of what kind oferror performance to expect for a giventransmitted bit energy and noise environ-ment. These so-called bit-error-ratecurves are commonly used to comparedifferent digital communication tech-niques. For example, for transmitting digi-tized voice, a bit-error probability of 10−3

(1 error per 1000 bits, on average) isgenerally considered sufficient. Whenchoosing between two possible transmis-sion techniques, a designer might usebit-error-rate curves to determine whichtechnique would require the smallesttransmitted bit energy to achieve the de-sired bit-error probability of 10−3.

Power spectral density is anothercommon characteristic used to comparethe bandwidth of communication signals.Power spectral density curves show thedistribution of signal power in the fre-quency domain. Often, a system requiresthat multiple user signals coexist within agiven bandwidth. If the system uses frequency-division multiplex techniques,each signal, or channel, is assigned aunique center-transmission frequency, and channels are placed adjacent to each other.

As the spacing of these signals getscloser, their power spectra start to over-lap, and power from one signal spills intoanother. This phenomenon, known asadjacent channel interference, increasesthe bit-error rate (which, in practicalterms, means a more-complex receiveror more bit-signal-to-noise power isneeded to accommodate the interfer-ence). A limited number of signals canbe packed within a specified frequencyband, given a maximum allowable signal-to-noise degradation; this number isknown as the channel packing efficiency.When packing signals, communicationsystem designers need to trade channelspacing (in frequency) with performancedegradation caused by adjacent channelinterference.

Performance Measures for Digital Communication Systems

Amplitude

TimeTb

+1

–1

2Tb 3Tb 4Tb

Originalbit stream

Bit error

Amplitude

TimeTb

Tb = Bit period

+1

–1

2Tb3Tb 4Tb

Amplitude

Time

+1

–1

Bit errors in digital communication systems canbe caused by unwanted noise in the hardwareand environment. In this figure, the top graphrepresents the bit stream and the middle graphrepresents the noise. The bottom graph com-bines the two, and shows how noise can causean error in the bit stream.

Gaussian minimum shift keying is notnew—the technique has been used exten-sively in Europe for cell-phone applica-tions with a bandwidth–bit-time product of0.3. But system designs using very smallbandwidth–bit-time products such as 1/5or 1/8 are new—and challenging. Aero-space became interested in these smallerbandwidth–bit-time products because of

Feasibility TestingWhen Aerospace first proposed the use ofGaussian minimum shift keying for theAEHF program, the milsatcom commu-nity reacted with considerable skepticism.By all accounts, much work still needed tobe done. In particular, researchers neededto figure out how to demodulate Gaussianminimum shift keying waveforms havingsmall bandwidth–bit-time products. Also,they had to devise a method to acquire andtrack the frequency, phase, and bit timingof the Gaussian signals. Other unknownsincluded how closely channels could bespaced and how the technique would per-form in the real world, with imperfect timeand frequency synchronization. The mil-satcom technical community was also con-cerned about the implementation complex-ity of the demodulator, and had to be

Bit-signal-to-noise ratio (decibels)

Bit-

erro

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C-code simulation

Breadboard test

120 2 4 6 8 1010–5

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BPSK:Theoretical

GMSK:Perfect timeand phase

GMSK: Imperfecttime and phase

Aerospace developed a demodulator algorithmspecifically optimized for Gaussian minimumshift keying signals.The program was first com-piled in C for simulation purposes, then sub-jected to breadboard testing. The predicted re-sults are closely matched. Here, thebandwidth–bit-time product is 0.2.

In demonstrating the effectiveness of Gaussianminimum shift keying, Aerospace developed thetracking loops needed to maintain time, fre-quency, and phase synchronization. As shownhere, the simulations exhibited negligible degra-dation on demodulator bit-error rate, when com-pared to ideal synchronization.

convinced that the risk was acceptable. Inaddition, researchers needed to investigatehow Gaussian minimum shift keying per-formed under special operating environ-ments associated with AEHF satellites,such as jamming. Backward compatibilitywith existing Milstar terminals was also animportant consideration.

DemodulationAs a first step, Aerospace researchers de-veloped a demodulator algorithm specifi-cally optimized for Gaussian minimum

Before it can be transmitted or received ina satellite system, a data stream must beencoded onto a carrier signal that willpropagate by means of an electromagneticwave. This process, called modulation, ap-plies to both conventional radio-frequencyand newer optical techniques. In eithercase, the data stream is converted to awaveform that is compatible with the trans-mission technology.

A typical spectrum of a digital signalconsists of a main lobe, centered on thecarrier frequency, where the majority of thesignal power is contained, and sidelobes,where the remaining signal power is con-tained.

Carrier signals can be completely de-scribed by three parameters: amplitude,frequency, and phase. Any one of thesethree can be manipulated to suit the datatransmission requirements. Amplitude-shiftkeying, for example, modifies the ampli-tude of the carrier wave. Frequency-shiftkeying adjusts the frequency, and phase-shift keying manipulates the phase.

Amplitude-shift keying is used exten-sively for commercial terrestrial applica-tions, but its usefulness for satellite appli-cations is limited. Space systems typicallyemploy saturated power amplifiers, whichfunction at the maximum operating point tomaximize power efficiency. Around thismaximum point, however, amplifier per-formance is nonlinear, meaning the outputis no longer directly proportional to the in-put. When an amplitude-shifted keying sig-nal is passed through such a nonlinearamplifier, sidelobes can grow large enoughto interfere with the adjacent signals. As aresult, the amount of bandwidth or powerneeded for signal transmission increases.

Frequency-shift keying has some attrac-tive characteristics, but it generally doesnot use bandwidth very efficiently and isnot suitable for applications where band-width efficiency is crucial.

Phase-shift keying does not suffer thesame degradation through a saturated

Modulation Basicspower amplifier as amplitude-shift keyingand generally uses bandwidth more effi-ciently than frequency-shift keying. Somecommon formats include binary and quar-ternary phase-shift keying. Binary phase-shift keying takes each input bit individu-ally and chooses one of two possiblephases to represent that bit value—so forexample, to send a “0,” a phase of 0 de-grees might be chosen, and to send a “1,”a phase of 180 degrees might be chosen.

Quarternary phase-shift keying takestwo bits at a time and chooses one of fourpossible phases to represent them—so, tosend a “00,” a phase of 0 degrees mightbe chosen; to send a “01,” a phase of 90degrees might be chosen; to send a “11,” aphase of 180 degrees might be chosen;and to send a “10,” a phase of 270 de-grees might be chosen.

This method can be extended to takethree input bits, choosing one of eight (or23) phases, or four bits, choosing one ofsixteen (or 24) phases. This pattern can beextended to a generalized M phase-shiftkeying signal, and all these waveformswould have the same power spectral den-sity. But the improvement in bandwidth ef-ficiency comes at the cost of decreasedpower efficiency. The receiver must decidewhich phase, out of a possible M choices,was transmitted. Those M phases are only

separated by 360/M degrees. The phaseseparation for binary phase-shift keying,for example, is 360/2 or 180 degrees. Ahigher M value would have a correspond-ingly smaller phase separation. Thissmaller separation means it takes lessnoise to corrupt the signal and cause thereceiver to make an error. To combat this,more power must be transmitted.

Another approach is to shape the wave-form spectra coming out of the modulator.This can be accomplished by filtering thewaveform to narrow the main lobe and re-duce sidelobes. The downside to filteredwaveforms is that the filtering process dis-torts the amplitude of the signal, resultingin the same distortion through a saturatedpower amplifier found in amplitude-shiftkeying.

Another option is to use continuousphase modulation. Waveforms produced inthis manner exhibit smooth phase transi-tions rather than the abrupt phase transi-tions produced through binary or quar-ternary phase-shift keying. This isimportant because smooth phase transi-tions require less bandwidth for signaltransmission. Gaussian minimum shiftkeying, the waveform that Aerospace pro-posed for the Advanced Extremely HighFrequency program, is a form of continu-ous phase modulation.

Digital datasource

Mappingdevice

00

01

11

10

0 deg

90 deg

180 deg

270 deg

→

→

→

→

Phaseshiftingdevice

0 deg

90 deg

180 deg

270 deg

Carrier-wave

generator

Processedsignal

shift keying signals. The algorithm wasinitially created and validated using com-mercial modeling software and was latertranslated into C, a programming languagethat runs very fast on a personal computer,to shorten the simulation times. Optimiz-ing the demodulator was not an easy task.Receiver noise is inevitable, and becausenoise is a random process, designers ingeneral can only seek to maximize theprobability that a demodulator will cor-rectly discern the value of each bit that is

transmitted. Moreover, to compensate forthe intentional intersymbol interference in-troduced by the Gaussian minimum shiftkeying modulation process, the optimal re-ceiver would have to look at sequences ofbits, not individual bits, to decide whatdata were sent. Ultimately, Aerospace de-veloped a demodulator, called a MaximumLikelihood Sequence Estimator, that re-ceives a signal in white Gaussian noise andoutputs an estimate of the most likely datasequence transmitted.

This graph shows the measured power spectral density for Gaussian minimum shift keying whenpassed though a standard (linear) and saturated (nonlinear) power amplifier. Even in scenarios involv-ing saturated amplifiers, the technique does not give rise to significant sidelobes. In this example, thebandwidth–bit-time product is 0.125, and the data rate is 1 megabit per second.

Measured data showing growth of sidelobes in the power spectral density of offset quarternaryphase-shift keying. The pink curve indicates performance through a standard (linear) amplifier, whilethe green curve shows the poorer performance though a saturated (nonlinear) amplifier.

In developing the demodulator, Aero-space researchers also discovered that bypreparing or “preprocessing” the datathrough a specialized algorithm known asthe precoding algorithm, the demodulatorperformance could be improved. As a re-sult, the transmission power requirementscould be reduced by 2 to 2.5 decibels. Sim-ulations using this precoding algorithm andthe Maximum Likelihood Sequence Esti-mator demonstrated that, assuming idealsynchronization, the power performance ofGaussian minimum shift keying is superiorto differential phase-shift keying by 0.5 to1 decibel at the bit-error rate required forthe AEHF program.

More recently, Aerospace improved onthe Gaussian minimum shift keying de-modulator with the development of a so-called soft-decision demodulator. Whilethe Maximum Likelihood Sequence Esti-mator provides only hard yes-no decisions(was a 1 sent or not?), the soft-output de-modulator yields both the bit decision anda reliability measure of that decision(there’s a 90 percent likelihood that a 1 wassent). The use of this soft-output demodu-lator enables soft-decision decoding, whichprovides an additional 2.5-decibel poweradvantage over hard-decision decoding(see related article, “Forward Error Correc-tion Coding,” in this issue).

Tracking and AcquisitionHaving simulated the performance of theGaussian minimum shift keying signalswith ideal transmitter and receiver synchro-nization, Aerospace researchers then designed the tracking loops needed tomaintain time, frequency, and phase syn-chronization. Using a mathematical modelof the received signal, they derived timing-error and phase-error information using thereceived random data and designed track-ing algorithms to track and correct these er-rors. The tracking algorithms were vali-dated in C-code simulations and werefound to have negligible degradation on de-modulator bit-error rate, when compared toideal synchronization.

Researchers then implemented a signal-frequency acquisition scheme that allowsacquisition of the Gaussian minimum shiftkeying signal within a relatively wide win-dow of frequency uncertainty. A series ofC-code simulations demonstrated that itsresidual frequency error could be easilyhandled by the phase-tracking algorithm. Itshould also be noted that although both thesignal-acquisition and tracking functions

are needed for a stand-alone application ofthe Gaussian minimum shift keying signal,the AEHF system would only need phasetracking at the satellite payload because thesignal timing and frequency of the trans-mitting terminal are already synchronizedto the satellite while operating in commu-nication mode.

Next, Aerospace assessed the jammingvulnerability of the demodulator with itsattendant phase-tracking algorithm. Simu-lations were conducted using jammers that

Saturated

Linear

Nor

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pow

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deci

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)–100

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Frequency from carrier (megahertz)–4 –2 0 2 41 3–1–3

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Frequency from carrier (megahertz)–2 –1 0 1 2

were representative of any anticipatedthreat. Results showed that none of thejammers had a serious effect on demodula-tor performance. In addition, the issue ofbackward compatibility with existing Mil-star terminals was addressed. Aerospaceverified, by simulation and analysis, that aproperly designed Gaussian minimum shiftkeying downlink could be tracked and de-modulated, with acceptable degradation,by existing Milstar terminals designed fordifferential phase-shift keying signals.

The measured power spectral density for Gaussian minimum shift keying.The yellow curve shows theperformance calculation derived from the C-code computation. The orange curve shows the resultsfrom a breadboard test (the noise floor is higher because of the test-equipment limitations). In this ex-ample, the bandwidth–bit-time product is 0.2, and the data rate is 1 megabit per second.

This schematic diagram shows the prototype Gaussian minimum shift keying modulator/demodulator test configuration, with time and phase tracking.

Nor

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)

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C-code computation

Noise floor

Frequency from carrier (megahertz)–2 –1 0 1 2

Receiveddata

Received data

Transmitteddata

Quadrature

Clock

In-phase

Digital/analog

Digital/analogQuadrature

In-phaseIn-phase/

quadratureconverter

Timing clockgenerator

Amplifier 118 megahertz modulated signal

Noise generator

X

Power dividerSpectrumanalyzer

118 megahertz modulated signal with noise

Timingreference

Frequencyreference

118 megahertz reference signal

Gaussian minimumshift keying

demodulator with timeand phase tracking

Gaussianminimum shift

keying modulator

Data generator/bit-error-rate

testerVector signal

generator

Digital/analog

Digital/analog

Band-pass filter

PrototypingAfter the simulation stage, Aerospace re-searchers moved on to the prototypingstage. Field-programmable gate arrays(customizable integrated circuits that arecommonly used in hardware development)were combined with components such asanalog-to-digital and digital-to-analogconverters to create a laboratory prototypeof the modulator and demodulator, withsignal acquisition and tracking functions.Radio-frequency components were addedto convert the signal up and down asneeded, and a high-power amplifier wasinserted to demonstrate how Gaussianminimum shift keying performed througha saturated power amplifier. This prototypemodem allowed Aerospace and the larger

milsatcom community to confirm the sim-ulation results, obtain hardware complex-ity estimates, and gain confidence inGaussian minimum shift keying as a viableoption for both the downlinks and high-data-rate uplinks of the AEHF system.

ConclusionAerospace has worked closely with theAEHF contractors and other agencies totransfer knowledge and experience aboutGaussian minimum shift keying—particu-larly for signals with small bandwidth–bit-time products. Extensive simulations andanalyses performed by Aerospace and thelarger milsatcom community have con-firmed that the technique is suitable for theAEHF program and that the proposeddownlink waveform can in fact meet the

new system requirements with acceptableimplementation complexity and risk. As aresult, the AEHF program has adopted theGaussian minimum shift keying techniquefor both of its high-data-rate uplink anddownlink components.

AcknowledgmentThe authors would like to thank Dr. GeeLui for his guidance and assistance inpreparing this article.

Further ReadingJ. B. Anderson, T. Aulin, and C. E. Sundberg,Digital Phase Modulation (Plenum Press, NewYork, 1986).

J. B. Anderson and C. E. Sundberg, “Advancesin Constant Envelope Coded Modulation,”IEEE Communications Magazine, pp. 36–45(December 1991).

G. K. Kaleh, “Simple Coherent Receivers forPartial Response Continuous Phase Modula-tion,” IEEE Journal on Selected Areas in Com-munications, pp. 1427–1436 (December 1989).

G. L. Lui, “Threshold Detection Performanceof GMSK Signal with BT=0.5,” Proceedings of1999 Military Communications Conference(Atlantic City, NJ, November, 1999).

G. L. Lui and K. Tsai, “Viterbi and Serial De-modulators for Pre-coded Binary GMSK,” Pro-ceedings of the 1999 International TelemetryConference (October 1999).

P. Michel, “The Occupied Bandwidth andSpectral Characteristics of Filtered/UnfilteredPSK, MSK, GMSK,” CCSDS Report of theProceedings of the RF and Modulation, ed. T. M. Nguyen (GSOC, Germany, Sept.1993).

B. Sklar, Digital Communications (Prentice-Hall, Englewood Cliffs, NJ, 1988).

K. Tsai and G. L. Lui, “Binary GMSK: Charac-teristics and Performance,” Proceedings of the1999 International Telemetry Conference(October 1999).

Digital communication systems, particularly for military use, need to perform accuratelyand reliably in the presence of noise and interference. Among many possible ways toachieve this goal, forward error-correction coding is the most effective and economical.

Forward error-correction coding(also called channel coding) is atype of digital signal processingthat improves data reliability by

introducing a known structure into a datasequence prior to transmission or storage.This structure enables a receiving systemto detect and possibly correct errorscaused by corruption from the channeland the receiver. As the name implies, thiscoding technique enables the decoder tocorrect errors without requesting retrans-mission of the original information.

The Aerospace Corporation has de-voted considerable effort to research anddevelopment of forward error-correctiontechniques, with particular emphasis onone method known as turbo coding. Thiswork has played an important role in sup-porting several government programs, in-cluding the Advanced Extremely HighFrequency program, the Advanced Wide-band System, and the Geostationary Op-erational Environmental Satellite System.

The Evolution of Forward ErrorCorrectionIn a communication system that employsforward error-correction coding, a digitalinformation source sends a data sequenceto an encoder. The encoder inserts redun-dant (or parity) bits, thereby outputting alonger sequence of code bits, called acodeword. Such codewords can then betransmitted to a receiver, which uses asuitable decoder to extract the originaldata sequence.

Codes that introduce a large measure ofredundancy convey relatively little infor-mation per each individual code bit. Thisis advantageous because it reduces thelikelihood that all of the original data will

be wiped out during a single transmission.On the other hand, the addition of paritybits will generally increase transmissionbandwidth requirements or message delay(or both).

Algebraic coding (also known as blockcoding) was the only type of forward error-correction coding in use whenClaude Shannon published his seminalMathematical Theory of Communicationin 1948. With this technique, the encoderintersperses parity bits into the data se-quence using a particular algebraic algo-rithm. On the receiving end, the decoderapplies an inverse of the algebraic algo-rithm to identify and correct any errorscaused by channel corruption.

Another forward error-correcting tech-nique, known as convolutional coding,was first introduced in 1955. Convolu-tional codes process the incoming bits instreams rather than in blocks. The para-mount feature of such codes is that the en-coding of any bit is strongly influenced bythe bits that preceded it (that is, the mem-ory of past bits). A convolutional decodertakes into account such memory when try-ing to estimate the most likely sequenceof data that produced the received se-quence of code bits. Historically, the firsttype of convolutional decoding, known assequential decoding, used a systematic

procedure to search for a good estimate ofthe message sequence; however, suchprocedures require a great deal of mem-ory, and typically suffer from buffer over-flow and nongraceful degradation.

In 1967, Andrew Viterbi developed adecoding technique that has since becomethe standard for decoding convolutionalcodes. At each bit-interval, the Viterbi de-coding algorithm compares the actual re-ceived code bits with the code bits thatmight have been generated for each possi-ble memory-state transition. It chooses,based on metrics of similarity, the mostlikely sequence within a specific timeframe.

The Viterbi decoding algorithm re-quires less memory than sequential de-coding because unlikely sequences aredismissed early, leaving a relatively smallnumber of candidate sequences that needto be stored.

Some types of algebraic coding aremost effective in combating “bursty”errors (errors that arrive in bursts). Con-volutional coding is generally more ro-bust when faced with random errors orwhite noise; however, any decoding er-rors occurring in the convolutional de-coder are likely to occur in bursts. In1974, Joseph Odenwalder combinedthese two coding techniques to form a

Charles Wang, Dean Sklar, and Diana Johnson

Error-CorrectionCodingForward

110 101010 000011 000 110011010

Messagesequence Codeword

Paritybits

Messagebits

(6,3)Block

encoder

An example of a (6, 3) algebraic encoder that produces a six-bit codeword for every three-bit mes-sage sequence. In this example, each six-bit output codeword is composed of the original three-bitmessage sequence and a three-bit parity sequence.This codeword format is known as systematic.

concatenated code. In this arrangement, theencoder linked together an algebraic codefollowed by a convolutional code. The de-coder, a mirror image of the encoding op-eration, consisted of a convolutional de-coder followed by an algebraic decoder.Thus, any bursty errors resulting from theconvolutional decoder could be effectivelycorrected by the algebraic decoder. Perfor-mance was further enhanced by using aninterleaver between the two encodingstages to mitigate any bursts that might betoo long for the algebraic decoder to han-dle. This particular structure demonstratedsignificant improvement over previouscoding systems and is currently being usedin the Deep Space Network and Air ForceSatellite Control Network as well as incommercial broadcasting services.

In 1993, Claude Berrou and his associ-ates developed the turbo code, the mostpowerful forward error-correction codeyet. Using the turbo code, communicationsystems can approach the theoretical limitof channel capacity, as characterized by theso-called Shannon Limit, which had beenconsidered unreachable for more than fourdecades.

Turbo CodingOne of the requirements of a turbo coder-decoder (or codec) configuration is that theencoder must include some arrangement ofat least two component encoders. Althougheach component encoder may employ al-gebraic coding or convolutional coding, theoverall encoder can be considered a blockencoder because data are processed inblocks. The size of these blocks is dictated

by the length of the interleaver that sepa-rates each component encoder.

The interleaver in a turbo encoder servesa different purpose than interleavers usedby other parts of a communication system.Standard interleavers scramble code bitsamong multiple blocks so that they are notcontiguous when transmitted; as a result,any bursty errors caused by channel cor-ruption are transformed, or spread out, intomore-random errors after deinterleaving.The interleaver in a turbo encoder, on theother hand, is designed so that the secondencoder gets an interleaved version of thesame data block that went into the first en-coder; thus, the second encoder generatesan independent set of code bits. Doing soprovides diversity to the coded sequencebeing transmitted. Although any interleav-ing pattern can be adopted, different pat-terns can result in significant differences in

the bit-error rate; therefore, the interleaverdesign contributes significantly to the over-all error performance of the turbo-codesystem.

The turbo codec must have as manycomponent decoders on the receiving endas component encoders on the transmittingend. These decoders are concatenated inserial fashion and are joined by a series ofinterleavers and deinterleavers in a feed-back loop.

In a typical decoding operation, the firstdecoder generates statistical informationbased on the data received from the firstcomponent encoder. This information isthen fed to the second decoder, whichprocesses it along with the data receivedfrom the second component encoder. Afterdecoding, the improved and updated statis-tical information is fed back to the first de-coder, which starts the process again. This

An example of a turbo encoder using parallel concatenated component codes (the most commonlyimplemented configuration). Although the figure shows a block size of four bits for discussion pur-poses, typical block sizes are on the order of hundreds or thousands of bits. Bit times are indicated bysubscripts. The figure represents a rate-1/3 turbo encoder (that is, one data bit produces a three-bitcodeword), but other rates are possible with this identical configuration by the careful elimination ofselected code bits.

Turbo decoders have an iterative structure, composed of as many compo-nent decoders as there are component encoders, concatenated in a serial

fashion. The interleavers and deinterleavers are used to ensure the correctordering among the various types of information processed.

Code-word 4

c4c4d4,2 1

Code-word 3

c3c3d3,2 1

Code-word 2

c2c2d2,2 1

Code-word 1{d4,d3,d2,d1}

Input data block

{d3,d1,d4,d2}

Scrambleddata block

{d4,d3,d2,d1}

Original data block

Code bits(from firstencoder)

Code bits(from second

encoder)

To modulator

{c4,c3,c2,c1}1 1 1 1

{c4,c3,c2,c1}2 2 2 2

Encoder 1

Interleaver

Encoder 2 Cod

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n

c1c1d12 1

Receiveddatablock

Receivedcode bits(encoder 1)

Receivedcode bits(encoder 2)

StatisticalInformation

StatisticalInformation

StatisticalInformation

Final statisticalinformation

Statistical informationfor next interation

Data estimates

Feedback loop

Interleaver Decoder2 DeinterleaverDecoder

1

Interleaver

Deinterleaver

Bit estimation(”hard decision”)

Switch down afterlast iteration

{d4,d3,d2,d1}^ ^ ^ ^

process typically continues for six to ten it-erations for each block of data, at whichpoint a switch is triggered and the actualdata estimates are produced.

Thanks to the iterative decoding process,turbo codes can achieve a bit-error rate thatapproaches the Shannon limit. For exam-ple, at a desired bit-error rate of 10-6, con-volutional codes can typically provide a5.5-decibel improvement (72 percentpower savings) and concatenated codes a7.75-decibel improvement (83 percentpower savings) over an uncoded system.Using a turbo code, an additional 2.25-decibel improvement over the concate-nated code can be attained, resulting in atotal coding gain of 10 decibels (90 percentpower savings) compared with the un-coded system. For a rate-1/2 turbo-codedsystem, the error performance comeswithin about 1 decibel of the Shannonlimit.

Aerospace Achievement in Turbo-Code DevelopmentDuring the last five years, Aerospace hasmade significant contributions to the ad-vancement of turbo codes, which can po-tentially benefit all government satelliteprograms. For example, because a turbocode’s performance is sensitive to elementsin the code’s structure (such as component

channel. Furthermore, Aerospace showedthat using conventional demodulationschemes for most of these waveforms pre-vents turbo codes from achieving their op-timal performance. Aerospace thereforedeveloped and is patenting various uniquedemodulation schemes that permit betterturbo-code performance.

Some programs, such as the AdvancedWideband System, require high-data-ratetransmission as well as good bit-error-rateperformance. To achieve this goal, Aero-space began investigating turbo codes in-volving high-rate component codes. Itturned out, however, that most common de-coding algorithms were unsuitable becausethey were originally developed for turbocodes employing low-rate componentcodes. Low-rate codes introduce more par-ity bits than do high-rate codes, and there-fore require faster transmission and largerbandwidth to compensate (for real-time ap-plications). Aerospace therefore developeda decoding algorithm specifically opti-mized for high-rate component codes.

Such developments have made signifi-cant—in some cases, critical—contribu-tions to the success of these programs.

ConclusionForward error-correction coding representsthe most efficient, economical, and pre-dictable way of improving the reliability oftransmitted or stored data. The turbo codein particular offers designers a powerfultool for ensuring robust communicationsdespite adverse conditions. As concernsover spectrum management and bandwidthefficiency increase, the ability to maximizechannel capacity without sacrificing datareliability becomes all the more important.Thanks in part to Aerospace developments,forward error-correcting codes should playa larger role in helping military communi-cation satellites to keep up with increasingsystem demands.

Further ReadingC. Berrou, A. Glavieux, and P. Thitimajshima,“Near Shannon Limit Error-Correcting Code:Turbo Code,” Proceedings of the 1993 IEEE In-ternational Conference on Communications,1064–1070 (Geneva, Switzerland, May 1993).

G. Lui and K. Tsai, “A Soft-Output Viterbi Al-gorithm Demodulation for Pre-coded BinaryCPM Signal,” to appear in Proceedings of 20thAIAA International Satellite System Conference(May 2002).

D. Sklar and C. C. Wang, “On the Performanceof High Rate Turbo Codes,” to be published in

code configuration and interleaving pat-tern), Aerospace has developed softwarethat searches for optimal turbo-code struc-tures. The bit-error rate improvement ofturbo codes tends to saturate beyond somerange of signal-to-noise ratio; beyond thispoint, known as the error floor, the bit-errorrate cannot be reduced by simply increas-ing the signal-to-noise ratio. Aerospace hasdeveloped various schemes to mitigate thiserror-floor problem (resulting in three U.S.patents).

Aerospace’s work in turbo codes hasbeen directly applied in support of the Ad-vanced Extremely High Frequency pro-gram, the Advanced Wideband System, theGlobal Positioning System, and the Geo-stationary Operational EnvironmentalSatellite System. These efforts have culmi-nated in the development of a powerfulcomputer simulation tool to evaluate the end-to-end user bit-error rate over aprocessed or unprocessed satellite channelthat uses turbo codes. This simulation toolmodels the turbo codec, different channeldisturbances (e.g., Gaussian noise, scintil-lation, and jamming), and various modula-tion schemes (such as phase-shift keying,frequency-shift keying, quadrature ampli-tude modulation, and Gaussian minimumshift keying) that can be independently selected for either the uplink or downlink

Bit-

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Bit-energy-to-noise ratio (decibels)Shannon limit(for rate – 1/2 code)

Concatenated

Convolutional

Uncoded

Turbo

100

10–1

10–2

10–4

10–3

10–5

10–6

The bit-error rate of a rate-1/2 turbo coded system after 10 iterations as compared with systems thatuse no coding, convolutional coding, and concatenated coding. In this example, at a desired bit-errorrate of 10-6, the use of convolutional and concatenated codes provides a 5.5-decibel (72 percentpower savings) and 7.75-decibel (83 percent power savings) respective improvement compared withthe uncoded system. Here, the use of a turbo code imparts an additional 2.25-decibel improvementcompared with the concatenated code, resulting in a total coding gain of 10 decibels (90 percentpower savings) compared with the uncoded system. With the use of a rate-1/2 turbo code, system er-ror performance can approach levels within about 1 decibel of the Shannon limit.

Proceedings of 2002 IEEE Aerospace Confer-ence (March 2002).

M. R. Shane and R. Wesel, “Parallel Concate-nated Turbo Codes for Continuous Phase Mod-ulation,” Proceedings of 2000 IEEE WirelessCommunications and Networking Conference(September 2000).

B. Vucetic and J. Yuan, Turbo Codes (KluwerAcademic Publishers, Boston, 2000).

C. C. Wang, “Mitigating the Error Floor forTurbo Codes,” Proceedings of Globecom ’98(November 1998).

C. C. Wang, “Improving Faded Turbo CodePerformance Using Biased Channel Side Infor-mation,” Proceedings of Military Communica-tions Conference 1999 (October 1999).

C. C. Wang and D. Sklar, “A Novel MetricTransformation to Improve Performance of theTurbo Coded System with DPSK Modulation in

Fading Environments,” Proceedings of MilitaryCommunications Conference 2001 (October2001).

C. C. Wang and D. Sklar, “Performance of theTurbo Coded System with DPSK ModulationUsing Enhanced Decoding Metrics andMatched Channel Side Information,” to appearin Proceedings of 2002 International Communi-cations Conference (April 2002).

In a communication system that em-ploys forward error-correction coding,a digital information source sends adata sequence comprising k bits ofdata to an encoder. The encoder in-serts redundant (or parity) bits,thereby outputting a longer sequenceof n code bits called a codeword. Onthe receiving end, codewords areused by a suitable decoder to extractthe original data sequence.

Codes are designated with the no-tation (n, k) according to the numberof n output code bits and k input databits. The ratio k/n is called the rate, R,of the code and is a measure of thefraction of information contained ineach code bit. For example, each codebit produced by a (6, 3) encoder con-tains 1/2 bit of information.

Another metric often used to char-acterize code bits is redundancy, ex-pressed as (n–k)/n. Codes introducinglarge redundancy (that is, large n–k orsmall k/n) convey relatively little infor-mation per code bit. Codes that intro-duce less redundancy have highercode rates (up to a maximum of 1)and convey more information per codebit. Large redundancy is advanta-geous because it reducesthe likelihood that all ofthe original data will bewiped out during a singletransmission.

On the down side, theaddition of parity bits willgenerally increase thetransmission bandwidth orthe message delay (orboth). For real-time appli-cations, such as voicecommunications, thecode-bit rate must beincreased by a factor ofn/k = 1/R to avoid a reduc-tion in data throughput.

Hence, for a given modulation scheme,the transmission bandwidth increases bythat same factor n/k. If, however, thecommunication application does not re-quire the real-time transfer of information,then additional message delay (ratherthan increased bandwidth) is the usualtrade-off.

Represented graphically, the generalerror-performance characteristics ofmost digital communication systemshave a waterfall-shaped appearance.System performance improves (i.e., bit-error rate decreases) as the signal-to-noise ratio increases. The two curvesshown below compare the performanceof a typical system with and without for-ward error-correction coding. The codedsystem, operating with a received signal-

to-noise ratio of 8 decibels, has asmaller bit-error rate by a factor of 100compared with the uncoded system atthe same signal-to-noise ratio.

Viewed another way, the graphs in-dicate that the coded system canachieve the same bit-error rate as theuncoded system at a lower signal-to-noise ratio. This reduction in requiredsignal-to-noise ratio, called the codinggain, is a common metric used tomeasure the performance of differentcoding schemes.

The importance of coding gain isevident when the system is viewedfrom the designer’s perspective. Forexample, to obtain the same level ofimproved bit-error rate without the useof coding, a designer would have to

achieve a larger signal-to-noise ratio (12 decibels in-stead of 8). To do so wouldrequire the use of largerpower supplies, bigger an-tennas, or higher-qualitycomponents that introduceless noise. If none of thesemodifications can be pro-vided, then the designer willhave to tolerate some type ofperformance degradation—such as reduced serviceranges or lower operatingmargins—to obtain the sameimprovement.

How Forward Error-Correcting Codes Work

Coded

Uncoded

10–2

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Code bitestimates

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Nulling AntennasAdaptive antennas dynamically respond to interference by “nulling out” orcanceling the interference energy. The Aerospace Corporation has successfully applied the technology to satellites and ground terminals for two Air Force Spaceand Missile Systems Center programs.

Designers and operators of mili-tary communication satelliteshave long sought to reduce thevulnerability of their systems

to intentional and unintentional interfer-ence. Their efforts have resulted in threedistinct technologies for interference mit-igation: spread-spectrum modulation, an-tenna sidelobe reduction, and adaptive in-terference cancellation.

Spread-spectrum modulation, the mostwidely used, modulates the communica-tion signal using a code that is known toboth the sender and receiver. This addi-tional modulation expands the bandwidthof the transmitted signal, thereby spread-ing the interference over a wider fre-quency range and reducing its effect.

Antenna sidelobe reduction seeks tominimize interference received beyond theantenna’s desired field of view by applyingantenna design techniques to reduce side-lobes—the minor lobes separated from theantenna’s main beam. This technique ef-fectively reduces interference both re-ceived and transmitted by the antenna.

Adaptive interference cancellationmonitors the received signal and identifiesinterference when present. Several an-tenna elements—the actual radiating andreceiving components—are combined toform a null or cancellation in the directionof the interference. The adaptive systemautomatically responds or adapts tochanging interference patterns, withouthuman intervention.

Practical system designs must employan appropriate combination of these threetechniques to achieve the desired level ofinterference protection in the most eco-nomical way.

Adaptive Antenna DesignsAdaptive antenna systems contain fivemajor components: a means of detectinginterference, a means of distinguishingdesired signals from interference, a con-trol processor for determining how tocombine the antenna elements, antennaelements and circuitry to respond to com-mands from the control processor, and aperformance monitor to identify changesin the interference environment and re-spond accordingly.

Development of adaptive systems be-gan in the 1950s with two nearly concur-rent design approaches. The first focusedon sidelobe cancellation, an approach thatassumes interference arrives through theantenna sidelobes while the desired signalarrives through the main beam. Auxiliarybroad-coverage antenna elements are usedto sample the interference. The correlationbetween the main and auxiliary antennaelements quantifies the received interfer-ence power, which is treated like an errorsignal in an automatic control system.

Minimizing the error signal is equivalentto minimizing the interference.

The second approach applies to arrayantennas—units equipped with severalprimary receiving or radiating elements;in this case, the array elements are com-bined under adaptive control to maintaindesired signal reception and form nulls inthe direction of interference sources.

This process of canceling interferenceis inherently a subtraction operation. Theradiation patterns or spatial variations ofthe antenna elements are subtracted fromone another to form an overall null in thedirection of the interference.

It’s a demanding process, and the for-mation of deep nulls requires extremelyprecise matching of amplitude and phase

Don J. Hinshilwood and Robert B. Dybdal

Adaptive

for Military Communications

NullingantennaNullingantenna

NullMainbeam

UserJammer

An adaptive uplink antenna forms a null in the direction of a jamming signal, preventing it from interfering with communications to intended users.

This figure illustrates the difference between a standard (left) and an offset (right) reflectorantenna. The feed of the offset antenna is directed toward the dish at an offset from itsaxis of symmetry.

characteristics across the operating band-width. How precise the system can be de-pends on the frequency variation of the an-tenna elements and the amplitude-trackingand phase-tracking performance of theelectronics. The level of design toleranceneeded to maintain adequate precisionposes a significant challenge for practicaladaptive antennas.

Technological advances in recent yearshave helped. Low-noise system compo-nents, for example, are available with thenecessary unit-to-unit reproducibility at alow weight with high performance. Adap-tive weighting circuitry, which is needed tocombine the antenna elements, providesthe high accuracy needed to meet the strin-gent tolerances. Digital technology for theadaptive processing and control functions

is well developed. Capable instrumentationis available to evaluate and diagnose adap-tive system performance. Simulation tech-niques have also been developed to providehigh-fidelity modeling of practical hard-ware and evaluate system performance ona statistical basis.

Satellite Uplink AntennasAerospace studies of multiple-beam anten-nas in the 1960s led to the Defense Satel-lite Communications System III satellite.These studies also spawned the idea ofadaptively processing the individual beamsin order to cancel interference from groundsources; in fact, they are believed to be the origin of adaptive uplink designs using multiple-beam antennas. Subsequent development by both Aerospace and the re-search community advanced the concept.

Aerospace efforts in the Extremely HighFrequency Follow-On program, for exam-ple, sought to develop a design with suffi-cient simplicity to allow practical imple-mentation; these efforts focused ondemonstrating the performance of a seven-beam design.

The results are apparent in Milstar MDR(medium data rate), a system that providessubstantial interference protection. The de-sign combines all three protection tech-niques for interference mitigation. Spread-spectrum modulation, for example, helpsreduce interference susceptibility; the useof low-sidelobe multiple-beam antennasreduces interference received beyond thedesign coverage area; and adaptive antennaprocessing serves to protect users from in-terference within the design coverage area.

The traditional satellite uplink antennauses an offset reflector that producesmultiple beams from a set of focal radiators. Each focal radiator pro-duces a component beam in a differ-ent direction. When interference is notpresent, these component beams arecombined according to a predeter-mined scheme that assigns values or“weights” to each one using the an-tenna’s weighting circuitry. The combi-nation of these component beamsforms the quiescent pattern thatmatches the design coverage area.

Adaptive Uplink Antennas: How Do They Work?Highest sensitivity or “gain” is foundwithin the antenna’s main beam. Be-yond the main beam, sensitivity falls off considerably at the antenna’s minorlobes, called sidelobes. The offset re-flector is designed to minimize thesesidelobes, which helps to reduce inter-ference generated outside of the designcoverage area.

When interference is present, thecomponent beams are combined in adynamic fashion to produce a null in thedirection of the interference. These nullsprevent interruption of communications,

but also reduce the antenna’s effectivecoverage area. Uplink antennas typi-cally distinguish interference from desired signals by detecting signalsthat do not employ the proper spread-spectrum coding.

Since spread-spectrum coding pro-vides a portion of the interference pro-tection, a threshold for the received interference power must also be estab-lished. In this way, the adaptive pro-cessing and reduction in the quiescentcoverage will occur only when adaptivecancellation is required for additionalinterference protection. The processor

derives the values for theweighting circuitry from interference correlation between the componentbeams. Continued monitor-ing of the correlation valuesallows the system to iden-tify changes in the interfer-ence environment and au-tomatically generate newadaptive weight values.

Interference cancellationis a subtractive process.Ideal cancellation requirescomponents that are ex-actly matched in amplitudeand phase—a goal that isnot achievable in practicalsystems.

Processor

Focalradiators

To satellite transponder

Designcoverage

area

Preamplification

Weightingcircuitry

Designcoverage

Individualcomponentbeams

Side-lobes

Angle

Quiescentpattern

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Interference

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Quiescentpattern

The system’s predecessor, Milstar LDR(low data rate), achieved adequate interfer-ence protection simply by combining low-sidelobe multiple-beam antennas andspread-spectrum modulation; however,spread-spectrum modulation becomes lesseffective at higher data rates (because ofthe decreased ratio of signal bandwidth totransmission bandwidth). Thus, for theMDR system, adaptively processed uplinkantennas were needed to maintain a highlevel of interference protection.

The uplink antennas on the satellites usean offset reflector and a cluster of focal-field radiators to obtain multiple beamswithin a spot-beam coverage area. Sevenbeams are adaptively combined in this de-sign. When interference is not present, apredetermined combination of the individ-ual beams provides the quiescent pattern.The antenna is mechanically steered to itsdesired location and can be changed in or-bit as communication needs vary. When in-terference begins, the individual beamswithin the cluster are adaptively combinedto create pattern nulls to cancel the inter-ference. Meanwhile, the low sidelobes pro-vided by the multiple-beam design preventinterference beyond the coverage area. In-terference can be detected in various ways.For example, any received signals that do

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The satellite antenna is configured tocommunicate with a specified coveragearea. When interference is present, how-ever, the nulls created by the antenna sys-tem reduce the available coverage. Thepercentage of design coverage area stillavailable during interference provides auseful metric for assessing the perform-ance of candidate designs and specifyingsystem performance.

Protecting Ground TerminalsAerospace also helped develop a sidelobe-cancellation system for the Defense Satel-lite Program (DSP) as a part of the SatelliteReadout Station Upgrade program. Pro-tecting ground receiving stations fromnearby interference was the primary goal.The waveforms for this system were al-ready well established, so spread-spectrum modulation could not be employed. Also,the large antennas at the ground terminalswere enclosed in radomes—protective ra-diolucent shells for housing antennas—sosidelobe-reduction techniques could not beused, either. Therefore, the only feasibleoption was an adaptive sidelobe canceller.

A fundamental challenge in designingthis system was the sidelobe response ofthe main antenna. The sidelobe response

Sep

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Separation

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The minimum separation between inter-ference and desired user signals is lim-ited by the sharpness of the antennapattern and the tolerable performanceloss by the user. For multiple-beam an-tennas, the beamwidth of the compo-nent beams measures the patternsharpness, and narrower beamwidthsresult in less required separation be-tween interference and user signals. Inother words, the narrower the beams,the closer the user can be to the sourceof interference without losing communi-cation. The reduced separation values,however, come at the expense of alarger antenna and an increased num-ber of component beams to service thedesign coverage area. Thus, practicaldesigns must balance complexity andsize with separation between interfer-ence and user signals.

For example, consider a nominalbeamwidth of 1 degree in the diagramat left. If the coverage area is directly

beneath the satellite (as shown by thecurves marked “Subsatellite”), and if theuser can sacrifice 10 decibels of gain,then the interference can be as close as50 nautical miles to the user. By con-trast, if the user can only tolerate a 3-decibel loss in uplink antenna gain, thenthe source of interference can be nocloser than 150 nautical miles.

If the uplink antenna is pointedstraight down at the subsatellite point,the design coverage area forms a cir-cle. As the antenna is pointed towardsthe edge of Earth, the design coveragearea spreads out because of Earth’scurvature. This spread in the coveragearea also increases the required sepa-ration values. For example, the mini-mum separation value for an uplink an-tenna pointed down at a 20-degreeangle would be around 150 nauticalmiles with a 10-decibel loss in gain andmore than 400 nautical miles with a 3-decibel loss in gain.

A flight unit of the Milstar 2 medium-data-rateadaptive nulling antenna. Behind the main re-flector (the large disk at the bottom of the photo)sits the radio-frequency correlator, beamformingnetwork, uplink-feed elements, and control elec-tronics. The downlink feed and dichroic subre-flector are positioned further above. Uplink userand interference signals are focused by the mainreflector and subreflector onto the cluster of feedelements. Under the direction of the nullingprocessor (not shown), the beamforming net-work produces the nulls in the antenna’s pattern.The correlator receives samples of the uplinksignals from the beamformer, measures theirstrengths, and sends this information to theprocessor to finish the adaptive feedback loop.

How Close Can the Interference Be?

Shown here are representative patterns processed by a sidelobe canceller, which assumes interfer-ence arrives though the antenna sidelobes, not the main beam. By correlating the main and auxiliaryantenna elements, the system can identify interference and automatically begin countermeasures.

Main beam

Reflector antenna pattern

Auxiliary antenna patterns

Side lobe

AnglePattern null Interference direction

This parametric plot illustrates the tolerances forthe amplitude and phase matching in achievinga specified null depth. Maintaining a near-zerointerference level is demanding, and the re-quired tolerances become more stringent as thecancellation performance increases. For exam-ple, the requirements for a 40-decibel cancella-tion (1/10,000 of the interference power) are sig-nificantly tighter than those for a 30-decibelcancellation (1/1,000 of the interference power).

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describes the sum of individual sidelobecontributions from various sources, includ-ing direct radiation from the feed, diffrac-tion from the reflector rim, and scatteringfrom feed supports. Each sidelobe sourcearises at a different physical location on theantenna and has a corresponding time de-lay. As a result, the sidelobe response ofthe reflector antenna varies with frequencyover the operating bandwidth. This fre-quency dependence must be matched bythe frequency dependence of the auxiliaryantenna elements and adaptive circuitry.The greater the cancellation required, thetighter the tolerance in matching these fre-quency responses.

At the ground receiving station, the fre-quency responses of the main reflector antenna and the auxiliary antennas differedgreatly, so the adaptive circuitry needed toprovide significant equalization to achieveeffective cancellation over the requiredbandwidth. In this case, an adaptive trans-versal filter was successfully used. This fil-ter combined time-delay components withadaptive weighting circuitry. These time-delay components were initially measuredusing a scale-model antenna and later veri-fied with the full-scale antenna. The meas-urements were performed using a general-purpose vector network analyzer. Themeasured amplitude and phase responsewas then processed by the network ana-lyzer, which converts measured frequency-domain data into time-domain data. Thistime-domain response of the antenna side-lobes provided the required time-delayspread for the transversal filter.

One of the earliest adaptive antenna de-signs uses a main reflector antenna anda set of auxiliary antennas. The designassumes interference arrives through thesidelobes of the main antenna, and wheninterference is present, the main antennareceives both desired signals and inter-ference components. The auxiliary anten-nas primarily receive interference powerbecause their gain in the direction of thereflector’s main beam is much lower. Acorrelation between the main reflector an-tenna and the auxiliary antennas indi-cates that interference power is being re-ceived. The processor functions like acontrol system, minimizing the interfer-ence correlation, and thus produces acomposite antenna pattern with a null in

the interference direction. The differencesin the frequency responses of the mainand auxiliary antennas require equaliza-tion to match the amplitude and phase re-sponses in order to meet the cancellationtolerances over the required bandwidth.

Adaptive Sidelobe Cancellation

Main reflectorantenna

Auxiliaryantenna

Adaptiveequalizer

Processor

–

This sidelobe canceller subsystem hasbeen installed at DSP ground sites and hasbeen successful in protecting mission datafrom terrestrial interference.

Adaptive Antenna SimulationSimulation analyses are an important partof adaptive antenna development. Initialsimulations, based on a scenario that de-scribes the anticipated interference envi-ronment, are used to develop a hardwaredesign capable of complying with systemrequirements. As system development pro-ceeds, actual hardware data can be incor-porated into the simulation, thereby im-proving its usefulness.

Simulation also plays an essential role intesting adaptive antennas. Although thesimulation can be run repeatedly with vary-ing scenario parameters to provide statisti-cal answers for system performance ques-tions, such testing can take a long time. Amore reasonable approach is to use thesimulation to derive representative testcases, which can then be measured to vali-date the simulation. Once validated, thesimulation provides the required statisticalanswers to questions about system per-formance.

In fact, Aerospace developed a detailedsimulation package, called NullerSim, to

Image A is a snapshot of the quies-cent pattern (no interference) of asimple, seven-element nulling an-tenna pointed at Washington, DC.The seven element beams, indi-cated in yellow, are weighted andsummed to provide a beam overthe coverage area, shown by theblue dotted line. The colors of thepattern indicate the available sys-tem margin. Light blue corresponds

to the weather margin desired overthe coverage area; the darker theblue, the higher the margin. Greencorresponds to positive link mar-gins; communication is possible inthis region, though with less toler-ance of weather-related outages.Red indicates that communicationis not possible; the darker the red,the worse the performance. Thenulls of the antenna appear black.

In image B, a source of interfer-ence located in Boston has turnedon. Note that user services oversome of the coverage area hasbeen lost (turned red), and that anull (black area around Boston) hasbeen placed on the interferencesource. In the figures C, D, and E,the nuller is pointed successivelyfarther along a line running fromWashington, DC, to Boston. The

A

FED

CB

Adaptive Nulling Antennas:

The Next Wave in Jamproof Communications

interference source remains fixed inBoston. Moving the nuller brings thecoverage area closer to the interfer-ence source. In image F, the nulleris pointed directly at the interfer-ence source. Sequence G throughL shows how the shape of the nullcan vary depending on where thejammer is within the beam. In thisseries of images, the nuller remainspointed at Washington, DC. Two

interference sources are randomlylocated within the coverage area.The adapted patterns for six pairsof randomly selected locations areshown. In the top-left image (G), thenuller has adapted to two interfer-ence sources located at roughlyequal distances northeast andsouthwest of the city. Distinct nullshave been placed on the interfer-ence sources. In this case, it’s clear

where the interference sources arelocated. As shown in the remainingimages, it’s not always clear fromthe patterns exactly where the inter-ference sources are. For example,the jammers in figure I are both lo-cated to the north. In figure K, oneis to the west, and one to the north-west. In figure L, one jammer ap-pears to be located to the south,while the other lies farther north.

G H I

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In this pair of figures, the second jammer turns on after a delay relative to the first jammer.The percentcoverage area falls when the first jammer turns on, but then recovers. It again falls when the secondjammer turns on, and recovers once again.

This pair of figures shows the performance of the adaptive antenna system as time passes after twojammers turn on simultaneously. The upper plot shows the drop in jammer-power-to-noise ratio as thesystem cancels the jamming. The lower plot shows the percent coverage area recovered for usercommunications. When jamming begins, percent coverage area falls but quickly recovers.These timevariations illustrate the adaptive system’s transient response—the time it takes for the system toreach a steady state after interference begins.

facilitate this process. Originally intendedas an independent means of verifying con-tractor analyses, NullerSim proved usefulin addressing system performance ques-tions that arose during development. In thecase of the Milstar MDR nuller, the simu-lation has been fully validated by meas-ured performance and has furnished sup-port for the on-orbit testing of the MDRdesign. More recently, NullerSim wasused to derive requirements and providevalidation and benchmark values for con-tractors’ simulations for the Advanced Ex-tremely High Frequency program.

Adaptive System TestingIn evaluating an antenna’s ability to rejectinterference, the researcher must answertwo fundamental questions: What is thesteady-state loss in performance when in-terference is present, and how soon afterthe start of interference does the systemreach steady-state performance? Answer-ing these questions requires testing at thesystem level rather than at the componentlevel.

Performance measures for adaptive an-tennas differ from those of conventionalantennas. Adaptive operation is generallytested by measuring signal-fidelity param-eters. For example, measuring the bit-errorrate of the desired signals in the presenceof interference provides a good metric ofsteady-state performance. By measuringthe time required for the adaptive weight-ing values to reach their steady-state levelsafter the start of interference, the transientresponse of the adaptive system can be ob-tained. The ability to freeze the adaptiveweight settings and measure the antennapattern also provides useful information,such as percent coverage area for an up-link antenna or gain and system-noise tem-perature variations for a sidelobe canceller.

Evaluation of adaptive antennas also re-quires the ability to generate desired andinterfering signals arriving from differentdirections. To test the satellite readout sta-tion for the DSP upgrade, interfering sig-nals were obtained from small antennas lo-cated close to the ground terminal antenna.Evaluating the uplink adaptive antenna forthe Milstar MDR program was more chal-lenging because of the high risk involvedin testing flight hardware outdoors. Aero-space developed a novel concept that notonly solved the problem, but achieved sig-nificant cost and time savings as well. Thisconcept extends so-called compact-range

technology, a technique that provides testsignals in an indoor environment.

A compact range normally uses theplane wave generated in the near field ofan offset reflector whose diameter is atleast twice as big as the antenna beingtested. If the compact-range reflector is en-visioned as an antenna, a subreflector canbe used to provide high-fidelity beams off-axis from the focal point. This technique iscommonly used to obtain the required fieldof view for uplink antennas, as indeed itwas for the Milstar MDR design. When ap-plied to the compact range, such a designresults in plane-wave components arrivingfrom different directions—as required toevaluate the uplink adaptive design. Dis-crete illuminators placed within the focalregion simultaneously produce test fieldshaving different arrival directions, whileindependent signal sources provide desiredand interference signal components.

ConclusionsFurther developments in adaptive antennaswill improve communications in adverseenvironments. Development of associatedtechnology in high-performance radio-frequency components, digital processing,and control electronics as well as increased

Research Endeavors

Adaptive antenna technology has enjoyedconsiderable development since its intro-duction in the 1950s.The Aerospace Corporation has followed this researchand has received several U.S. patents forinnovative concepts.

One development devised a means toincorporate a priori antenna design infor-mation into the adaptive cancellationprocess. For example, an array of an-tenna elements can be arranged in astraight line to produce a low-sidelobepattern.The positions and levels of thesesidelobes are known from the array de-sign; therefore, if interference is detected,the system can determine which sidelobecontains the interference by sequentiallysampling them all with a separate an-tenna beam obtained from the same ar-ray of elements.This beam can then besubtracted from the quiescent array pat-tern.The technique, known as “sidelobeannihilation,” avoids the usual time-consuming recursive determination of theadaptive weight values and has potentialapplication in future wireless systems.

Other research has focused on pro-tecting a very-high-data-rate (greater than

500 megabits per second) uplink an-tenna. In this case, the spacecraft has alow-sidelobe, high-gain antenna with avery narrow beamwidth.The combinationprotects against interference not gener-ated in the immediate vicinity of theground terminal. Interference that is gen-erated near the terminal is reduced byrepositioning the uplink antenna so thatinterference arrives at a low-gain portionof the main beam.This repositioning isguided by a low-level coded signal attached to the desired signal. In interference-free conditions, this signalcan be used to maintain beam pointing tooffset variations in satellite attitude.Thisapproach is advantageous because it ob-viates the need for auxiliary antennas andadaptive circuitry and avoids the problemof achieving effective cancellation overthe wide bandwidth used by the high-data-rate communications.

More recently, a patent was granted fora technique that reduces interference tosignals transmitted by an antenna. Aero-space researchers believe it is the firsttime that adaptive antenna concepts havebeen applied to transmitting antennas.

This development was prompted by theadvent of wireless services that usecode-division multiple-access modulation,a technique in which multiple channelsare independently coded for transmissionover a wideband channel.The capacity ofsuch systems can be impaired by multi-path components—signals that follow dif-ferent paths to the receiver.The time de-lays of multipath signals interfere with thedirect signal path, which in turn de-creases the isolation between systemusers and increases channel interfer-ence.The standard approach—use of aso-called RAKE receiver—provides thenecessary time-delay alignment of the di-rect and multipath signals.The new Aero-space approach uses the transmitted sig-nals as “radar” signals to detect signalcomponents bouncing off nearby obsta-cles, such as buildings.These same ob-stacles that reflect the transmitted signalalso generate multipath components. Byadaptively processing these signals andreducing their magnitude, the multipathgeneration will be reduced along with thedegradation to other transmitted signals.

understanding of simulation and testingtechniques will lead to more capable andeconomical adaptive antenna designs. Re-search at Aerospace has already helped re-duce the vulnerability of defense commu-nications satellites. Future military andcommercial systems—particularly in thearea of wireless communications—canlook forward to similar improvements inreliability and security.

Further ReadingR. B. Dybdal, “Multiple Beam CommunicationSatellite Antenna Systems,” Proceedings of the1974 IEEE International Conference on Com-munications (Minneapolis, MN, June 17–19,1974).

R. B. Dybdal and S. J. Curry, “Adaptive Receiv-ing Antenna for Beam Repositioning” (U.S.Patent 5,739,788, April, 1998).

R. B. Dybdal and S. J. Curry, “Adaptive Trans-mitting Antenna” (U.S. Patent 5,781,845, July,1998).

R. B. Dybdal and D. J. Hinshilwood,“DEADEN: A New Adaptive CancellationTechnique,” Proceedings of the 1995 IEEE Mil-itary Communications Symposium (San Diego,CA, November 5–8, 1995).

R. B. Dybdal, D. J. Hinshilwood, and K. M.Soo Hoo, “Development Considerations in the

Design and Simulation of Adaptive MBAs forSatellite Communications,” Proceedings of the1993 IEEE Military Communications Sympo-sium (Boston, MA, October 11–14, 1993).

R. B. Dybdal and R. H. Ott, “Apparatus andMethod for Employing Adaptive InterferenceCancellation over a Wide Bandwidth” (U.S.Patent 5,440,306, August 8, 1995).

R. B. Dybdal and K. M. Soo Hoo, “Evaluationof Adaptive Multiple Beam Antennas,” Pro-ceedings of the 1990 AMTA Symposium(Philadelphia, PA, October 8–11, 1990).

D. J. Hinshilwood, “Performance Measures forAdaptive Antenna Systems,” Proceedings of the1996 IEEE Military Communications Sympo-sium (McLean, VA, October 21–24, 1996).

D. J. Hinshilwood, “The Simulation of Adap-tive Antennas,” Proceedings of the 1996 IEEEMilitary Communications Symposium(McLean, VA, October 21–24, 1996).

D. J. Hinshilwood, “Steering Vector Optimiza-tion for Adaptive MBA Satellite Communica-tions,” Proceedings of the 1993 IEEE MilitaryCommunications Symposium (Boston, MA, Oc-tober 11–14, 1993).

K. M. Soo Hoo and R. B. Dybdal, “ResolutionPerformance of Adaptive Multiple Beam Anten-nas,” Proceedings of the 1989 IEEE MilitaryCommunications Symposium (Boston, MA, Oc-tober 15–18, 1989).

The Aerospace Corporation is applying satellite communication technologies to improvethe communication capabilities of the Federal Interagency Communications Center and thefour federal agencies it serves to provide an effective communication system for the safety of personnel in remote regions in Southern California.

In Southern California, where terrainin remote regions includes expansivedesert and rugged mountains, brushand chaparral, and both deciduous

and evergreen forests, several hundredrangers and firefighters from four federalagencies work to combat fires and protectnatural and cultural resources and thepublic from natural and human-caused in-cidents. Because the terrain may be roughand personnel may be spread over largeareas—more than 125,000 square kilome-ters in Southern California—guardians ofthese regions frequently work in isolation.What if they need backup resources whendealing with critical incidents?

Those faced with such a crisis call adispatcher, other rangers and firefighters,or interagency personnel, if they can do sousing conventional terrestrial radio or cel-lular telephone communications. Butwhat if their VHF (very high frequency)radio is out of coverage and they can’tconnect to a cell site? What happens whenthe dispatcher can’t talk to rangers or fire-fighters and doesn’t know their locations?The Aerospace Corporation’s answer is tobetter design and integrate communica-tion networks, including satellite commu-nications, to serve these workers in re-mote areas.

Aerospace provides technical solutionsto such communication problems to assistthe Federal Interagency CommunicationsCenter (FICC) in better serving rangersand firefighters from four federal agenciesin Southern California. FICC is a dispatch center that provides command,control, and communication functions

to fire-management and law-enforcementpersonnel from the four federal agencies,which include the Bureau of Land Management (BLM), Forest Service,National Park Service, and Bureau of Indian Affairs.

Aerospace has been working during the past year and a half to improve theFICC communications system. Aerospaceworks directly for the BLM in an environ-ment characterized by a very high level ofinteragency coordination and contribu-tion. It has assisted the FICC agencies notonly in determining requirements, but alsoin designing, implementing, installing,and integrating a number of communica-tions enhancements, for example, to theterrestrial repeaters and base stations thatmake up the FICC radio system. The bureau and the other FICC agencies havenoted dramatic improvements to their existing VHF radio system.

A Pilot SystemBeyond improving existing communica-tions, Aerospace has recommended new kinds of equipment and services to make workers in these remote regionsmore self-sufficient and, at the same time,tie them more closely to the dispatchersand to each other. With the approval of theFICC agencies, Aerospace has imple-mented these recommendations in a pilotsystem for nine vehicles and the dispatchcenter. It includes satellite telephones, lap-top computers, Global Positioning System(GPS) receivers for vehicle location re-porting, and access to the AT&T CellularDigital Packet Data (CDPD) network fordata communications.

Prior to the installation of the pilot sys-tem, VHF radios equipped with the usualpush-to-talk (PTT) microphones providedthe only means of communication be-tween rangers, firefighters, and dispatch-ers, except for ordinary cell phones. Now,vehicles in the pilot system are equippedwith satellite telephones for improvedcoverage. Two types of satellite phonesare being installed: one uses the Motientsystem, which includes only a geosyn-chronous satellite; the other uses theGlobalstar system, which works with aconstellation of 48 low Earth orbit satel-lites. Because Globalstar satellites are inconstant motion relative to the user, thesatellite system automatically selects thesatellite that’s best for the user.

Each satellite telephone system has itsown advantages. The Globalstar device

Winfred L. Battig, Ronald G. Nishinaga, and Leonard L. Domenic

RemoteRegionsCommunication Technologies for

A BLM ranger uses his new satellite phonewhile Aerospace program manager Wynn Battig looks on. The vehicle-mounted laptopdisplays GPS positions and map references.

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includes cell-phone capability and can beused either in the vehicle or remotely. TheMotient system phone can extend the rangeof PTT radio functionality for the user. Itprovides a satellite bounce to connect tothe dispatch center as if it were the VHFradio system. Thus, even when rangers andfirefighters are outside normal VHF radiorange, using the satellite bounce they canstill connect to dispatch with PTT micro-phone functionality. Dispatchers can hearand talk to them with the efficiency of di-rect VHF radio, except for the slight delayintroduced by the satellite bounce. The pi-lot system evaluation period will allow

users to determine the value of these fea-tures in real situations.

A “ruggedized” laptop computer—onethat is strengthened for resistance to wearand stress—is installed in each vehicle but

removable for work in the field outside thevehicle or in an office. Two types of lap-tops are being installed, each with slightlydifferent features for evaluation. Whatmakes them especially helpful to therangers and firefighters is that they are in-terconnected via a wireless CDPD net-work, which allows access to the Internetand gives instant-messaging capability toremote personnel and FICC dispatchers.They can send emergency messages tosummon help without a microphone. Thisfeature, along with the rest of the pilot sys-tem, enables those responsible for protect-ing remote regions to pass critical datamore reliably.

Vehicles are also equipped with GPS re-ceivers and software on the laptops to dis-play positions of all pilot-system vehicleson detailed roadmaps. The pilot systemprovides better communication coverage

and allows greater precision for the FICCdispatchers and rangers and firefighters inlocating each other.

For rangers with law-enforcement re-sponsibility, the pilot system providesdirect access to the California Law En-forcement Telecommunication Systemdatabase. Rangers can run license-platechecks, personal-identity checks, and gun-registration checks directly from their ownlaptops, rather than relaying such requeststhrough dispatch. Besides increasingranger effectiveness and safety, this featureautomates a responsibility of dispatch per-sonnel who handle more than 10,000 inci-dents from rangers each year, helping to re-duce the dispatch center’s intense peak-load requirements.

Future Communication ImprovementsThe results of a formal evaluation, currentlyunder way, will guide the expansion of thepilot system into a completed system serv-ing approximately 300 law-enforcementrangers and firefighters in Southern Califor-nia. The offices of a number of other remoteregions in the United States are expressinginterest in similarly improving their capa-bilities. Many who are part of the pilot ef-fort regularly express their satisfaction withthe system and their hopes to see it ex-tended throughout the country. The Aero-space pilot system may well become amodel for integrating satellite and terrestrialcommunication technologies to improvecommunications for workers in national remote regions.

Coverage patterns of a single VHF radio repeater site in Southern California before and after im-provements recommended by Aerospace. Each color represents a 10-decibel step, moving from yel-low (maximum gain) to blue (fringe coverage).

Communications architecture established for the FICC pilot system and anticipated for the futurecompleted system. The pilot system installed improvements in only nine vehicles, while the com-pleted system is planned for well over 100.

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Dispatchers in the FICC Dispatch Center usesatellite phones and a wireless data link to com-municate with rangers and firefighters, whosepositions dispatchers can now identify.

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SharedEuropean military satellite communication technologies have never reached the level of their U.S. counterparts—and the gulf appears to be widening. As a result, the UnitedStates and its allies will have to work harder to share military communications andcoordinate front-line operations.

Recent world events have highlighted the importanceof coordinated action between the United States andits primary allies in achieving common military andpolitical objectives. Central to improved coordination

is the free flow of information, and military planners have cometo rely on military satellite communications (milsatcom) to pro-vide the needed services; however, the level of U.S. funding formilsatcom projects and the advances made by the U.S. commu-nications industry have created a technology gap that could itselfhinder the flow of information during allied military campaigns.As the British journal The Economist reported in June 2001,“With more than $1 billion a day to spend, the Pentagon’s budgetdwarfs those of any of America’s allies or antagonists. Expendi-ture on defense research alone is four times the combined Euro-pean total. America’s lead in the use of sensors and telecoms is sowide, and growing, that one of its biggest problems is stoopinglow enough to fight alongside its crudely armed allies.”

Milsatcom is not cheap. Since the end of the Cold War, newand emerging milsatcom programs have felt the same budgetpressures as all other military procurements. As a result, procure-ment schedules have slipped, and systems have been redefined toaccommodate budget restrictions. Nonetheless, progress in theUnited States has been steady, albeit slower than originally antic-ipated. On the other hand, the milsatcom systems of major Euro-pean allies have lagged behind. A looming gulf in relative capa-bility may make front-line communications between U.S. andallied forces nearly impossible. The importance of this observa-tion is itself controversial; not all European allies believe front-line “shooter-to-shooter” communication is necessary or even de-sirable. Their alternative viewpoint is that headquarters-to-headquarters communication is adequate and more appropriate.

In spite of this controversy—or possibly because of it—theUnited States has pushed for international agreements in hopes offacilitating future interoperability of allied milsatcom systems.These efforts have taken the forms of direct technical negotia-tions, attempts to gain acceptance of U.S. waveforms as NATO

(North Atlantic Treaty Organization) standards, andmore recently, bilateral and multilateral negotiations forthe joint development of future milsatcom systems.

The Birth of European MilsatcomThe first European milsatcom systems were actuallybuilt by the United States, which invited certain allies toparticipate in the Initial Defense Communication Satel-lite Program (IDCSP), begun in the mid-1960s. The pro-gram provided relatively narrow-bandwidth transponderservice in the X-band (7.2–8.4 gigahertz) from a spin-stabilized platform. In 1969, the United States launchedan augmented version of the IDCSP system for theUnited Kingdom, called Skynet. NATO also undertookoperation of two IDCSP satellites, followed by two morein 1970 and 1971. (See milsatcom timeline, page 56.) Allof these U.S.-built systems were fully interoperable withIDCSP, communicating to both land-based and ship-borne terminals.

During the 1970s, the United States replaced IDCSPwith the Defense Satellite Communications System(DSCS) program, which provided wider-bandwidthchannels as well as a higher-power payload. A parallel

The Challenge of

Mak King and Malina M. HillsMilitary Communications

Skynet 2, the first European-built milsatcom system, marked a turning point in thehistory of European milsatcom. From then on, the desire for indigenous nationalspace systems, combined with limited budgets and expertise, drove Europe awayfrom interoperability with U.S. milsatcom systems.

series of milsatcom developments in the ultrahigh-frequency (UHF) range and beyond—including Tacsat, theLincoln Experimental Satellite series, the UHF Gapfillerprogram, Marisat, and FLTSATCOM—to provide bothUHF (0.3–3 gigahertz) and SHF (3–30 gigahertz) links tomobile and shipborne terminals. The United States also ex-panded military frequency use into the extremely high fre-quency (EHF) range (30–300 gigahertz) with the develop-ment of the FLTSATCOM EHF Package. This system sawthe introduction of sophisticated onboard processing sub-systems that provided frequency hopping for antijam com-munications, low probability of interception/detection, andmitigation of natural and induced atmospheric scintillation.It provided the proof of concept for the highly secure on-board waveform processing that was subsequently used inthe Milstar satellites launched in the 1990s.

The DSCS III satellites, first launched in 1982, furtherenhanced U.S. military communications by providing newfeatures—such as adaptive nulling—to counteract jam-

ming (see related article, “Adaptive Nulling Antennas for MilitaryCommunications”). These features were enhanced even further inthe Milstar satellites. By the late 1980s, U.S. milsatcom advance-ments had greatly outpaced European developments.

Separate EvolutionsDuring the 1980s and 1990s, European nations pursued au-tonomous milsatcom capabilities along two independent paths: TheSkynet and NATO programs acquired dedicated satellites thatmaintained a level of interoperability with U.S. systems, whileother nations such as France, Spain, and Italy chose to go their separate ways.

The Skynet 4 satellites (Skynet 3 was cancelled in 1974) en-hanced the United Kingdom’s X-band coverage with higher-powerchannels; it also added two UHF channels for connectivity to mo-bile platforms and housed an experimental EHF uplink for in-creased jam resistance. The Skynet 4 series provided increasedpower output for the two UHF and four SHF transponders, a steer-able high-power SHF spot beam, wider SHF channel bandwidths,

and two S-band (1.7–2.7 gigahertz) transponders. This wasthe first payload that was built by British contractors withno U.S. assistance.

NATO also replenished its satellite resources, launchingNATO IVA and IVB in 1991 and 1993. The NATO IV satel-lites were almost identical to Skynet 4 because they wereprocured by the UK Ministry of Defense.

In France, Spain, and Italy, military and commercialcommunication satellite programs were combined to offseteach country’s relatively limited space-technology capabil-

ities and to provide support to their national indus-tries. For example, the first French milsat-com system, Syracuse, was built by theFrench Ministry of Defense and hosted on

the commercial Telecom communication satellites.Syracuse provided X-band “bent-pipe” transponders,

which provide no onboard processing of uplink and down-link signals. Spread-spectrum modulation was used to pro-vide jam resistance, and Telecom used a standard bus to

evolution occurred in the U.S.-built NATO II satellites, which alsooperated at higher powers and wider bandwidths. Separately, theBritish government developed the Skynet 2 satellites, which in-corporated a higher-power payload while maintaining the nar-rower bandwidth of the first Skynet. The Skynet 2 system was thefirst European-built milsatcom system (with U.S. assistance on thepayload), and it marked a turning point in the history of Europeanmilsatcom. From then on, the desire for indigenous national spacesystems, combined with limited budgets and expertise, drove Eu-ropean countries away from interoperability with U.S. milsatcomsystems.

While Europe remained committed to these spin-stabilized,low-capacity superhigh-frequency (SHF) milsatcom systemsthrough the 1970s and most of the 1980s, the United States fieldedan enhanced array of milsatcom systems. For example, U.S. ad-vances in satellite bus technology led to the development of three-axis-stabilized spacecraft, which greatly facilitated improvementsin many system applications. The U.S. Navy embarked upon a

The Spanish Hispasat satellites are owned and operated by a combined public-privatecompany formed by the Spanish government. Hispasat provides X-band servicethrough bent-pipe transponders.

The French Telecom communication satellites hosted the Syracusepayloads, built by the French Ministry of Defense. In Europe, such

ventures combining government and commercial funding and management are not un-common. The Telecom program was directed and operated by the French nationaltelecommunications agency.

help minimize cost (Syracuse 2 was only a $2 billion pro-gram). Syracuse blended not only government and pri-vate system operations and funding, but also manage-ment. The Telecom program was directed and operatedby the French national telecommunications agency. Incontrast, U.S. laws and regulations generally prohibit agovernment role in ownership or management of com-mercial satellite communications systems.

Similar to the French acquisition model, the Spanishacquisition model combines federal government andcommercial oversight. Spain’s Hispasat satellites areowned and operated by a combined public-private com-pany formed by the Spanish government. Like Syracuse,the Hispasat satellites provide X-band service throughbent-pipe transponders (3 channels of 125 megahertzbandwidth), with directive antenna coverage that is provided by a simple steerable horn-antenna assemblyfeeding a parabolic dish.

Civil SystemsIn the United States, public expenditures for space arebalanced more or less equally between the civil and defense sec-tors, and defense funding for space research and development isfour to five times that of commercial funding, according to figurespublished by the National Science Foundation and various re-search groups. Thus, defense and civil space technology develop-ments have both played a large role in U.S. commercial space ad-vances. In Europe, on the other hand, public expenditures forspace come predominantly from the European Space Agency (52percent), followed by the national space agencies (31 percent) andmilitary programs (17 percent). The funding balance reflects a Eu-ropean emphasis on developing a strong regional space industryrather than military space systems. As a result, advancements incivil space systems tend to enable military and commercial spacesystems, and not vice versa. Civil space programs that have

assisted European milsatcom developments include the Italsat,Olympus, Artemis, and Stentor programs.

The Italsat program provided the developmentplatform for Italy’s dual-use military and com-

mercial communication satellite technology.

While the first Italsat satellites did not include overt military pay-loads, they supported the Italian milsatcom program, known asSICRAL, by providing the first European implementation of on-board processing. They also included an L-band (1.5–1.7 giga-hertz) and Ku-band (10.7–14.5 gigahertz) mobile communicationspackage (Italsat 2) and an EHF-propagation experiment (Italsat 1).

The Olympus program began in 1982 and ended with a launch in1989. The satellite improved upon previous European communica-tions satellites by providing higher-power capabilities (3.5 kilo-watts), steerable beams at the Ka band (27–31 gigahertz), a Ka-band propagation experiment, and higher bandwidths—includingone 700-megahertz channel. Sponsored by the European SpaceAgency, Olympus was truly a pan-European effort, involving proj-ect members from 11 European countries (and Canada).

Artemis, also sponsored by the European Space Agency, in-cluded Ka-band and laser crosslinks, spot beams, and L-band ca-pabilities to mobile users that were somewhat derivative of the Ital-sat payloads. Unfortunately, the failure of the upper stage duringthe 2001 launch left Artemis in an inclined orbit far below its tar-

geted geostationary course. Efforts to salvage the operationwere partly successful, and Artemis demonstrated its lasercrosslinks in November 2001, establishing an optical data-transmission link with a French Earth-observation satellite,SPOT 4.

The Stentor program, developed by the French SpaceAgency, was designed to demonstrate advanced bus tech-nologies, including ion propulsion, lithium-ion batteries,deployable thermal radiators, fluid loops for heat transport,and autonomous stationkeeping using the Global Position-ing System. In addition, the program contains communica-tion technology demonstrations such as its active-array an-tennas and onboard digital processing. The Ku-bandpayload includes one wideband transponder, threetransponders with surface-acoustic-wave filters and on-board multiplexers, a 48-element phased-array antenna, adeployed reflector, and a steerable spot-beam antenna.Stentor also has an EHF-propagation experiment. Thesecommunication technologies are expected to benefit theFrench Syracuse 3 and 4 programs.

The Italsat program provided the development platform for Italy’s dual-use military andcommercial communication satellite technology. It provided the first European imple-mentation of onboard processing.

Sponsored by the European Space Agency, Olympus improved upon previous Euro-pean communication satellites by providing higher-power capabilities, steerablebeams at the Ka band, a Ka-band propagation experiment, and higher bandwidths.

allies to introduce systems that are interoperable with U.S. milsat-com systems.

For example, Italy’s SICRAL, first launched in 2001, uses tech-nologies that were tested on Italsat. Consequently, 90 percent of thetechnology was developed in Italy. The system boasts the first op-erational EHF communications capacity produced in Europe; how-ever, it does not include onboard demodulation and remodulationand therefore is not interoperable with U.S. systems or compatiblewith recently approved NATO EHF Standardization Agreements.Only the SHF and UHF military capabilities were built in accor-dance with NATO Standardization Agreements. The next genera-tion of SICRAL is expected to include an onboard SHF processingcapability and frequency-hopping protocols compatible with theUniversal Modem, a multinational effort to provide jamproof satel-lite communications using the nonprocessing transponders onDSCS III, NATO, or Skynet 4. Of course, the United States stopped working on equipment employing the Universal Modem standard

several years ago, deciding instead to focus on EHF systems forantijam communications.

The United Kingdom is replacing the aging Skynet 4 constella-tion with Skynet 5, which will provide UHF, SHF, and EHF com-munication capabilities, though not necessarily from a single satel-lite design. British requirements do not explicitly call for processedEHF, although they do require interoperability with U.S. net-works—especially U.S. Navy networks. EHF service will mostlikely be provided as part of an anticipated bilateral agreement in-volving participation in the Advanced Extremely High Frequency(AEHF) program, the successor to Milstar; but this EHF servicecould potentially be a separate payload on a Skynet 5 bus. In thiscase, a U.S. contractor would probably build the EHF payload.

The Telecom 2 satellite, which hosts the Syracuse 2 payload, willreach the end of its service life in 2006; thus, a Syracuse 3A launchis required in 2003, a Syracuse 3B launch in 2006. The Syracuse 3 will include SHF channels cross-strapped with EHFfeeder-links, but the EHF capability will not be processed onboardand therefore will not be compatible with U.S. systems.

The Spanish government is also seeking a replacement for itsHispasat satellites, to be called XTAR/SpainSat. It appears that the

Pan-European Milsatcom EffortsGiven the cooperation seen on civil programs such as Olympus,one might have anticipated the creation of a pan-European milsat-com system; however, the path toward a multinational milsatcomsystem has proved tortuous.

In 1992, the Western European governments began collabora-tive discussions geared toward initiating a pan-European system.In 1993, three separate programs were being studied: EuMilSat-Com, which would involve eight countries; BiMilSatCom, involv-ing the United Kingdom and France; and InMilSat, involving theUnited Kingdom, France, and the United States.

By 1995, the EuMilSatCom discussions were discontinuedwhen Italy dropped out, citing the program’s estimated costs. In-MilSat was dropped because of differences between U.S. and Eu-ropean operational requirements. At the same time, two new po-tential programs emerged: GEFsatcom, involving Germany andFrance; and TriMilSatCom, involving the United Kingdom,France, and Germany. Discussions for both ended unsuccessfullyin 1996.

In 1997, the United Kingdom, France, and Germanysigned a TriMilSatCom Memorandum of Understanding,and it appeared that a European milsatcom system was onthe horizon. One year later, however, the United Kingdomwithdrew after deciding to focus on a separate national system (Skynet 5).

At this point, given the substantial difficulties in recon-ciling performance requirements, achieving interoperabil-ity with legacy systems, controlling cost, and apportioningnational workloads, most other countries also decided tofocus on national systems. The single remaining exceptionis a potential collaboration between France and Germanyin the Syracuse 3 system.

Current DevelopmentsThe United States has been actively encouraging milsat-com interoperability with major European allies since the1980s and has been aggressively promoting the use ofshared waveforms. The model for this effort is the Milstarsystem, which achieved interoperability among three U.S. mili-tary branches by imposing an interface control document—in ef-fect, a waveform interoperability standard. In the early and mid-1980s, Aerospace began to document the Milstar low-data-rate(LDR) waveform as a standard that could be made available to avariety of U.S. space programs. This effort was eventually adoptedby the Air Force and resulted in the official military EHF LDRWaveform Interoperability Standard. When medium-data-rate(MDR) capability was added to the Milstar program in the early1990s, the idea of a waveform standard was already well accepted,and Aerospace contributed significantly to a broad-based govern-ment effort to document the new waveform as the official militaryEHF MDR Waveform Interoperability Standard.

These two EHF U.S. military waveform standards, along withstandards associated with other frequency bands, have been sharedwith selected major allies over the years in hopes that they wouldbe used in systems developed by those allies. In addition, with thehelp of allies, several U.S. waveform standards have been adaptedand ratified as NATO Standardization Agreements. Still, while theUnited States has had some success in influencing NATO Stan-dardization Agreements, it’s had very little success in convincing

Artemis is sponsored by the European Space Agency. Despite initial launchdifficulties Artemis, successfully demonstrated its laser crosslinks.

military payload will be nine SHF bent-pipe transponders, with noonboard processing.

The desire to enhance indigenous space capabilities has fosteredthe development of four separate European milsatcom systems,none of which will be fully interoperable with U.S. systems orwith each other. Only the Skynet 5 EHF capability is expected tobe interoperable with U.S. systems, thanks to the anticipated U.S.involvement in the payload’s design and manufacture.

Future Prospects for InteroperabilityThe NATO IV satellites are reaching the end of their service lives,and NATO must decide soon on a replacement strategy. Possibleacquisition methods for NATO V include a standard acquisitioncontract, shared capacity with a national system, or a publicly fi-nanced (fee-for-service) initiative. Separately, the governments ofthe United States, United Kingdom, France, and Italy have all pro-posed that their national systems be used as the basis for either agapfiller to NATO V or for the NATO V system itself. The UnitedStates also proposed a partnership in the AEHF program as a solu-tion to NATO’s future highly protected milsatcom requirements.

The AEHF program is already international, as the UnitedStates and Canada have signed a bilateral Memorandum of Under-standing detailing a partnership in the system’s development. TheAerospace Corporation was heavily involved in the developmentof this document, providing most of the technical support to theU.S. delegations in these discussions. Aerospace also provided di-rect contracted support to the Canadian Department of National

Defense in its evaluation of the program’s ability to meet Canadian national communication requirements.

In addition, similar agreements with the United Kingdom andthe Netherlands are nearing completion. Aerospace has also playeda central technical support role in these negotiations. The ItalianMinistry of Defense has stated that it would consider cooperativedevelopments with the United States, but asserts that this would re-quire transfer of U.S. technology to Europe—presumably a benefitto European industry. While NATO has documented needs forUHF, SHF, and EHF systems, it may not implement processed-EHF service until 2007 or later. The path toward resolution of theNATO V acquisition decisions and the potential for direct NATOinteroperability with U.S. systems is not clear.

Hopes for allied milsatcom cooperation may ride on recent European Union statements calling for a common European rapid-reaction force. Further transatlantic cooperation may also drive theperceived need for an interoperable (at least by NATO standards)European milsatcom system. Cooperative space efforts are not nec-essarily viewed as more economical; indeed, the Italian Ministry ofDefense estimates that its SICRAL system was cheaper to buildthan any cooperative system could have been.

ConclusionsThe need for milsatcom interoperability with America’s major Eu-ropean allies was clearly established during the Gulf War and re-mains a crucial factor for future development. As one path towardthe goal of interoperability, the United States has tried to obtain

Left: The Stentor program, developed by the French Space Agency, was de-signed to demonstrate advanced bus technologies, including ion propulsion,lithium-ion batteries, deployable thermal radiators, fluid loops for heat trans-port, and autonomous stationkeeping. The Stentor program also has anEHF-propagation experiment. Right: France will launch Syracuse 3 in 2003.

The satellite will include SHF channels cross-strapped with EHF feeder-links, but the EHF capability will not be compatible with U.S. systems. TheSHF payload will have four spot beams, one global beam, one beam for met-ropolitan France, and nine 40-megahertz channels.The EHF payload will in-clude two spot beams, one global beam, and six 40-megahertz channels.

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Skynet 1NATO IISkynet 2NATO IIIOlympusSyracuse 1 Skynet 4Italsat 1NATO IVSyracuse 2 Hispasat 1Italsat 2ArtemisSICRALStentorSpainsatSyracuse 3Skynet 5

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U.S. Systems

European Systems

The historical record of U.S. and European milsatcom launches shows atrend toward a diversity of European systems beginning in the 1990s. This

trend reflects a move away from pan-European and European-U.S.milsatcom interoperability.

agreements on waveform standardization. These efforts have beenmoderately successful on paper, but they have not led to the devel-opment of interoperable systems—probably because of Europe’sinability to match America’s relatively high funding levels and fastdevelopment schedules.

Similarly, efforts to pursue future system developments on anequal-partner basis have also failed because of the lack of equalityin the capability, requirement levels, and funds available. Interna-tional interest in America’s AEHF program appears to provide thebest hope yet for eventual front-line milsatcom interoperability.While the value of true interoperability that spans all levels of mil-itary operations is not yet universally accepted, its desirability ap-pears clear to U.S. decision makers, who will undoubtedly empha-size allied interoperability in all future milsatcom developments.

Further ReadingC. Bildt, J. Peyrelevade, and L. Spath, “Towards a Space Agency for theEuropean Union,” Report of the Wise Men to the ESA Director-General,Annex 1, 14 (November 9, 2000).

L. de Haro, M. Calvo, J. C. Vargas, M. Sanchez, M. J. Marin, and G.Crone, “ASYRIO: A Reconfigurable Antenna for a Second Generation ofthe Spanish Satellite (Hispasat-II),” Proceedings of 8th International Con-ference on Antennas and Propagation (IEEE, Vol. 1, pp. 92–94, 1993).

M. Garreau, “Hispasat 1: A Breakthrough in Communications Satellites,”Proceedings of Second European Conference on Satellite Communica-tions, ESA SP-332 (Liège, Belgium, October 22–24, 1991), pp. 47–51.

Global MILSATCOM 2000, Proceedings (The SMi Group, The Hatton,London, November 20–21, 2000).

“In an Age without Heroes,” The Economist, Vol. 359, No. 8224 (June 2,2001).

R. A. Lacy, “Observations on the Private Finance Initiative (PFI) andSkynet 5,” The Aerospace Corporation, TOR-2001(3000)-0982e (January 2001).

C. Lardier, “For a European Space Defense,” Air & Cosmos (July 13,2001), p. 36.

C. Lardier, “New Generation of Military Satellites,” Air & Cosmos (Febru-ary 16, 2001), pp. 34–35.

C. Lardier, “Satellite Communications in the 21st Century,” Air & Cosmos (April 20, 2001), p. 37.

C. Lardier, “Syracuse 3 Satellite Planned for 2004,” Air & Cosmos(December 15, 2001), p. 38.

Lehman Brothers, “Satellite Communications, Industry Update”(July 3, 2001).

D. H. Martin, Communication Satellites, 4th ed. (The Aerospace Press,El Segundo, CA, and AIAA, Reston, VA, 2000).

D. Michaels, “Europe Lags Behind U.S. in Surveillance From Space,”The Wall Street Journal (June 19, 2001), p. A16.

National Science Foundation, Division of Science Resources Statistics,http://www.nsf.gov/sbe/srs/, accessed Dec. 13, 2001.

M. Nones, “Industry and Defense Ministry Backing Italy’s SICRAL Satellite,” Il Sole (February 9, 2001), p. 13.

M. A. Taverna, “Twin Milsats Enhance Europe’s Telecom Net,” AviationWeek & Space Technology (February 12, 2001), p. 66.

FutureThe current military satellite communications network represents decades-old technology. To meet the heightened demands of national security in the coming years, newer and more powerful systems are being developed.

Advances in information tech-nology are fundamentallychanging the way militaryconflicts are resolved. The

ability to transmit detailed informationquickly and reliably to and from all partsof the globe will help streamline militarycommand and control and ensure informa-tion superiority, enabling faster deploy-ment of highly mobile forces capable ofadapting quickly to changing conditions inthe field. Satellite communications play apivotal role in providing the interoperable,robust, “network-centric” communica-tions needed for future operations.

Military satellite communications (ormilsatcom) systems are typically catego-rized as wideband, protected, or narrow-band. Wideband systems emphasize highcapacity. Protected systems stress antijamfeatures, covertness, and nuclear surviv-ability. Narrowband systems emphasizesupport to users who need voice or low-data-rate communications and who alsomay be mobile or otherwise disadvan-taged (because of limited terminal capabil-ity, antenna size, environment, etc.).

Milsatcom is a system of systems thatprovides balanced wideband, narrowband,and protected communications capabilityfor a broad range of users across diversemission areas. The anticipated implemen-tation of advanced architectures, sup-ported by heightened connectivity in spaceas well as on the ground, will enable na-tional security space communications totake advantage of commercially devel-oped Internet-like communications, butwith greater assurance and security.

For wideband communication needs,the Wideband Gapfiller Satellite program

and the Advanced Wideband System willaugment and eventually replace the De-fense Satellite Communications System(DSCS). These satellites will transmit sev-eral gigabits of data per second—up to tentimes the data flow of the satellites beingreplaced. Protected communications willbe addressed by a global extremely highfrequency (EHF) system, composed of theAdvanced Extremely High FrequencySystem and Advanced Polar System.These systems are expected to provideabout ten times the capacity of currentprotected satellites (the Milstar satellites).Narrowband needs are supported by theUFO (Ultrahigh-frequency Follow-On)

constellation, which will be replaced by acomponent of the Advanced NarrowbandSystem (see timeline, page 56).

Capacity gains in these systems willalso be matched by improved features,such as multiple high-gain spot beams thatare particularly important for small termi-nal and mobile users. Satellite, terminal,control, and planning segments will utilizeemerging technology to ensure the best ca-pability for the cost. Coordination amongground, air, and space segments and between government and commercial assets will help ensure deployment of themost efficient, effective, and affordablecommunications systems.

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In 1997, the Senior Warfighters’ Forum established a road map charting the course of military satellite communications through 2010. In 2002, there will be course corrections as the Department of Defense pursues an aggressive acceleration in the delivery of improved communications capability.

communication payload will include state-of-the-art technology and provide a majorleap in capability. Preliminary estimates in-dicate that one Wideband Gapfiller space-craft will provide transmission capacity upto 2.4 gigabits per second. This capabilityalone exceeds the capacity of the entire ex-isting DSCS and Global Broadcast Serviceconstellations.

Throughput capacity is divided amongnine X-band beams and ten Ka-bandbeams. Eight of the X-band beams areformed by separate transmitting and receiv-ing phased-array antennas, which providethe ability to shape and scale coverage ar-eas. The ninth X-band beam provides Earthcoverage. The ten Ka-band beams areformed by gimbaled dish antennas and in-clude three beams with reversible polariza-tion. (Polarization—the direction of theelectric field of an antenna—plays an im-portant part in optimizing reception or re-ducing the effects of jamming).

The key to the very flexible payload isthe digital channelizer (or digital signalprocessor). The channelizer divides thecommunications capacity into 1872 sub-channels of 2.6 megahertz each andswitches and routes these subchannels. Thesignals can be cross-banded from one fre-quency band to another and any uplinkcoverage can be connected to any downlinkcoverage. Also, any uplink signal withinone coverage area can be connected to anyor all downlink coverages.

The implementation plan calls for a min-imum of three geosynchronous spacecraftand associated ground control software,with an option for up to three additionalspacecraft. The payload will be integratedinto a commercial spacecraft bus. Eachsatellite will weigh approximately 5900kilograms at launch and use more than10,000 watts of power. This design usesbipropellant chemical propulsion for orbitraising and xenon ion propulsion to remove

Wideband Communications Assured capacity is the primary goal of themilitary’s wideband communications sec-tor. Wideband data rates are defined asthose greater than 64 kilobits per second,although the line between wideband andnarrowband is blurring as commercial datarates to disadvantaged users move higher.The military’s wideband requirements arecurrently supported by DSCS and theGlobal Broadcast Service, as well as com-mercial systems. These military systems,together with the planned Wideband Gap-filler satellites, will form the Interim Wide-band System, which will eventually giveway to the Advanced Wideband System.

Wideband Gapfiller SatellitesThe Wideband Gapfiller Satellite programwill provide the next generation of wide-band communications for the Departmentof Defense (DOD). The constellation willsupplement the military X-band (roughly7–8 gigahertz) communications capabilitynow provided by the Defense SatelliteCommunications System and the militaryKa-band (about 20–21 gigahertz down,30–31 gigahertz up) capability of theGlobal Broadcast Service. In addition, theWideband Gapfiller Satellite program willinclude a high-capacity two-way Ka-bandcapability to support mobile and tacticalpersonnel.

The name “Gapfiller” is somewhat mis-leading because this very capable wideband

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The military satellite communications framework is a system of systemsthat provides connectivity for a broad range of users across diverse missionareas. In the future the framework will support “network-centric” warfare

through an architecture that promotes the interconnection of satellites andconstellations in space, as well as through ground nodes.

The Wideband Gapfiller Satellite program will provide the next generation of widebandcommunications for the Department of Defense.

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orbit eccentricity and for station keeping.The mean mission duration for each space-craft is 11.8 years.

Synchronization of the various Wide-band Gapfiller Satellite segments is underway, and 1700 operational wideband termi-nals are expected by 2010. Terminals capa-ble of operating within several frequencybands are a fundamental piece of the wide-band architecture, and a recent contractawarded to Harris Corporation for up to200 lightweight, high-capacity quad-bandGround Multiband Terminals (GMTs) willhelp ensure the delivery of communicationsservices through the Wideband Gapfillersatellites, as well as through the currentDSCS, future Advanced Wideband System,and commercial satellite systems. Also, theArmy’s Multiband/multimode IntegratedSatellite Terminal (MIST) will provide upto megabits-per-second capacity for mobilecommunications in the next decade.

Responsibility for control of the satel-lites will be shared among various branchesof the armed services. Network control willrely on existing worldwide ground facilitiesoperated by the Army. Spacecraft controlwill be conducted by Air Force operatorsusing the Command and Control System—Consolidated (CCS-C). The CCS-C is the

integrated command and control system be-ing developed to support all milsatcomsatellite constellations, legacy and future. Itwill replace the current command and con-trol functions of the Air Force SatelliteControl Network.

The Wideband Gapfiller Satellite con-tract was awarded to Boeing Satellite Sys-tems in January 2001, and the first satellitelaunch is planned for the second quarter offiscal year 2004—just three years after thecontract award.

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The Global Broadcast Service replaces the superhigh-frequency X-band payload with four 130-wattmilitary Ka-band transponders. Each transponder can be accessed through either of the receivepaths, configured by ground command. Data are transmitted through three spot-beam antennas oneach spacecraft. Two of the beams each cover an area 500 nautical miles in diameter, and the thirdcovers an area of 2000 nautical miles in diameter.

The position of the National SecuritySpace Architect (NSSA) was establishedin March 1998, with implementationguidance provided in a July 1998 Memo-randum of Understanding between theSecretary of Defense and the Director ofCentral Intelligence Agency. The memo-randum codified the presidential directivePDD/NSC-49, which commits the Secre-tary of Defense and Director of CentralIntelligence to ensuring that defense andintelligence space activities are coordi-nated and that space architectures areintegrated to the greatest extent feasible.

The history of the Space Architect’sinvolvement in military satellite communi-cations (milsatcom) begins in 1995,when NSSA’s predecessor, the Depart-ment of Defense (DOD) Space Architect,was established. Its first responsibilitywas to develop a future milsatcom archi-tecture that coordinated core DOD capa-bilities with commercial technologies andglobal broadcast capabilities. The resultsof that work influenced the developmentof the 1997 Senior Warfighters’ Forumand the Joint Requirement Oversight

Council’s “Course of Action” road mapthat has guided acquisition planning intothis decade. As its first task, NSSA iscompleting an architecture that will ex-tend the DOD Satellite Communicationsroad map from 2010 to 2015 and beyondand expand it to include the intelligencecommunity and NASA.

NSSA’s vision of the future is knownas the Mission Information ManagementCommunications Architecture. As in thecase of the DOD Space Architect’s mil-satcom effort, an Architecture Develop-ment Team has been assembled frommembers of government acquisition,user, and support communities. Thesestakeholders will take the architectureinto transition planning in 2002. Althoughthe objective Mission Information Man-agement Communications Architecturewon’t take shape until the next decade,the NSSA and stakeholder planning willaffect milsatcom decisions being madethroughout this decade.

In the past, the Space Architect re-ported to both the Assistant Secretary ofDefense (Command, Control, Communi-

cations, and Intelligence) and the headof the Director of Central Intelligence’scommunity management staff. TheSpace Architect developed and coordi-nated mid- to long-range space architec-tures, but had no direct influence overspace budgets or acquisition programs.However, under action initiated by theSecretary of Defense in response to theCommission to Assess United StatesNational Security Space Managementand Organization, the Space Architectwill report to the new Air Force Under-secretary/National Reconnaissance Of-fice Director and will be responsible forensuring that National ReconnaissanceOffice and Air Force program funding forspace is consistent with policy, planningguidance, and architectural decisions.The office will remain jointly staffed andthe Space Architect will retain the end-to-end architecture responsibility for allnational security space systems. The of-fice will also be responsible for assistingthe undersecretary with trade-offs be-tween space systems and non-spacesystems.

The Space Architect

Global Broadcast ServiceOperation Desert Storm clearly demon-strated the need for the rapid delivery oflarge volumes of information to users onthe front lines. During Desert Storm, air-tasking orders and intelligence reports weresometimes delivered by hand due to thelack of available communications band-width. This concern drove the creation ofthe Global Broadcast Service in the mid-1990s. With the advent of this service, mostcritical information could be transmitted inseconds. For example, a 1-megabyte airtasking order that might take up to an hourto transmit over Milstar or UFO (at 2.4kilobits per second) could now be sent inless than a second. The ability to pushmegabits of data to a small terminal wasmade possible by commercial advance-ments in high-power satellite transpondersand direct broadcast service technology.The first, and very successful, use of theGlobal Broadcast Service was in support ofoperations in Bosnia in 1996, where com-mercial satellites were used to broadcastmilitary data to modified commercial directbroadcast set-top receivers and decoders.

Today, the Global Broadcast Service isprovided through a series of four Ka-bandtransponders and three steerable beamshosted on the Navy’s UFO 8, 9, and 10spacecraft. Ground terminals with antennadiameters of 0.6 to 1 meter receive data atrates up to 24 megabits per second pertransponder from either of the two 500-nautical-mile diameter spot beams. Ratesup to 1.5 megabits per second can beachieved through the 2000-nautical-milediameter spot beam. Data are uplinked tothe transponders through fixed Primary In-jection Points and transportable Theater In-jection Points. The receiving suites andbroadcast-management suites supplied byRaytheon Company support military Ka-band and commercial Ku-band operations.

In the future, the Wideband GapfillerSatellite will provide the Global BroadcastService through Ka-band transponders.This is the second hosted Global Broadcast

Service implementation, and its migrationpath is still under consideration with regardto the Advanced Wideband System.

Advanced Wideband System The successor to the Defense SatelliteCommunications System and the Wide-band Gapfiller Satellite program is the Ad-vanced Wideband System. The system’s fi-nal configuration has not yet solidifiedunder ongoing milsatcom transformationalefforts, but the concept is one of appliedtechnology and engineering that will re-move capacity as a constraint on warfarecommunications. Analyses by the DefenseInformation Systems Agency and JointStaff indicate that a global wideband satel-lite communications capacity in excess of15 megabits per second will be needed bythe middle of the next decade.

The Advanced Wideband System willtake advantage of the commercial and gov-ernment technology advances of the firsthalf of this decade to meet expected needs.Laser crosslinks, space-based data process-ing and routing systems, and highly agilemultibeam/phased-array antennas willmost likely be included. A con-stellation of advanced

wideband-capable satellites is planned witha first launch at the end of this decade.

Capacity in the right place is the overallrequirement, but getting adequate capacityto ever-smaller terminals worldwide is be-coming increasingly difficult because ofthe limits on the amount of internationallyallocated bandwidth in the X and Ka bandsfor DOD use (see related article, “CriticalIssues in Spectrum Management for De-fense Space Systems,” in this issue). Sev-eral options for mitigating the current limi-tations are under consideration, includingthe use of higher frequencies (notably inthe 40–75-gigahertz range, and possiblymuch higher). Also, increasing the numberof wideband-capable satellites over a re-gion would enable users with directionalantennas to use an allocated frequencyband on more than one satellite in view.Another approach would increase effectivebandwidth by simultaneously reusing allo-cated frequencies through the use of smallindependent beams or cells, achievablethrough multibeam/phased-array antennas.Frequency reuse is an important character-istic of terrestrial and space-based cellularsystems. Radio-frequency componentswith more efficiency and power will also beused to get more data to small terminals,similar to the way commercial directbroadcast service transponder technologywas adopted for the Global Broadcast Ser-vice a decade earlier.

Synchronization of the various Ad-vanced Wideband System segments is beginning. To support these efforts, newterminals, such as the GMT, will be intro-duced, and the CCS-C will be employed,

Through the Global Broadcast Service, informa-tion such as video, maps, charts, weather pat-terns, and digital data can be transmitted to mo-bile users equipped with small tactical terminals.

The Defense Satellite Communications System (DSCS) is part of the Interim Wideband System,along with the Global Broadcast Service and the Wideband Gapfiller System.

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but with significant additional capability toaddress the increased complexity in provid-ing high capacity, tailored communicationsto highly mobile forces.

Protected CommunicationsProtected systems have the ability to avoid,prevent, negate, or mitigate the degradation,disruption, denial, unauthorized access, orexploitation of communications services byadversaries or the environment. Future pro-tected systems include the Advanced Ex-tremely High Frequency System and Ad-vanced Polar System.

Advanced EHFThe loss of Milstar Flight 3 in 1999 and thelast deployment of a Milstar satellite (Flight6) in fiscal year 2003 have increased theneed for a successor system with full opera-tional capability by 2010. Consequently, inNovember 2001, the Advanced ExtremelyHigh Frequency (AEHF) System contractwas awarded to the Lockheed Martin SpaceSystems and TRW Space and Electronicsteam for the System Development andDemonstration phase of the new program.Under this contract, three satellites and theassociated ground command and controlsegment will be produced. Under DODtransformational initiatives, other protectedmilsatcom options are being considered tocomplete the needed protected strategicand tactical capability; however, if full op-erational capability cannot be achieved intime with the transformational options,then the original program to acquire fourAEHF satellites plus one spare will be re-stored. All new protected satellites will beinteroperable with the Milstar satellites.

The AEHF System will have up to 12times the total throughput of Milstar, insome scenarios. Single-user data rates willincrease from a maximum of 1.544megabits per second (medium data rate) to8 megabits per second (high data rate).Along with capacity, the new system willprovide an almost tenfold increase in thenumber of spot beams for improved useraccess. These small beams will focus powerto improve reliability and data rates to smalland large terminals and to minimize inter-ception and interference opportunities forregional adversaries. Overall, the AEHFSystem network will support twice as manytactical networks as Milstar. Improvementsin network capability will also help ensurecompatibility with international partners.

As in Milstar, the AEHF Systemcrosslinks will enhance routing and reducevulnerability to terrestrial disruption. The

new crosslinks will operate at several timesthe current Milstar data rate.

By 2010, about 2500 terminals are ex-pected in the protected communications in-ventory for the Air Force, Navy, Army, andMarines. Portable, mobile, and fixed termi-nals with low, medium, and high data rateswill support ground units, aircraft, surfaceships, and submarines. Standard antennaswill range in size from a few centimeters toabout 3 meters. Applicable milsatcom ter-minals include the Family of Advanced Be-yond line-of-sight Terminals (FAB-T), theSingle-Channel Antijam Man-Portable Ter-minal (SCAMP), Secure Mobile AntijamReliable Tactical Terminal (SMART-T),

and Submarine High Data Rate (Sub HDR)system. The FAB-T combines two previousprograms, the Airborne Wideband Termi-nal and Command Post Terminal Replace-ment, and establishes a family of terminalswith a common open architecture for air-borne and ground applications.

For mission control, the system willhave a dedicated segment consisting ofcommunications management, mobilecommand and control centers, SatelliteGround Link Standard/Unified S-Band(SGLS/USB) satellite control, and EHF in-band satellite control. The CCS-C will in-terface with the AEHF satellite control toprovide SGLS/USB command capabilities.

Milstar provides protected communications and offers advanced features such as onboard signal pro-cessing and satellite-to-satellite crosslinks. The system will eventually give way to the AEHF system.

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Advanced Polar SystemThe demand for protected polar satellitecommunications to support submarines,aircraft, and other platforms and forces op-erating in the high northern latitudes hassteadily increased over the last twentyyears. In 1995, the Pentagon’s Joint Re-quirements Oversight Council approved thePolar Operational Requirements Docu-ment, which paved the way for a programto address the polar communications de-mand. Subsequently, the decision was madeto place a series of modified EHF payloadsonto host satellites. The first package waslaunched in 1997, and the remaining twoare scheduled for launch within the nextthree years. Although this hosted capabilitywill provide a critical service to the end ofthis decade, it only meets a small fraction ofthe requirements spelled out in that 1995Operational Requirements Document. Con-sequently, a replacement system is beingconsidered for the 2008–2010 timeframe.The Air Force Space Command and theMILSATCOM Joint Program Office re-cently completed a polar concept study thatcovered 35 wide-ranging options for a fu-ture polar capability. As a result of thisstudy, two satellites in highly inclined,highly elliptical molniya orbits have beenrecommended. In addition, transforma-tional initiatives within the Department ofDefense have put forward a proposed Na-tional Strategic SATCOM System thatwould combine worldwide and polar cov-erage for highly survivable communica-tions, all in one system.

Narrowband CommunicationsIn the past, the term “narrowband” implieddata rates of less than 64 kilobits per sec-ond, but a higher boundary could apply inthe future as higher data rates to small ter-minals become possible. Mobile and othersmall terminal users depend on high-power,low-data-rate satellite systems to receivedata via broadcast (as in the Navy’s FleetBroadcast) and for two-way communica-tions. Narrowband needs—generally trans-mitted in the ultrahigh-frequency (UHF)range—are supported by the UFO constel-lation, which will be replaced by a compo-nent of the Advanced Narrowband System.

Advanced Narrowband SystemThe Advanced Narrowband System isDOD’s next-generation narrowband tacticalsatellite communications system, and itsgoal is to provide global narrowband com-munications services to tactical users (whoare typically quite mobile). The Advanced

Narrowband System consists of six seg-ments: DOD space; commercial space;telemetry, tracking, and command; networkcontrol; user entry; and gateway.

The Mobile User Objective System is thesuccessor to the Navy’s current Boeing-built UFO system and is the key transportelement in the Advanced Narrowband Sys-tem. The Mobile User Objective Systemwill provide beyond-line-of-sight commu-nication to support mission objectivesacross all branches of the military.

The Communications Satellite ProgramOffice of the Space and Naval Warfare Sys-tems Command has completed conceptstudies resulting in several approaches toaddressing narrowband needs. Aerospacehas supported the Navy in evaluating theseapproaches and has collaborated, from anAdvanced Narrowband System perspec-tive, on possible commercial satellite com-munications augmentation aspects.

The current UFO constellation has eightsatellites, plus one on-orbit spare, each ofwhich provides a mix of 38 UHF commu-nication channels at 5 and 25 kilohertz andone 25-kilohertz fleet broadcast channel.About 7500 UHF terminals are in use to-day. The capacity of this system will fall farshort of anticipated needs by the end of thisdecade, considering that the estimated 2010Combined Major Theaters of War require-ment is about 42 megabits per second with

over 2,300 simultaneous accesses—hence,the urgent need for the Advanced Narrow-band and Mobile User Objective Systems.Launches could begin before the end of thedecade, paving the way for full operationalcapability by 2013. The number of narrow-band satellite communications terminals ofall types is expected to approach 82,000 in2010. About 50 percent of those will behandheld Combat Survivor Evader Locatorunits, and the remainder will be predomi-nately legacy and advanced Joint TacticalRadio System terminals.

The Mobile User Objective System willemploy commercial technology to enablecommunications with users of large termi-nals and small or handheld terminals. Com-mercial systems such as Thuraya in theMiddle East and AceS in Southeast Asiahave shown that more than 10,000 low-data-rate handheld terminals can be serv-iced over a region with one satellite. Largemultibeam antennas, some more than 12meters in diameter, enable the use of sev-eral hundred spot beams to improve signal-to-noise levels and achieve up to 30 timesfrequency reuse. Systems with these capa-bilities currently operate at L-band (1.5 gi-gahertz downlink) frequencies.

In addition to the Mobile User ObjectiveSystem, the Navy is keeping other alterna-tives open for meeting Advanced Narrow-band System requirements. One alternative

The Advanced Extremely High Frequency system will have as much as 12 times the total throughputof Milstar, in some scenarios. Single-user data rates will increase to 8 megabits per second. The sys-tem will also provide a large increase in the number of spot beams for improved user access.

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would be to field or lease commercial sys-tems, if the commercial market proves suf-ficiently mature. Another option would beto field additional evolved UFO satellites toallow the commercial sector to mature andimprove government options. The Navyhas dubbed this alternative “UFO-E,” indi-cating that the Navy would consider con-tinuing the UFO constellation with gradualimprovements.

Accelerating CapabilityIn early fiscal year 2002, DOD initiated aTransformational Communications Studyto accelerate the delivery of advanced capa-bilities with state-of-the art technology tothe field. The study is led by the NationalSecurity Space Architect (NSSA) and isspringboarding off the NSSA’s Mission In-formation Management CommunicationsArchitecture. The study is examining in-creased intersystem connectivity via opti-cal crosslinks, greater reliance on groundfiber where possible, and the use of com-mercial assets as appropriate. Potentially,all U.S. government satellite communica-tions programs in planning or developmentcould be affected.

A large part of achieving advanced ca-pabilities involves applying the best tech-nology to emerging programs. To ensuremilsatcom’s technological edge in worldsatellite communications, the MILSAT-COM Joint Program Office has establisheda Milsatcom Innovation Center to acceler-ate the insertion of emerging technologies

into new systems. Aerospace,MITRE, MIT Lincoln Labora-tory, and NASA’s Jet PropulsionLaboratory are contributing on-site to the Center’s activities.Milsatcom will most definitelyhave a new look in the future.

Further Reading“Advanced Military Satellite Com-munications Capstone Require-ments Document,” HQ, U.S. SpaceCommand/J6S (April 1998).

Vice Admiral A. K. Cebrowski, U.S.Navy, and J. J. Garstka, “Network-Centric Warfare: Its Origin and Fu-ture,” Jan. 1998, http://www.usni.org/Proceedings/Articles98/PROce-browski.htm, accessed Nov. 12, 2001.

W. S. Cohen, Secretary of Defense, “Informa-tion Superiority and Space,” Ch. 8 in Annual Re-port to the President and the Congress 2000,http://www.dtic.mil/execsec/adr2000/chap8.html,accessed Nov. 12, 2001.

Captain J. Loiselle, USN, R. Tarleton, DeputyProgram Manager, and J. Ingerski, “The Next-Generation Mobile User Objective System(MUOS),” Communications Satellite ProgramOffice Space & Naval Warfare Systems Com-mand, San Diego, CA, May 2001, http://enterprise.spawar.navy.mil/spawarpublicsite/docs/next_gen_muos.pdf, accessed Nov. 12, 2001.

MILSATCOM Joint Program Office Web site,http://www.losangeles.af.mil/SMC/MC,accessed Nov. 12, 2001.

H. J. Mitchell, Maj. Gen., USAF, National Secu-rity Space Architect, K. A. Johnson, CDR, USN,

Mission Information Management Study Lead,S. S. Jenkins, Lt. Col., USAF, P. R. Axup, Lt.Col., USAF, MIM Study Architecture Engineers,2000, “Architecting Information Management: aKey Enabler for Information Superiority,” http://www.dodccrp.org/2000CCRTS/cd/html/pdf_pa-pers/ Track_7/046.pdf, accessed Nov. 12, 2001.

“MUOS Fact Sheet,” Navy CommunicationsSatellite Programs, Office of Congressional andPublic Affairs, Space & Naval Warfare SystemsCommand, September 1999, available in the factsheets section of http://enterprise.spawar.navy.mil/spawarpublicsite, accessed Nov. 12, 2001.

“Report of the Commission to Assess UnitedStates National Security Space Management andOrganization,” January 2001, http://www. de-fenselink.mil/pubs/space20010111.html,accessed Nov. 12, 2001.

The Space Architect’s vision of the future closely integrates government satellite communications into a system of systems. Additionally, ittreats communications as an enterprise and balances air, space, and ground communications capabilities.

The Ground Multiband Terminal is a tactical satellite communi-cations ground terminal that will support operations in the X, C,Ku, and military Ka bands.

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Recommended linksOpen for trades

In-theater fiberentry points

Whitehouse

Bookmarks Recent Publications and Patents by the Technical Staff

PatentsW. F. Buell and B. Jaduszliwer, “Continuous

Cold Atom Beam Atomic System,” U.S.Patent No. 6,303,928, Oct. 2001.A single-laser-beam magneto-optic trap isused to generate a slow, continuous, high-flux beam of atoms for use in a cold-atomclock. The highly stable device uses gradientmagnetic fields and a single circularly polar-ized laser beam directed toward a right-angle conical mirror with an opening at theapex. The continuous cold-atom beam and acentral portion of the incident laser beamexit through the opening along the axis ofthe cone. A collimating and deflecting laserprovides transverse cooling of the atomicbeam to bend, brighten, and separate it fromthe entrapping laser light, thereby reducingshifts in the operating frequency of theatomic clock. The continuous beam relaxesrequirements on the local oscillator, whichcan be stringent for clocks using pulsedcold-atom streams.

D. J. Chang and P. R. Valenzuela, “PressureVessel Testing Fixture,” U.S. Patent No.6,253,599, July 2001.This method for sealing a cylindrical com-posite tube improves pressure testing. Theends of the tube are inserted into fixtureshaving a vertically extending flange thatslides inside the tube. A ferrule is placedaround the outer wall of the tube, and a re-taining ring is placed around the ferrule.Both the ferrule and retaining ring have mat-ing slanted abutting surfaces. The retainingring has an inner diameter slightly smallerthan the outer diameter of the ferrule. As theretaining ring is clamped down upon the endfixtures, the slanted abutting surfaces slideagainst each other to apply a normal radialcompression between the tube and flangeand between the tube and ferrule. This com-pression generates a mechanical friction orshear force when the tube and end fitting arepulled apart longitudinally under high inter-nal pressure during testing. The radial com-pression secures the tube in the fixture andalso prevents leaks.

Z. H. Duron, “Wave Speed Bridge Damage De-tection Method,” U.S. Patent No. 6,257,064,July 2001.This nondestructive method for testing thestructural integrity of bridges and other largestructures is based on the accurate character-ization of an impact wave traveling throughthe bridge or structure. Accelerometers areplaced at each end of a support girder, and acalibrated impact hammer is used to strikethe girder at a specific location. For each im-pact location, a series of several data sets canbe generated to ensure statistical accuracywith high coherence in results. Wave-speedestimates are based on measured arrivaltimes and distances between impact andmeasurement locations. Large changes in a

wave-speed index indicate a high probabilityof a structural change in the bridge, indicat-ing possible damage. Standard computerand communications technologies are usedto acquire and process the measured re-sponses. The method can be used to deter-mine distinct damage to a bridge caused bysignificant events or to track structuralchanges over time.

J. A. Gelbwachs, “Cloud Base MeasurementMethod,” U.S. Patent No. 6,281,969, Aug.2001.This technique describes how an instrumentpositioned above a cloud can determine thealtitude of the base of the cloud. The high-altitude or space-based lidar method is notrestricted by cloud attenuation but relies oncloud porosity and diffuse reflections fromland or water. First, a pulsed laser beam issent through a clearing in the cloud towardthe ground, where it is diffusely reflected orscattered. Some of the reflected laser lightreturns directly to the lidar system, and someof it strikes the base of the cloud. Some ofthe light that hits the cloud is reflected backto Earth, where it is again reflected diffuselytoward the sky. Some of this light reachesthe lidar system, which calculates the alti-tude of the cloud base by computing thetime difference between the first and secondreturns.

L. K. Herman, C. M. Heatwole, G. M. Manke,B. T. Hamada, “Pseudo Gyro With Unmod-eled Disturbance Torque Estimation,” U.S.Patent No. 6,263,264, July 2001.Using software processes, this pseudo gyroemulates the functions of a hardware gyro.Used in place of a traditional gyro, the in-strument can increase the lifetime and relia-bility of a satellite system while reducingpower requirements. Computations arebased on the principle of conservation ofmomentum. By accounting for the momen-tum from external torques and the transferof momentum between a satellite bus and itsappendages and momentum-storage de-vices, the pseudo gyro calculates the angularvelocity of the vehicular bus. Informationconcerning relative position and rate is takenfrom the attitude and appendage controllers and tachometers onboard thesatellite.

R. P. Patera, “Space Vehicular Fly-By Guid-ance Method,” U.S. Patent No. 6,302,354,Oct. 2001.A fly-by guidance algorithm can be used tomaneuver a space vehicle into a desired fly-by trajectory near another space vehicle orobject. The guidance algorithm consists of apredictive phase and a modified proportionalnavigation phase. Using time data from anonboard clock and angular line-of-sight datafrom an onboard sensor, the predictive guid-ance phase maneuvers the space vehicle inpreparation for the modified proportional

navigation phase, during which the fly-byvehicle accelerates toward the object on anintercept course. The algorithm determinesthe precise time to terminate maneuvering toachieve the desired fly-by distance. The algorithm is well suited for monitoring andimaging of space vehicles and planetary objects.

E. J. Simburger, D. A. Hinkley, E. Y. Robinson,D. G. Gilmore, J. V. Osborn, “Power SphereNanosatellite,” U.S. Patent No. 6,284,966,Sept. 2001.A power source for a nanosatellite is formedfrom several flat polygonal solar panels con-nected by rotating hinges. The apparatus re-mains stowed until deployment, when itforms a sphere that encloses the payload.The solar-array panels are supported by anextending internal strut. The spherical shapeprovides attitude-insensitive solar energycollection as well as passive heat radiation.

E. J. Simburger, J. H. Matsumoto, T. W. Giants,A. Garcia III, F. R. Jeffrey, P. A. Gierow,“Integrated Solar Power Module,” U.S.Patent No. 6,300,158, Oct. 2001.A flexible 3-D printed circuit can be manu-factured from multiple layers of a clear insu-lating material, such as polyimide, with hor-izontal metalization layers between themand vertical metal traces through them.Semiconductor devices, such as thin-film so-lar cells, can be directly deposited on thissubstrate to form a flexible electronic mod-ule. Additionally, discrete components canbe bonded and electrically connected to thecircuit. In a typical configuration, a flexiblepower module could be made with thin-filmsolar cells deposited on one side and powerconverters bonded to the other side. Theflexible printed circuit board is well suitedfor forming electronic systems about acurved surface, such as a power sphere for ananosatellite.

K. Siri, “Maximum Power Tracking SolarPower System,” U.S. Patent No. 6,281,485,Aug. 2001.This maximum-power tracking system en-ables a power source, such as a solar array,to deliver maximum peak power. The sys-tem uses a set-point signal, modulated by adither signal, to control increasing, decreas-ing, and steady states of the solar-array volt-age. This setup achieves stabilized regularpower tracking during periods of low de-mand and maximum power tracking duringperiods of high demand. The system can beapplied to a constant power load or a highlypulsating load by using two bus stabilizerscoupled across the power-input and power-output ports. At a frequency above the cen-tral frequency of the bus stabilizers, the sys-tem provides proper damping to ensurestability in both input and output voltageswithout unexpected oscillation. Multipleconverters and maximum-power tracking

units can be connected in parallel usingshared bus control for fault-tolerant equal-ized power conversion.

C. Sve, P. R. Valenzuela, T. S. Wall, R. W. Fran-cis, R. B. Pan, S. J. VanWormer, “Compart-mental Fast Thermal Cycler,” U.S. PatentNo. 6,271,024, Aug. 2001.This instrument provides fast thermal-cycletesting of solar cells and similar compo-nents. The apparatus has a temperaturechamber divided into two compartments.The upper is warmed by heating lamps, andthe lower is cooled by liquid nitrogen. Bothuse pressurized nitrogen gas for thermalconduction and regulation. The test device ismounted to a panel, which is raised and low-ered along a vertical track joining the twocompartments. The entire chamber is insu-lated, and the two compartments are ther-mally isolated from one another, except forthe opening through which the test devicetravels. As a result, neither compartment ex-pends any time recovering to its original op-erating temperature during use. Computercontrol enables in-situ electrical testing, fail-safe heating, precision temperature control,operator notification, and thermal-gradientcontrol.

PublicationsR. N. Abernathy, B. L. Lundblad, and M. P.

Keough, “Summary of the Model Valida-tion Program for Rocket Launch Clouds,37th AIAA/ASME/SAE/ASEE Joint Propul-sion Conference and Exhibit (Salt LakeCity, UT, July 8–11, 2001), AIAA Paper2001–3728.

W. Ailor, “Space Traffic Control—A View ofthe Future,” IAF, 52nd International Astro-nautical Congress (Toulouse, France,Oct.1–5, 2001), IAF Paper 01-U501.

S. T. Amimoto, D. C. Johannsen, and M. A.Kwok, “Recent Results From the AlphaLaser Performance Test Program,” 32ndAIAA Plasmadynamics and Lasers Confer-ence (Anaheim, CA, June 11–14, 2001),AIAA Paper 2001–2867.

E. J. Beiting, “Design and Performance of a Fa-cility to Measure Electromagnetic Emis-sions from Electric Satellite Thrusters, 37thAIAA/ASME/SAE/ASEE Joint PropulsionConference and Exhibit (Salt Lake City,UT, July 8–11, 2001), AIAA Paper2001–3344.

R. Bitten, N. Lao, and J. Muhle, “Joint Govern-ment/Industry Space Programs—LessonsLearned and Recommendations,” AIAASpace 2001 Conference and Exposition (Al-buquerque, NM, Aug. 28–30, 2001), AIAAPaper 2001–4630.

J. B. Blake et al., “The Global Efficiency ofRelativistic Electron Production in theEarth’s Magnetosphere,” Journal of Geo-physical Research, Vol. 106, No. A9, pp.19,169–19,178 (Sept. 2001).

J. B. Blake, R. S. Selesnick, et al., “ProtonSpectra Detected by the Proton Switches onthe CRRES Satellite,” Journal of Spacecraftand Rockets, Vol. 38, No. 4, pp. 584–589(Aug. 2001).

J. B. Blake, R. S. Selesnick, et al., “Studies ofRelativistic Electron Injection Events in1997 and 1998,” Journal of GeophysicalResearch, Vol. 106, No. A9, pp.19,157–19,168 (Sept. 2001).

J. C. Camparo, “Stellar Scintillation and the At-mosphere’s Vertical Turbulence Profile,”Journal of the Optical Society of America A,Vol. 18, No. 3, pp. 631–637 (Mar. 2001).

W. S. Campbell, M. E. Sorge, A. B. Jenkin, etal., “Orbital Debris Hazard AssessmentMethodologies for Satellite Constellations,”Journal of Spacecraft and Rockets, Vol. 38,No. 1, pp. 120–125 (Feb. 2001).

D. W. Chen and K. M. Masters, “Continuous-Wave 4.3-mm Intracavity Difference Fre-quency Generation in an Optical Paramet-ric Oscillator,” Optics Letters, Vol. 26, No.1, pp. 25–27 (Jan. 2001).

J. B. Clark, “Equivalence of Fractional Facto-rial Designs,” Statistica Sinica, Vol. 11, No.2, pp. 537–547 (Apr. 2001).

J. H. Clemmons, “Acceleration Signatures inthe Dayside Boundary Layer and the Cusp,”Physics Chemical Earth C, Vol. 26, No.1–3,pp. 195–200 (2001).

J. H. Clemmons, “Inhomogeneous TransverseElectric Fields and Wave Generation in theAuroral Region: A Statistical Study,” Jour-nal of Geophysical Research, Vol. 106, No.A6, pp. 10,803–10,816 (June 2001).

J. H. Clemmons and R. Pfaff, “The CollisionMeter: An Experimental Technique to Mea-sure Charged-Neutral Interactions and GasComposition in the Upper Atmosphere,”Physics and Chemistry of the Earth, Part C,Vol. 26, No. 4, pp. 247–252 (2001).

K. Coste, “Qualification of the Arc 5-lbf Bipro-pellant Thruster for Deep Pressure Blow-down Operation,” 37th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Ex-hibit (Salt Lake City, UT, July 8–11, 2001),AIAA Paper 2001–3988.

M. W. Crofton, “Grid Erosion Analysis of theT5 Ion Thruster,” 37th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Ex-hibit (Salt Lake City, UT, July 8–11, 2001),AIAA Paper 2001–3781.

M. W. Crofton and I. D. Boyd, “Plume Mea-surement and Modeling Results for a Hol-low Cathode Micro-Thruster,” 37th AIAA/ASME/SAE/ASEE Joint Propulsion Confer-ence and Exhibit (Salt Lake City, UT, July8–11, 2001), AIAA Paper 2001–3795.

M. W. Crofton, T. A. Moore, and I. D. Boyd,“Near-Field Measurement and ModelingResults for Flight-Type Arcjet: HydrogenAtom,” Journal of Spacecraft and Rockets,Vol. 38, No. 3, pp. 417–425 (May–June2001).

K. D. Diamant, J. J. Brandenburg, and R. B.Cohen, “Performance Measurements of aWater Fed Microwave ElectrothermalThruster,” 37th AIAA/ASME/SAE/ASEEJoint Propulsion Conference and Exhibit(Salt Lake City, UT, July 8–11, 2001),AIAA Paper 2001–3900.

S. V. Didziulis and P. P. Frantz, “The Coordina-tion Chemistry of Transition Metal CarbideSurfaces: Detailed Spectroscopic and Theo-retical Investigations of CO Adsorption onTiC And VC (100) Surfaces,” Journal ofPhysical Chemistry B, Vol. 105, No. 22, pp.5196–5209 (2001).

L. Drake and P. Portanova, “Evolved Expend-able Launch Vehicle System—the NextStep in Affordable Space Transportation,”IAF, 52nd International Astronautical Con-gress (Toulouse, France, Oct.1–5, 2001),IAF Paper 01-V104.

W. A. Engblom, “A Front-Capture Scheme forthe Simulation of Homogeneous and Parti-cle-Laden Gravity Currents,” InternationalJournal of Numerical Methods in Fluids,Vol. 35, No. 8, pp. 961–982 (Apr. 2001).

J. F. Fennell, “Dawn-Dusk Asymmetry in Parti-cles of Solar Wind Origin within the Mag-netosphere,” Annales de Geophysique, Vol.19, pp. 1–9 (2001).

J. F. Fennell et al., “MeV Magnetosheath IonsEnergized at the Bow Shock,” Journal ofGeophysical Research, Vol. 106, No. A9,pp. 19,101–19,115 (Sept. 2001).

H. F. Fliegel, L. F. Warner, et al., “Photometryof Global Positioning System Block II andIIA Satellites on Orbit,” Journal of Space-craft and Rockets, Vol. 38, No. 4, pp.609–616 (Aug. 2001).

P. P. Frantz and S. V. Didziulis, “ChemicalModification of the Interfacial FrictionalProperties of Vanadium Carbide ThroughEthanol Adsorption Surface,” Science, Vol.481, pp. 185–197 (2001).

S. Goldstein, J. A. Lopez, et al., “Testing andAnalysis of Detonation Transfer Across aGap Within a Confined Volume,” 37thAIAA/ASME/SAE/ASEE Joint PropulsionConference and Exhibit (Salt Lake City,UT, July 8–11, 2001), AIAA Paper2001–3221.

D. F. Hall et al., “Midcourse Space Experiment(MSX) Satellite Measurements of Contami-nant Films Using QCMs—5 Years inSpace,” 35th AIAA Thermophysics Confer-ence (Anaheim, CA, June 11–14, 2001),AIAA Paper 2001–2956.

M. P. Hickey, G. Schubert, and R. L. Walter-scheid, “Acoustic Wave Heating of theThermosphere,” Journal of Geophysical Re-search, Vol. 106, No. A10, pp.21,543–21,548 (Oct. 2001).

J. A. Kechichian, “Computational Aspects ofTransfer Trajectories to Halo Orbits,” Jour-nal of Guidance, Control, and Dynamics,Vol. 24, No. 4, pp. 796–804 (Aug. 2001).

Bookmarks Continued

K. K. Khurana, “Energetic Ion Dynamics inJupiter’s Plasma Sheet,” Journal of Geo-physical Research, Vol. 106, No. A9, pp.18,895–18,905 (Sept. 2001).

E. E. King, R. C. Lacoe, and J. Wang-Ratkovic,“Influence of the Lightly Doped Drain Re-sistance on the Worst-Case Hot-CarrierStress Condition for NMOS Devices,” Mi-croelectronics Reliability, Vol. 41, No. 5,pp. 649–660 (May 2001).

H. C. Koons, “Statistical Analysis of ExtremeValues in Space Science,” Journal of Geo-physical Research, Vol. 106, No. A6, pp.10,915–10,921 (June 2001).

M. A. Kwok, “A Model Predictor for ChemicalLaser Combustors,” 32nd AIAA Plasmady-namics and Lasers Conference (Anaheim,CA, June 11–14, 2001), AIAA Paper2001–2868.

C. A. Landauer and K. L. Bellman, “New Ar-chitectures for Constructed Complex Sys-tems,” Applied Mathematics and Computa-tion, Vol. 120, No. 1–3, pp. 149–163 (May10, 2001).

C. A. Landauer and K. L. Bellman, “VirtualWorlds as Meeting Places for Formal Sys-tems,” Applied Mathematics and Computa-tion, Vol. 120, No. 1–3, pp. 165–173 (May10, 2001).

K. R. Lorentzen, M. D. Looper, and J. B.Blake, “Relativistic Electron MicroburstsDuring the GEM Storms,” Geophysical Re-search Letters, Vol. 28, No. 13, pp.2573–2576 (July 2001).

M. W. Maier, “Software Architecture: Introduc-ing IEEE Standard 1471,” Computer, Vol.34, No. 4, pp. 107–109 (Apr. 2001).

J. W. Murdock and R. P. Welle, “DownstreamGas Effect on Nozzle Flow-Separation Lo-cation,” Journal of Propulsion and Power,Vol. 17, No. 4, pp. 936–938 (Aug. 2001).

D. L. Oltrogge and R. G. Gist, “Experienceswith Situational Awareness for Communi-cations Satellite Operations,” 19th AIAA In-ternational Communications Satellite Sys-tems Conference (Toulouse, France, Apr.17–20, 2001), AIAA Paper 2001–0075.

J. P. Penn et al., “SPST Collaborative Prioriti-zation of Advanced RLV Technologies De-rived from a Bottom-up Process,” 37thAIAA/ASME/SAE/ASEE Joint PropulsionConference and Exhibit (Salt Lake City,UT, July 8–11, 2001), AIAA Paper2001–3983.

E. L. Petersen, “An Improved TurbulentBoundary-Layer Model for Shock Tubes,”31st AIAA Fluid Dynamics Conference andExhibit (Anaheim, CA, June 11–14, 2001),AIAA Paper 2001–2855.

E. L. Petersen, “Measurements of Reflected-Shock Bifurcation in a High-PressureShock Tube,” 23rd International Sympo-sium on Shock Waves (Arlington, TX, July22–27, 2001), Paper No. 2377.

E. L. Petersen, “Nonideal Effects Behind Re-flected Shock Waves in a High-PressureShock Tube,” Shock Waves, Vol. 10, pp.405–420 (2001).

E. L. Petersen, “On the Accuracy of Heteroge-neous Shock-Tube Measurements InvolvingAerosols,” 2nd Joint Meeting, U.S. Sections,Combustion Institute (Oakland, CA, Mar.25–28, 2001), Paper No. 23.

J. E. Pollard, “A Hall Effect Thruster PlumeModel Including Large-Angle Elastic Scat-tering,” 37th AIAA/ASME/SAE/ASEE JointPropulsion Conference and Exhibit (SaltLake City, UT, July 8–11, 2001), AIAA Pa-per 2001–3355.

J. E. Pollard, “Plume Mass Spectrometry with aHydrazine Arcjet Thruster,” Journal ofSpacecraft and Rockets, Vol. 38, No. 3, pp.411–416 (May–June 2001).

J. E. Pollard, “Validation of Hall ThrusterPlume Sputter Model,” 37th AIAA/ASME/SAE/ASEE Joint Propulsion Conferenceand Exhibit (Salt Lake City, UT, July 8–11,2001), AIAA Paper 2001–3986.

J. E. Pollard and K. D. Diamant, “Ion Flux, En-ergy, and Charge-State Measurement for theBPT-4000 Hall Thruster,” 37th AIAA/ASME/SAE/ASEE Joint Propulsion Confer-ence and Exhibit (Salt Lake City, UT, July8–11, 2001), AIAA Paper 2001–3351.

J. L. Roeder, “Dominant Role of the Asymmet-ric Ring Current in Producing the Storm-time Dst,” Journal of Geophysical Re-search, Vol. 106, No. A6, pp. 10,883–10,904 (June 1, 2001).

J. L. Roeder, “Ground-Based and Polar Space-craft Observation of a Giant (Pg) Pulsationand Its Association Source Mechanism,”Journal of Geophysical Research, Vol. 106,No. A6, pp. 10,837–10,852 (June 1, 2001).

J. L. Roeder, J. F. Fennell, et al., “Energy Con-tent in the Storm Time Ring CurrentTurner,” Journal of Geophysical Research,Vol. 106, No. A9, pp.19,149–19,156 (Sept.2001).

M. N. Ross, “Chance Encounter with theStratospheric Kerosene Rocket Plume fromRussia over California,” Geophysical Re-search Letters, Vol. 28, No. 6, pp. 959–962(Mar. 15, 2001).

R. J. Rudy, S. M. Mazuk, D. K. Lynch, R. C.Puetter, and D. S. P. Dearborn, “The Near-Infrared Spectrum of the Planetary NebulaIC 5117,” Astronomical Journal, Vol. 121,No. 1), pp. 362–370 (Jan. 2001).

B. A. Shadwick and W. F. Buell, “Unitary Inte-gration with Operator Splitting for WeaklyDissipative Systems,” Journal of Physics A:Mathematical and General, Vol. 34, No. 22,pp. 4771–4781 (June 2001).

K. Siri and K. A. Conner, “Fault-TolerantScaleable Solar Power Bus Architectureswith Maximum Power Tracking,” 16th An-nual IEEE Applied Power Electronics Con-

ference (Anaheim, CA, Mar. 3–8, 2001),Vol. 2, pp. 1009–1014.

C. Truong, K. Siri, E. J. Simburger, R. C. La-coe, J. Ross, S. Brown, and A. Prater, “Ra-diation Test on a Small DC-DC Converter,”Government Microcircuit ApplicationsConference (San Antonio, TX, Mar. 3–8,2001), pp. 262–265.

G. C. Valley, “Modeling Transient Gain Dy-namics in a Cladding-Pumped Yb-DopedFiber Amplifier Pulsed at Low RepetitionRates,” Technical Digest for the Conferenceon Lasers and Electro-Optics (CLEO),(Baltimore, MD, May 6–11, 2001), pp.300–301.

M. W. Vanik, “Monitoring Structural HealthUsing a Probabilistic Measure,” Computer-Aided Civil and Infrastructure Engineering,Vol. 16, No. 1, pp. 1–11 (Jan. 2001).

R. L. Walterscheid and M. P. Hickey, “SecularVariation of OI 5577A Airglow in theMesopause Region Induced by TransientGravity Wave Packets,” Geophysical Re-search Letters, Vol. 28, No. 4, pp. 701–704(Feb. 15, 2001).

R. L. Walterscheid and G. G. Sivjee, “ZonallySymmetric Oscillations Observed in theAirglow from South Pole Station,” Journalof Geophysical Research, Vol. 106, No. A3,pp. 3645–3654 (Mar. 1, 2001).

J. C. T. Wang, “Modem SRM Ignition Tran-sient Modeling. V-Prospective Develop-ments in CFD Simulation,” 37th AIAA/ASME/SAE/ASEE Joint Propulsion Confer-ence and Exhibit (Salt Lake City, UT, July8–11, 2001), AIAA Paper 2001–3447.

J. D. White, M. L. La Grassa, M. S. Marlow,and S. Herrin, “Evaluation of Flight Perfor-mance of Low Heritage Spacecraft Hard-ware on STP Vehicles,” AIAA Space 2001Conference and Exposition (Albuquerque,NM, Aug. 28–30, 2001), AIAA Paper2001–4706.

A. M. Young and S. S. Osofsky, “Active Feed-back Circuit for Minimization of VoltageTransients During Pulsed Measurements ofSemiconductor Devices,” IEEE Transac-tions on Instrumentation and Measurement,Vol. 50, No.1, pp. 72–76 (Feb. 2001).

Corrections (Crosslink, Summer 2001)

The astronaut in the photo on p. 12 is AlanShepard, not Scott Carpenter, as identifiedin the caption.

The photo on p. 22 shows the Delta Starsatellite atop the booster at the launchpad atCape Canaveral Air Force Station.

The U.S. and Soviet Union space hookupin July 1975 involved a Soyuz and anApollo, not a Gemini, as stated in the articleon p. 23.

45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77

Arthur C. Clarke publishes first majorcommunication satellite article

Nat

iona

l Spa

ce S

ocie

ty

Army achieves radar contact with moon

Navy communications experiments using moon as reflector

SCORE, first communicationsatellite, launched;

transmissions included President Eisenhower’s

Christmas message

Project West Ford, copper-wirecommunications reflector

First IDCSP launch

NATO uses IDCSP (NATO I satellite system)

U.S./U.K. milsatcom agreement,forerunner of U.K. Skynet program

LES-5, -6, and Tacsat: forerunners of milsatcom’s narrowband segment

IDCSP becomes first operational DODcommunication satellite system; forerunnerof milsatcom’s wideband segment

U.K. Skynet I and NATO II satellites;interoperable with DSCS

First of 16 DSCS II satellites; first operational satellite in milsatcom’s wideband segment

Skynet II

First DOD milsatcom architecture published

First NATO III

U.S

.Nat

iona

l Arc

hive

s

Additive white Gaussian noise (white noise):Statistically random radio noise character-ized by a wide frequency spectrum that iscontinuous and uniform over a specifiedfrequency band.

Adjacent channel interference: Disruptionsin a signal channel caused by power leak-ing from an adjacent channel, typically re-sulting from insufficient filtering, tuning, orfrequency control in the reference or inter-fering channel (or both).

Antenna gain: A measure of the change in anantenna’s signal power amplification basedon orientation or signal direction.

Attenuation: A decrease in signal magnitudebetween two points.

Bandwidth: The range of frequencies occu-pied by a signal; the information-carryingcapacity of a communications channel.

Bandwidth–bit-time product: The result ob-tained by multiplying the filter bandwidthby the bit time.

Binary phase-shift keying: A modulationtechnique whereby a data stream is im-printed on a carrier wave using two wavephases (generally 0 and 180 degrees) to sig-nal zeros and ones.

Bit: A binary digit, the basic element of a sig-nal, often rendered as zero or one, plus orminus, on or off.

Bit-error rate: A measure of performance fora digital receiver; defined as the probabilitythat a bit will be received incorrectly or asthe ratio of bits received in error to totalnumber of bits transmitted.

Bit time: The period of time used to transmit asingle bit.

Block coding: A forward error-correctionscheme characterized by a one-to-one cor-

respondence between a finite set of inputsymbols and output codewords.

Channel: The propagating medium (i.e., cableor electromagnetic path) connecting thetransmitter and receiver.

Channel coding: The application of process-ing algorithms, prior to transmission via achannel (and reverse algorithms at the re-ceiver), used to improve data reliability.

Codec (coder/decoder): The encoding devicethat converts a basic digital signal to acoded signal, and its reverse counterpart.

Codeword: A sequence of symbols assembledin accordance with specific rules of a cod-ing scheme that are assigned a uniquemeaning.

Coding gain: The increase in efficiency that acoded signal provides over an uncoded sig-nal. Expressed in decibels, the coding gainindicates a level of power reduction thatcan be achieved.

Concatenated code: A forward error-correc-tion method to achieve a relatively largecoding gain by combining two or more rel-atively simple codes. A concatenatedscheme such as a block code followed by aconvolution code is particularly effective incombating bursty errors.

Continuous phase modulation: A modulationtechnique achieving smooth phase transi-tions between signal states, thereby de-creasing bandwidth requirements.

Convolutional code: A forward error-correc-tion scheme, whereby the coded sequenceis algorithmically achieved through the useof current data bits plus some of the previ-ous data bits from the incoming stream.

Crosslink: A communication link betweentwo satellites.

Decibel: A unit for a logarithmic ratio, com-monly used as a measure of the ratio of twopowers.

Demodulation: The process of separating amodulated signal from its carrier signal andconverting it back to its original state.

Downlink: The portion of a communicationslink used to transmit signals from a satelliteto an Earth-based terminal (on land, ship,or aircraft).

Footprint: The area of Earth with sufficientantenna gain to receive a signal from asatellite.

Forward error correction: Error-correctionmethod allowing detection and correctionof bit or symbol errors without the need forrequesting retransmission of the originaldata.

Frequency hopping: The continual switchingof transmitted frequencies based on ashared algorithm to minimize unauthorizedinterception or jamming of a radio trans-mission.

Gaussian minimum shift keying: A phase-shift-keying modulation technique thatpasses the input stream through a Gaussianfilter before sending it to the phase modula-tor. The smooth Gaussian pulses requireless bandwidth to transmit and are less sus-ceptible to adjacent channel interference.

Geosynchronous orbit: An equatorial orbitroughly 35,800 kilometers above Earth inwhich a satellite can remain fixed relativeto Earth’s surface.

Hertz: A measure of radio frequency equal toone cycle per second; often rendered interms of kilohertz (one thousand cycles persecond), megahertz (one million cycles per

The Back Page Communication Satellite Glossary and Milsatcom Timeline

78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 00 01 02 03 04 05 06 07 08 09 10

FLTSATCOM, first operational satellite in milsatcom’s narrowband segment

First DSCS III;12 launchedthrough 2001

FLTSATCOM 7 with EHF package;preoperational satellite for

milsatcom’s protected segment

First Skynet IV;6 launched

through 2001 First

NATO IV First UFO;

10 launchedthrough 2001

Milstar, first operational satellite in milsatcom’s

protected segment

Contract for Wideband Gapfiller System

SICRAL, Italian military communication satellite,

launched

Estimated Advanced Wideband System launch;successor to DSCS, GBS,

and Wideband Gapfiller

©IL

S

UFO 8 with GBS payload; first of 3 GBSlaunches

second), and gigahertz (one billion cyclesper second).

Inclined orbit: A satellite orbit for which theorbital plane is not in Earth’s equatorialplane.

Interleaving: A process of scrambling the or-der of symbols to be transmitted over achannel in such a way that, when they aredescrambled (at the receiver), any burst ofchannel errors will be spread out in timeand thus appear as random errors to the de-coder.

Jammer-to-signal ratio: The ratio in decibelsof the power of a jamming signal to that ofa desired signal at a receiving antenna.

Jamming: The act of intentionally directingelectromagnetic energy at a communicationsystem to disrupt or prevent signal trans-mission.

Lobe: A discrete segment of an antenna radia-tion pattern, characterized by a localizedmaximum bounded by identifiable nulls.

Main lobe: The lobe of an antenna radiationpattern containing the highest gain.

Modulation: The process of changing or regu-lating the characteristics of a carrier waveso that the variations represent meaningfulinformation.

Multiplexing: The process of coordinatingmultiple message channels for simultane-ous transmission by multiple users.

Null: A point in an antenna’s radiation patternat which the gain is zero.

Offset reflector: A reflector antenna consist-ing of a feed placed at the focus of a parab-oloid and directed at the center of a primaryreflector cut from the paraboloid at an off-set from the paraboloid’s axis of symmetry.

This arrangement avoids any blockage ofthe reflector’s aperture by the feed.

Parity bit: An extra bit inserted into a data se-quence before transmission to enable errordetection.

Phase-shift keying: The broad class of modu-lation techniques that use discrete phases ofa carrier wave to denote signal bits—for ex-ample, phases of 0 and 180 to represent ze-ros and ones.

Phased array: A group of antenna elementswhose radiation patterns are coordinatedsuch that the radio waves are reinforced incertain directions and suppressed in others.

Power spectral density: The distribution ofsignal power in the frequency domain.

Quarternary phase-shift keying: A phase-shift keying modulation techniques thatuses four distinct phases of the carrier wave(usually offset by 90 degrees) to signify apair of bits.

Radio-frequency interference: An electro-magnetic disturbance that interrupts, ob-structs, or degrades the performance ofelectronic equipment.

Radio-frequency spectrum: The portion ofthe electromagnetic spectrum spanningroughly 3 kilohertz to 300 gigahertz. It in-cludes the very high (30–300 megahertz),ultrahigh (300 megahertz–3 gigahertz), su-perhigh (3–30 gigahertz), and extremelyhigh (30–300 gigahertz) frequency bands.

Rake receiver: A radio receiver having multi-ple receptors using offsets of a commonspreading code to receive and combine sev-eral multipath time-delayed signals.

Saturation: The point at which an amplifiercannot deliver more power despite a furtherincrease in input level.

Scintillation: A random fluctuation of the re-ceived field strength caused by irregularchanges in the transmission path over time.

Shannon limit: The theoretical minimum rela-tive power needed to provide reliable datatransmission at a given bit rate. Thus, for agiven noisy channel, this limit yields themaximum rate at which error-free commu-nication is possible.

Sidelobes: The distinct portions of an an-tenna’s radiation pattern in any area otherthan the main lobe.

Signal-to-noise ratio: A ratio of signal powerto noise power in a channel.

Spot beam: A narrow transmission beam froma satellite antenna focused at a limited areaof Earth.

Spread-spectrum modulation: A telecommu-nication technique in which a transmittedsignal is spread out across a bandwidthrange considerably greater than the fre-quency of the original information.

Transponder (transmitter/responder): A de-vice on a communication satellite that re-ceives signals from Earth, alters their fre-quency, amplifies them, and retransmitsthem to Earth on a different frequency.

Turbo code: A relatively new forward-errorcorrection technique made up of a concate-nated code structure plus an iterative feed-back algorithm. Turbo codes have beenshown to manifest error-performance veryclose to what is theoretically possible.

Uplink: The portion of a communications linkused to transmit signals from an Earth-based terminal (on land, ship, or aircraft) toa satellite.

Estimated Mobile User

Objective System launch;

successor to UFO

Estimated AdvancedEHF launch;

successor to Milstar

First Wideband

Gapfiller System launch

Editor in ChiefDonna J. Born

EditorMichael R. Hilton

Managing EditorGabriel Spera

Articles EditorJon Jackoway

Art DirectorThomas C. Hamilton

Graphic DesignerRichard M. Humphrey

IllustratorJohn A. Hoyem

PhotographerEric Hamburg

Editorial BoardWilliam C. Krenz, Chairman

David A. Bearden

Donna J. Born

Linda F. Brill

John E. Clark

David J. Evans

Isaac Ghozeil

Linda F. Halle

David R. Hickman

Michael R. Hilton

John P. Hurrell

Mark W. Maier

Mark E. Miller

John W. Murdock

Copyright 2002 The Aerospace Corporation. All rights reserved. Permission to copy orreprint is not required, but appropriate credit must be given to The Aerospace Corporation.

Crosslink (ISSN 1527-5264) is published by The Aerospace Corporation, an independent,nonprofit corporation dedicated to providing objective technical analyses and assessmentsfor military, civil, and commercial space programs. Founded in 1960, the corporation oper-ates a federally funded research and development center specializing in space systems archi-tecture, engineering, planning, analysis, and research, predominantly for programs managedby the Air Force Space and Missile Systems Center and the National Reconnaissance Office.

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For questions about Crosslink, send email to crosslink@aero.org or write to The Aero-space Press, P.O. Box 92957, Los Angeles, CA 90009-2957. Visit the new Crosslink Web siteat www.aero.org/publications/.

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