document resume corliss, william r. · the applications technology satellites (atss) were. built in...

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DOCUMENT RESUME

SE 013 388

Corliss, William R.

Satellites at Work, Space in the Seventies.

National Aeronautics and Space Adninistration,

Washington, D.C.

EP-84

Jun 71

31p.

Superintendent of Documents, U. S. Government

Printing Office, Washington, D. C. 20204 {Stock No.

3300-0405 $0.60)

MF -$0.65 HC -$3.29

*Aerospace Education; *Aerospace Technology;

*Communication Satellites; *Earth Science;

Instructional Materials' Reading Materials; Resource

Materials; Secondary School Science; *Space

Sciences

ABSTRACTThis publication in the "Space in the Seventies"

series describes current status and future plans for "working"

spacecraft, also called "application satellites." These spacecraft

serve the needs of communications, meteorology, geodesy, and

navigation. They also enable us to study earth resources from space.

Many scientific and technical concepts are presented without an

abundance of technical elaboration. Color illustrations and

photographs are utilized throughout. (Author/P1)

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SPACE IN THE SEVENTIES

Man has walked on the Moon, made scientificobservations there, and brought back to Earthsamples of the lunar surface.

Unmanned scientific spacecraft have probed forfacts about matter, radiation and magnetism inspace, and have collected data relating to theMoon, Venus, Mars, the Sun and some of the stars,and reported their findings to gruund stationson Earth.

Spacecraft have been put into orbit around theEarth as weather observation stations, ascommunications relay stations for a world-widetelephone and television network, and as aids tonavigation.

In addition, the space program has acceleratedthe advance of technology for science and industry,contributing many new ideas, processes andmaterials.

All this took place in the decade of the Sixties.

What next? What may be expected of spaceexploration in the Seventies?

NASA has prepared a series of publications andmotion pictures to provide a look forward toSPACE IN THE SEVENTIES. The topics covered in

this series include: Earth orbital science; planetaryexploration; practical applications of satellites;technology utilization; man in space; andaeronautics. SPACE IN THE SEVENTIES presentsthe planned programs of NASA for the comingdecade.

June, 1971

COVER:

This photograph of the Earth was taken by ATS 3 from itson-station position at 47 degrees west longitude on theequator over Brazil. Four continents can be seen; SouthAmerica is most prominent. Major weather over the centralUnited States consists of a cold front moving eastward.At bottom center, a tropical storm can be seen with a coldfront extending into Argentina.

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TABLE OF CONTENTS

4 INTRODUCTION6 A TESTING LABORATORY FOR

NEVV CONCEPTS (ATS)8 COMMUNICATION SATELUTES

11 THE BALLOON WATCHER (CAS)12 THE INTELSAT FAMILY13 THE BRICK MOON REVISITED15 LOCATiNG THE CONTINENTS17 WEATHER WATCHING VS. WEATHER PREDICTING19 LATEST IN A LONG LINE (ITOS)20 SEEING THE BIG PICTURE (SMS)22 NIMBUS, A TEST VEHICLE FOR

METEOROLOGICAL INSTRUMENTS24 HUSBANDING THE EARTH'S RESOURCES (ERTS)28 A GLANCE AT THE FUTURE

INTRODUCTION

What possible practical uses could there be for afew pounds of contrived metal located over 100 milesabove the Earth? Historians of technology can vouchthat the same kinds of questions were leveled at thetelegraph, the steam engine, and almost every otherimportant invention. "Get a horse!" was a commonjeer still within the recollection of millions.

Today, everyone accepts the ihtercontinentalsatellite relay of television programs and telephoneconversations, but these communication satellitesinitially had to overcome the same skepticism as theautomobile. We also expect to be warned of theapproach of dangerous hurricanes and to be toldtomorrow's weather with high reliability. Who hasn'tseen a satellite-taken picture of cloud cover? Weathersatellites and communication satellites represent justthe beginning. In the artificial satelhte, we have 3powerful tooi which is already being turned to thesolution of some of the world's pressing practicalproblems.

Offhand, it is difficult to see how a small,automated machine more than 100 miles up couldpossibly contribute to this planet's down-to-Earthproblems. A century ago, an Earth satellite wouldindeed have been of littie use, but today when we arenearing the limits of our planet's raw materials and itscapacity to recycle our wastes, Earth satellites proveto be invaluable pollution monitors, resource-finders,weather-watchers, and communication relay points.As the convention& tools and techniques falter intheir struggle with terrestrial problems, spacetechnology is coming forth with new ideas, new waysto solve problems. This is NASA's great challengeduring the next decade: to help understand the Earth'secology and develop space systems which will, on theone hand, solve old problems, and, on the other, notcreate additional new problems for mankind.

The first "applications satellite" was conceivedabout a century ago by Edward Everett Hale, bestknown for his story "The Man Without a Country." In1869, Hale's precocious tale, "The Brick Moon," waspublished in the Atlantic Monthly. Hale envisioneda large artificial satellite circling the Earth in an orbitover the poles and passing along the Greenwichmeridian. Ships at sea, he reasoned, could takebearings on this man-made moon and thus fix theirpositions more accurately. The Brick Moon, thoughonly fictional, was the first navigation satellite.

4

The Brick Moon had the advantage e height overthe usual terrestrial landmarks used by navigators.Like present-day satellites, it could be seen from afar.Almost all of the practical benefits of artificialsatellites stem from this single factorhP.ght and,consequently, a sweeping view of the Earth. Not onlycan terrestrial eyes, including radio antennas, locatedthousands of miles apart see the satellite, but, givenvision of its own, a satellite with a TV camera canscrutinize huge portions of the Earth's surface.Indeed, the applications satellite is an invaluableextension of our sense in two ways: (1) it sees muchmore territory; and (2) its sensors see well beyonathe narrow visible spectrum. Looking at the Earthbelow through infrared sensors is like looking throughmagic glasses; everything glows acccrding to itstemperature and chemical makeup. For this reason,infrared sensors and those sensitive to other wave-lengths can diagnose the Earth's surface environmentfrom afar.

NASA's AppronhBy the time NASA was created in 1958, various

government agencies had already made numerousstudies of the potential advantages of artificialsatellites in communications and weather forecasting.Some of the earliest sateliites launched by the UnitedStates were of these types; for example, Score (1958)and Tiros 1 (1960). In 1958, one of NASA's first jobswas obvious: develop communication and weathersatellites for practical use on a day-to-day basis.During the early 1960's, almost two dozen satelliteswere launched to test spacecraft techniques andsensors. As space hardware and the associated groundequipment were proven out, other governmentagencies and private industry began to play moredominant roles in the operational aspects of satellitecommunication and meteorology. The ( i.ornunications Satellite Corporation (COMSAT) Nt. a_ created byCongress in 1962 to establish a global communica-tion network employing communication satellites.Soon after, the Environmental Science ServicesAdministration (ESSA), now part of the NationaiOceanic and Atmospheric Administration (NOAA),assumed responsibility for operating weathersatellites. NASA's role is now that of providing launchand tracking services for COMSAT and NOAA on areimbursable basis, and, more pertinent to this

Fig. 1 Earth satellites carry a wide variety of infrared,- microwave, and visible light sensors for meteorological and

Earth resource studies. The Nimbus sensor ring is shownhere.

booklet, pursuing new ideas in the technology ofcommunication and weather satellites, for we can besure that we are just beginning to reap the practicalbenefits from the space program.

The technological foundation of applicationssatellites has three main parts:

1. The spacecraft itself, in particular the datahandling and attitude control subsystems.Satellites tend to maintain the sameorientation with respect to the fixed stars asthey circle the Earth; therefore, satellites mustbe designed deliberately to point continuouslytoward the Earth and must be deliberatelyturned on their axes once each orbit.

2. Sensors. The usefulness of satellites inforecasting the weather and in assessing theplanet's resources depends upon being ableto distinguish the many electromagneticsubtleties of the radiation reflected andemitted from the Earth's surface andatmosphere. (Fig. 1)

3. Ground operations. One of the most complextasks is the deployment of ground stations toreceive satellite transmissions and relay themto central processing facilities, where they areconverted into formats suitable to the ultimateconsumer. Ordinary telephone lines are notadequate; new worldwide networks of highcapacity circuits must be built.

NASA's efforts are directed primarily toward thedevelopment of new spacecraft and sensors, whilethe major operating agencies (NOAA and theCOMSAT Corporation) are concerned more withground operations and the practical applications ofthe data acquired by the satellites. Of course, NASAworks closely with these agencies to insure that thesatellites and sensors it develops meet the objectivesof the groups using the data.

For convenience, NASA divides its applicationssatellite programs into two groups: (1) the Commu-nications Programs, and (2) the Earth ObservationPrograms. Naturally, there is considerable transferof technology from one program area to the other.The Communications Programs include NASA's workon advanced communications satellites, geodeticsatellites, and the Applications Technology Satellite(ATS). The ATS also provides valuable technology forthe Earth Observation Programs, which include thedevelopment of advanced meteorological satellitesand the Earth Resources Technology Satellite (ERTS)Program.

5

Each of the program areas will bp covered in moredetail in ths.- subsequent pages. Table 1 iists thespecific satellites under development in er.ich areaand the launch schedules.

A TESTING LABORATORY FOR NEW CONCEPTS(ATS)

Aircraft manufacturers have long used flying "testbeds" to test out new ideas for improving the species.The Applications Technology Satellites (ATSs) werebuilt in accordance with this philosophy. A majordifference, of course, is that an ATS must be operatedby remote control from the Earth and no opportunityexists for actual physical examination of the equip-ment under test once it is in orbit.

The ATS is a project of NASA's Goddard SpaceFlight Center. The first five spacecraft in the ATSseries have been launched. The intention was to placeall five in synchronous or geostationary orbits, inwhich the satellites' periods cf rotation about theEarth are the same as the Earth's period of rotationabout its spin axis. Geostationary satellites orbit overthe Equator at altitudes of about 22,300 miles and,to an observer on the Earth below, appear to remainin the same spot in the sky. The first five ATSsattempted to solve such technological problems asspin stabilization at geostationary altitudes and theaccurate pointing of antenna beams and sensorpatterns at the Earth. Several communicationexperiments and tests of cameras and other ser.sorswere successfully concluded during these flights.

1 he designations and launch dates of these space-craft are given below:

Prelaunch PostlaunchDesignation Designation Launch Date Remarks

ATS A ATS 1 Dec. 7, 1966 First photo showingnearly entire Earthdisc; over 2500photos returned.

ATS B ATS 2 Apr. 6, 1967 Did not attaingeostationary orbitdue to launch-vehicle failure.

ATS C AI'S 3 Nov. 5, 1967 Performed nineexperiments.

ATS D ATS 11. Aug. 10, 1968 Did not attaingeostationary orbitdue to launch-vehicle failure.

ATS E ATS 5 Aug. 12, 1969 Spacecraft nutationdamper failed.Partial success.

ATS F and G represent a new generation ofspacecraft_ The basic spacecraft design, which isdominated by a 30-foot umbrella-like antenna, will beunique among the hundreds of manmade craft nowin orbit around the Earth. (Fig. 2) The unusualgeometry is dictated by the prime objectives of theATS F/G mission:

Demonstrate the feasibility of deploying acollapsible paraboloidal antenna 30 feet indiameter and which has good radio-frequency

TABLE 1. NASA Launch Schedule For Applications Satellites*

Satellite

ATS

CAS

Intelsat

Geos

70 71

A

IV-1

IV-2

72 73

F

c

74 75

G

Communications Programs

ITOS

Nimbus

SMS

ERTS

A(NOAA-D

B D,C

E

A

A

F

B

B

G

F

._

Earth ObservationPrograms

*The letters refer to specific satellite vehicles. When satellites are launched successfully, numbers are assigned.

6

Fig. 2 Conceptual drawing of the ATS spacecraft.

performance up to 6000 MHz.* (The majortechnical difficulty here lies in the fact that theantenna must be folded up to fit within thelaunch vehicle fairing.)Demonstrate precision pointing of the space-craft and its fixed antenna to within 0.1° andthe capability of slewing (turning) the antenna17.5° in 30 minutes. The spacecraft has to dothis if it is to provide service to different areasof the Earth below.

The other characteristics of the ATS F/G spacecraftare described in Table 2.

Every operational satellite is only a part of a largermachine which includes ground equipment connectedby radio links to the satellite and the people who usethe information transmitted by the satellite. TheATS F and G spacecraft will be supported by NASA'sSpace Tracking and Data Acquisition Network(STADAN) and four special ATS ground stations whichwill take part in several of the communicationexperiments. Two of the ATS stations will be at fixedlocationsRosman, N. C., and Barstow, Calif., bothof which are permanent NASA tracking station sites.The two other ground stations will be the MobileTerminal and the Transportable Ground Station.These will be relocated as the experiments demand.

The first satellite (ATS F) will first be placed in ageostationary orbit over the equator where it can seeand be seen from almost one third of the Earth'sarea. The spacecraft can, however, be moved from*A MegaHertz (MHz) is one million cycles per second. It is aunit of frequency.

one spot to another by its rocket motor as theexperiments require. These positions must always beover the equator. For example, Fig. 3 shows an ATSlocated just west of the South American coast, 22,300miles over the Equator. The antenna beam patterns inthe C Band and Ultrahigh Frequency (UHF) Band"illuminate" the southeastern U.S. with radio energyas shown by the contoureThe aiming point isNASA's Rosman, N. C., tracking station.

One objective of the ATS F and G experiments isthe improvement of communications betweenterrestrial terminals via satellite relay. In 1944, thenoted science fiction writer, Arthur C. Clarke,proposed satellite radio relays; so the idea itself isabout 30 years old. The new ATS satellites will carryClarke's idea several steps further with experimentsin educational TV broadcasting, satellite-to-satelliterelay, and air-traffic control. The plannedexperiments are described on pages 9 and 10. Notall ATS experiments deal with communications. Toillustrate, all ATS satellites carry some scientificinstrumentation because no scientific satellitespresently operate in geostationary orbits. The ATSs,therefore, provide a unique opportunity to positionscientific sensors at specific spots over the Earth'sequator at fixed altitudes.

CC-Band and UHF refer to portions of the electromagneticfrequency spectrum; specifically, 3900-6200 MHz and300-3000 MHz.

Fig. 3 ATS F, located over the equator just west of theSouth American coast, can concentrate its radio beams inthe regions indicated.

TABLE 2. Design

Spacecraft Functions

Communication%

Power supply

Attitude control

Propulsion

Thermal control

Guidance and control

Structure

Launch vehicle

Tracking and dataacquisition network

Features vital Statistics, ATS F and G*.....----...............------...

Design Feel fresvAavr.,rsatile trarlsoZderi. capable of receiving and replying to terrestrial transmissions at

to frePulse-codemodulated (PCM) telemetry sends data on spacecraft status

ntift r'raboloidal dish.

The ATh 5 de?1°Yable,Use Nickel.cadmflaiutmsobalartt-ceerlilesprels. The average power level will be about 500

initialbi.

-nenterro(or

eels and the propulsion unit described below.rt'a 01°'

On 011310Ys a controllable hydrazine rocket engine. An ammonia resistojet (an., e Concept e edheat rocket) has been proposed in the other concept.

,.,Passive poatifigs,akr1,51 insulations. Heat pipes used in places to help distribute heat.

' "erihni louvers wfinds) used in critical sections.

Sun 4ensr,ti M. ineriial reference for attitude determination. 512 pulse commands;32 die- itd aci-onrronos. Experimental radio interferometer will be tested as an attitude sensor.

30-foct antenna and solar panels. Earth-viewing and Aft-viewing EquipmentModuir_d_oular st-.TVs support all components in configuration shown in Fig. 2.

Weilahhutft 2°5° Pounds.

Titan

S.,Pace TmclOrIg ane Data

Acquisition Network (STADAN) and four special ATS ground

4tions.

*As the design of the spacecraft C'ontinues, some 01 the details Presented will change.

fA radio transponder autornatically to transmissions from stations knowing the proper code or frequency.

COMMUNICATION SATELI5 /ES

If one drew all of the Earth's electroniccommunication links on a globeairline routes are drawn), the fol(in thesre Il'

feciturevels

would emerge:1. The technically advanced are

covered with telepho. ne lines undergrovirtually

Lindcables, microwapveedini ak:

nations

varicus otherAhany,c. -1 andelectronic nerve ..-----..

2. The less-develonloation

corn.13,rativelyons arebare of these cornmu iirie' jvith thedensest coverage noted around 0 lew cities.

3. The planet's wide vast.expanses ofc'crearl and

Africa, Asia, Austrade, and scot' 141",ricaboast few if any communication term_inals.

4. The continents ara ancd°,1undersea cables

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The first commercial communication satellites, thegeostationary intelsats, have been jockeyed intopositions between the continents where they relayvoice conversations, TV programs, and data from onebusy communication terminal to another, much asundersea cables do. Point-to-point relay of signals,however, does not bring all the benefits of goodcommunications to everyone. For example, the ATSF/G educational television relay experiment willdemonstrate the feasibility of broadcasting educa-ti3nal programs to wide areas inthe less-developedcountries where ground-based transmitters are rare.

PRACTICAL EXPERIMENTS ON ATS F/G

The Laser ExperimentThe light emitted by lasers can carry much more

information than radio waves. Because the world'sdemand for more communication may outstrip thecapacity of radio systems, NASA is laying thegroundwork for laser communication links betweensatellites and Earth stations and from satellite tosatellite. (Laser experiment on ATS G only.)

VISIBLE LASERWAVES

MILLIMETERWAVES

X-RAYS ULTRAVIOLET INFRARED

0.0005 mm 1 10RIM RIM

MICROWAVES

The Millimeter Wave ExperimentIn addition to exploring optical wavelengths with

the laser described above, ATS F/G will help expandthe useful radio spectrum into the millimeter regionof the spectrum (10 mm is equivalent to 30,000MHz). The major objective is the precise definitionof communication channels at these high frequencies.

The Radio Dispersion ExperimentWhen radio signals pass through the Earth's

atmosphere and ionosphere, frequencies are changedslightly and so are the time relationships betweensignals. Called frequency and time dispersion,respectively, these phenomena cause trouble in thetransmission of digital dataas from one comPuterto another. This experiment is aimed at understand-ing dispersion better and finding ways to circumventit.

TIME

TIME

TIME DISPERSION OF SIGNALS

The Radio Frequency Interference ExperimentAt the frequencies used for the microwave relay

of conversations and business data (6000 MHz),there is significant interference from the radio noisecreated by lightning and other atmosphericphenomena. ATS F/G will carry a special receiver torecord this noise and help us understand andovercome it.

Radio Beacon ExperimentBy directing radio signals at several frequencies

toward the Earth, this ATS experiment will enablescientists to measure the effects of ionized particleson propagation paths beyond the atmosphere.

The Very High Resolution RadiometerExperiment (VHRRE)

In a later section on meteorology there is adiscussion of-the importance of cloud-cover pictures,taken day and night, for better weather forecasting.The VHRRE consists of a telescope with detectorssensitive to both visible and infrared radiation.Because clouds emit less infrared radiation than theground, the infrared detector can help make picturesof them even at night. The flight qualification of thisinstrument is important to the development of theSynchronous Meteorological Satellite (SMS). (Seelater.)

7TS

The Ion Engine ExperimentSmall thrusts can be generated without the

expenditure of much propellant by electricallyaccelerating cesium ions to high velocities. If theexperimental ion engine on ATS is successful, futuresynchronous satellites may use ion engines to helpthem maintain their stations over specific spots onthe Equator.

10

IONSURFACE

GRID

40E10

CESIUMIONS

9

Indian Educational Television TestATS F and G will broadcast educational TV

programs to villages and rural areas in a jointexperiment with India. See text and Fig. 4.

Integrated Scientific ExperimentsMost scientific satellites orbit well below the

22,300 mile geostationary orbits. Consequently,there is comparatively little data on the cha rged-particle environment at ATS altitudes. Becausecommunication and future meteorological satelliteswill operate in this region, it is imperative tounderstand this environment better. That is thepurpose of this experiment.

Other ATS ExperimentsIn another ATS experiment, the satellite will be

used as a communication relay during Apollo Moonflights. An air traffic control experiment is alsoplanned. The final experiment will involve an attemptto control the orientation of ATS F/G usingmeasurements made from the Earth.

(Fig. 4) Point-to-point relay and broadcasting fromsatellites actually represent only early steps in theevolution of instant worldwide communication foreveryone. Ideally, it would be possible to contact anydesired combination of people and/or machines fromany spot on Earth with a minimum of equipment andat low cost. Such service is available today only withinthe boundaries of the technologically advancedcountries. Communication satellites can help expandthis kind of service to other countries and to thosedesolate regions where wires and large communica-tion terminals do not exist.

Information networking is a concept wheresatellites can also help. It might be desirable, by wayof illustration, to link together all medical facilities onthe globe. (Fig. 5) All recorded knowledge could, inprinciple, be pooled and made available at theterminals of such a network. With several geostation-ary satellites placed where they can be seen from allparts of the planet, such a concept becomes possible.

Another concept is the "multi-access" communica-tion satellite wherein many small terminalsyourhome, for examplecan call upon the services of anycommunication satellite within view simultaneously.Individuals or unmanned instrument stations inremote areas with a modest radio transceiver couldbecome part of a global network at the flick of aswitch. Sensor fields, such as widely dispersed

Fig. 4 The United States-India educational television experiment will employ ATS F for broadcasting. Highfrequency signals can be focused onto specific cities.

ATS-FANTENNA COVERAGEFROM 200 E. LONG.

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NATIONAL LIBRARYOF MEDICINE

Fig. 5 Operational schematic of a possible medical information network.

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MEDICAL SCHOOLAUDITORIUM

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weather monitors, could be integrated bygeostationary satellites which have the sensors inview at all times. It is this kind of application thathas led to the joint Franco-American CooperativeApplication Satellite (CAS) program.

THE BALLOON WATCHER (CAS)

In 1971, the Government of France, through itsCentre National d'Etudes Spatiales (CNES),willrelease 500 instrumented balloons between thesouthern latitudes 25° and 55°. Floating between39,000 and 45,000 feet, the balloons will measuretemperature and pressures along the layer of constantdensity air in which they float. When interrogated bythe Cooperative Applications Satellite (CAS), theballoons will radio back their scientific measure-ments. In addition, Doppler tracking equipment onthe spacecraft will pinpoint the range and radialvelocity of each interrogated balloon to determinelarge-scale movements of the atmosphere. Orbiting atabout 560 miles at an inclination of 50°, the CAS willbe able to interrogate each balloon twice each day.(Fig. 6) Collectively, the satellite and balloon sensorfield constitute France's Eole Project. Eole isdoubtless the forerunner of many other large-area,sensor-field experiments now made possible bycommunication satellites.

The satellite itself and the balloons are being builtby France. NASA will provide a Scout launch vehicleand its launch facilities at Wallops Island, Virginia.NASA may also support the mission with tracking anddata acquisition services, although the CAS willincorporate a tape recorder, enabling it to recorddata from the interrogated balloons for retransmis-sion to French data acquisition stations when thesatellite swings over them in its orbit. Thearrangement between the United States and France issimilar to that made when the first French satellite,the FR-1, was launched from Wallops Island in 1965.

The CAS will be an octagonal cylinder 28 inches indiameter, weighing about 206 pounds. As illustratedin Fig. 6, a skirt of solar cell panels opens petal-likearound the upper rim of the spacecraft. The long rodextending upwards along the spacecraft axis is agravity-gradient stabilization boom that helps keepthe top of the satellite pointed at the Earth. A spiralantenna painted on the conical projection on the

11

Fig. 6 Artist's concept of CAS operation, showing balloon-borne sensor fieldand relay of stored data to ground stations.

spacecraft top sends directional signals thatinterrogate the balloons about 550 miles below. Thesame antenna receives their replies.

The first CAS launch is scheduled for late 1971.1naddition to launch services, the United States willhelp interpret the data telemetered from the balloons.The National Center for Atmospheric Research andthe University of California at Los Angeles willparticipate in this analysis.

THE INTELSAT FAMILY

The Intelsat communication satellites are nowstationed in geostationary orbits above the Atlantic,pacific, and Indian Oceans, providing thousands ofnew intercontinental radio links. (Fig. 7) Together,the Intelsats torm the Global Satellite System of theInternational Telecommunications SatelliteConsortium, which consists of more than 70 member

12

countries. Represented by the CommunicationsSatellite Corporation, the United States builds someof the Intelsats and operates some of the groundstations. NASA's responsibilities include launchservices and consulting on a reimbursable basis.

Based on technical developments made duringNASA's Syncom Program in the early 1960's, theIntelsats are all geostationary spacecraft carryingsmall rocket engines that move them to and keepthem at selected spots over the Equator. There arenow four generations of Intelsats, and they allbear family traits. Intelsat I, also called Early Bird,was launched in 1965 and was the founder of thefamily. Additional "birds" flew in 1966 and 1967with four spacecraft in the Intelsat-II generation. Asindicated in Table 3, the trend has been toward biggerand better satellites with each new generation.

All of the Intelsats are cylindrical in shape, withthe curved sides covered with solar cells. Weights,dimensions, and circuit capacities have risen

markedly ire less than a decade_ (Table 3) TheIntelsat-IV generation consists of satellites 17_6 feethigh (Fig.. 8) and 93_5 inches in diameter_ TheI ntelsat-IV's are so large that NASA had to switch tothe more powerful Atlas-Centaur launch vehicle_ Inkeeping with the trend toward more versatile world-wide communication, the new Intelsats providemultiple access capabilities.. Compared to the 1965Early Bird, which was not much bigger than a wastebasket, the lntelsat-lVs represent an importantadvance in terrestrial communications_ This advancewas pioneered by NASA's Syncoms and thecorn rnunication experiments aboard the ATSspacecraft_

THE BR.ICK MOON REVISITEDRather than the brick of Hale's fictional satellite,

the first navigational satellite was fabricated mainlyfrom lightweight metals_ Transit 1A was launched bythe U_S_ Navy in the fall of 1959 to help guide itssubmarines_ More Transits followed but there havebeen no satellites specifically designed to helpcommercial ships and aircraft locate themselves inareas where conventional navigational aids do notexist, especially over the oceans_ With the density ofhigh-speed, transoceanic air traffic rising rapidly,better schemes for locating and communicating withaircraft are urgently needed..

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Fig_<Comm

Fig_ 7 The global system of communication satellites a nd ground stat

TABLE 3. The Intelsat Family

Generation Successful Launches Circuits* Weight at Launch Launch Vehicle

Intelsat I Early Bird (1955) 240 150 lbs Delta

Intelsat Il F-2 (1969), F-3 (1967), F-4 (1967) 240 357 Delta

Intelsat H I F-2 (1968), F-3 (1969), F-4 (1969) 1,200 647 Delta

Intelsat IV F-6 (1970), F-7 (1970) 12,000 3080 Atlas-Centaur

COMSAT has contracted for eight INTELSAT IVsatellites (F-1 through F-8). F-2 was launchedJan. 25, 1971.

*Various operating modes are possible in which circuits can be combined to provide for TV channelsand other types of wide-band channels.

The classical concept of a navigation satellitedepends upon accurately knowing the satellite'sposition and then finding one's position relative to it.In other words, the satellite becomes a known land-mark; the only one visible on the broad oceans. Thestars, of course, play the same role in stellarnavigation, but they are not always visible and stellarfixes are too slow and difficult for aircraft flying nearthe speed of sound. Navigation satellites have theadvantage that the signals can be received automatic-ally and analyzed by computers, giving pilots theirpositions rapidly and continuously.

Although NASA is not building a navigation satelliteat the moment, it is conducting pertinent experimentswith its ATS and Nimbus spacecraft. To illustrate, theATS 3 was used in the Omega Position LocationExperiment, which allowed cooperating ships andaircraft to fix their positions within 3 and 5 miles,respectively. In another experiment, a jet aircraftlocated its position to within 4 miles by making radioranging measurements on ATS 1 and 3 simultane-ously. (Fig. 9) The positions of the ATSs are, ofcourse, well known and, being in geostationary orbits,they are visible from much of the world's surface.Experiments of these kinds are continuing.

14

Fig. 9 Schematic showing ATS 1 and ATS 3 in a dualranging experiment with a jet aircraft. The jet was located towithin four miles.

The Director General of the European SpaceResearch Organization (ESRO) has requested thatNASA and ESRO jointly explore the possibility ofcooperative air traffic control experiments onsatellites. Technical meetings have already begun.Air traffic control differs from navigation in that itinvolves the centralized control of aircraft positionsrelative to one another rather than the determinationof their absolute geographical positions. Air trafficcontrol is, therefore, very much a communicationproblem, especially over the oceans where conven-tional communication techniques falter. Thepotential value of communication satellites in airtraffic control was first demonstrated in November1964, when a Pan American World Airways planeused Syncom 3, then in a geostationary orbit overthe Pacific, for air-ground communication across theocean. The realization of practical, satellite-aided,air traffic control has been hampered by technicalproblems as well as the $200 million price tag on thesatellite system. NASA and ESRO are conductingfurtiler experiments with the ATSs and other satellitesto de:ermine the best communication technique for apermanent international air traffic control system.

LOCATING THE CONTINENTS

Despite the recent conclusion that the Earth'scontinents do indeed drift a few centimeters eachyear, one would expect their positions to be wellknown after all these centuries of mapmaking.However, when geodetic satellites arrived on thescene, the continents were discovered to be !Icatedaccurately in terms of latitude and longitude but notso well on a relative basis; that is, the distancesbetween the continents were not known to withinseveral miles. Some oceanic specks of land werefound to be dozens of miles from where maps saidthey should have been. To fix accurately the relativepositions of land masses, a landmark visible simul-taneously from ocean-separated islands and conti-nents is needed. In this sense, geodetic satellites arereally navigation satellites for the "floating"continents. In addition, geodetic satellites also helpdetermine the true shape of the Eartha planet withmany idiosyncrasies in shape.

Almost all satellites can be used for geodesy,providing they can be seen with optical or electronicinstruments from widely separated points. The firstsatellite to be launched solely for geodetic purposeswas ANNA 1B, in 1962, bythe U.S. Army, Navy,NASA, and Air Force. (ANNA is an acronym for Army,Navy, NASA, Air Force.) The Army's Secor(Sequential Collation of Range) satellites followed.Currently, the U.S. National Geodetic SatelliteProgram involves the joint efforts of NASA, the

Department of Defense, and the Department ofCommerce. Three satellites have already beenlaunched under the so-called Geos program:Prelaunch PostlounchDesignation Designations Launch Date

Geos A

Pageos

Geos B

Geos 3. or Explorer 29

Pageos

Geos 2 or Explorer 36 .

Nov. 6, 1965

Jun. 23, 1966

Jan. 11, 1968

Another satellite, Geos C, m3y be scheduled forlaunch in 1973. NASA builds the spacecraft, providesthe launch and tracking services, and supplies someof the experiments. The agencies cooperate in theanalysis of the data.

The principal objectives of the Geos-C satellite areto:

1. Establish a single, common, worldwide datum(i.e., geodetic reference system) and improveglobal maps to an accuracy of about 10 meters.

2. improve the positional accuracy of geodeticcontrol stations and spacecraft trackingstations.

3. Define better the structure of the Earth'sgravitational field.

4. Correlate and compare the results obtainedfrom all spacecraft geodetic instrumentation.

The Geos satellites are essentially orbitinginstrument platforms similar to scientific satellites.Geos is very similar to the CAS satellite. Both aregravity-gradient stabilized by means of a long, axialboom, with their spiral antennas pointing downwardstoward the Earth. (Fig. 10) In this orientation,geodetic stations can simultaneously observe andinterrogate the satellite. Details on the Geosequipment and geodetic instrumentation arepresented in Table 4 and on page 16.

15

Fig. 10 The Geos spacecraft.

TABLE 4. Design Features and VitalStatistics, GEOS C


Communications

Power supply

Attitude control

Thermal control


Structure

Launch vehicle


16

Design Features

Sends housekeeping telemetryonly; command receiver. Conical,spiral, and turnstile antennas

Solar cells and battery: averagepower about 40 watts

Spin-stabilized. Long boom forgravity-gradient stabilization

Passive

Solar-aspect sensors andmagnetometers to determineattitude

Octagonal aluminum frame about49 inches between the sides.Weight: about 465 pounds

Delta

Space Tracking and DataAcquisition Network (STADAN)for routine tracking andacquisition of status telemetry.

kr\ Altb.A-1Fig. 11 The Geos B set up for a vibration test

PRACTICAL EXPERIMENTS ON GEOS

The Tracking AidsGeos must be readily "seen" by terrestrial tracking

stations if it is to achieve the objectives listed onpage 15. Therefore, mounted on Geos are severalreflectors that mirror laser and radar beams reachingthe satellite from Earth. The satellite is also madehighly visible by optical and radio beacons. Geos willalso carry a radar transponder, a tracking aid whichresponds to a pulse of radar energy by emitting areturn signal much stronger than possible by simplereflection.

Satellite-to-Satellite TrackingIn this experiment, Geos will be used in conjunction

with ATS F to determine whether a satellite with aprecisely known orbit (Geos) can help ground-based

tracking stations improve the accuracy with whichthey track another satellite (ATS).The Radar Altimeter Experiment

By di recti ng radarwaves downward from Geos tothe sea's surface, sea level will be measured towithin 10 cm_ This is scientifically important becausethe sea's surface is not perfectly spherical_ TheEarth's varying gravitational field makes broad hillsand depressions that Geos will map-

RADARwaves

WEATH ER WATCHING VS_ WEATH ER PREDICTINGIn August of 1969, the greatest storm tc. hit Nor-th

America in recorded history cut a swath from the GulfCoast to the Carolinas_ A hundred years ago,Hurricane Camille would have swept in from the Gulfunannounced, but in 1969 weather satellites helpedgive ample warning_ (Fig_ 12) It has been estimatedthat the timely warnings saved 50,000 lives_ Early

Pt-

hbEstoAt_

csr:h t.

thAtinh u

Fishc(EE

Mil

1

i

1

1

..._ _ __....--....

MC--IrliirdlOW_ 4111

'Illm 4111 Illom

mi a = ame=0.- _

warnings of dangerous storms have been an earlypayoff of weather satellites. Indeed, this practicalaspect of space meteorology has already repaid forthe rockets and spacecraft many times over in termsof lives and property saved.

It became apparent only a few months after WorldWar II, when captured German V-2 rockets carriedcameras to high altitudes, that cloud cover pictureswould be of great value to meteorologists. Many earlysatellites, such as Vanguard 2, 1958, and Explorer 7,1959, included meteorological instruments in theirpayloads. Today, thousands of pictures of the Earth'sspiraling cloud systems are taken every day. Thesepictures help forecast weather two or three days inadvance and also give meteorologists the big pictureso they can better understand the fair and foulweather systems that wheel across the planet'ssurface.

Meteorologists are not satisfied with theseaccomplishments; they would like to forecast weather

two weeks in advance and comprehend all theintricacies of the great air masses. These goalsrequire more than the simple watching of clouds fromorbit. Rather, accurate models of weather systemsmust be constructed. Such mathematical modelsrequire detailed knowledge of the temperature,pressure, water-vapor content, and the other factorsthat describe the atmosphere and its circulation. lt isimportant to know how these factors vary with heightabove the Earth's surface as well as geographicallocation. This requirement means that meteorologicalsatellites must be able to make vertical andhorizontal measurements from great distances.

The foregoing thoughts may be summarized interms of three objectives:

1. Obtain global cloud cover pictures (ageographic objective).

2. Observe the entire atmosphere continuously(a temporal objective).

3. Measure atmospheric variables quantitatively,vertically and horizontally.

Fig. 13 Schematic showing different amounts of infrared radiation emitted by different surfaces.ITOS satellite is shown overhead.

18

10"

The National Oceanic and Atmospheric Admin-istration (NOAA)* is responsible for the day-to-day operation of weather satellites, but NASA hasbeen assigned the task of developing and procuringnew spacecraft and meteorological sensors as well asproviding launch and tracking services. NASA nowhas three active weather satellite programs plus theATS effort, under which new cameras and othersensors are tested in space. There are three specificsatellite programs:

1. The Improved Tiros Operational Satellite (ITOS)Program, involving the development,procurement, testing, and launch of a newgeneration of operational meteorologicalsatellites for NOAA on a reimbursable basis.

2. The Nimbus Program, in which new techniquesand sensors are developed using the Nimbusspacecraft as a test vehicle.

3. The Synchronous Meteorological Satellite(SMS) Program, under which a newgeostationary weather satenite is beingdesigned to satisfy the requirements of NOAA'sNational Operational Meteorological SatelliteSystem (NOMSS).

LATEST IN A LONG LINE (ITOS)

As the decade of the 1970s began, NASA andESSA (now NOAA) were able to look back on two longand highly successful series of meteorologicalsatellites. Between 1960 and 1965, NASA hadorbited ten Tiros satellites without a single failure.To illustrate the productivity of these spacecraft,Tiros 8 operated for 31/2 years, sending back over100,000 cloud cover photographs. The first ESSAsatellite series (also called Tiros OperationalSatellites or TOS) began in 1966 and ended with thelaunch of ESSA 9 in 1969. Like the Tiros satellites,the ESSAs were primarily cloud photograpners. For

*Successor to the Environmental Science ServicesAdministration (ESSA).

Fig. 14 Diagram of the ITOS satellite. Meteorologicalsensors are mounted on the bottom, which is always pointedat the Earth.

SOLAR

MONITO/t CONTROL.

momerrumFLYRRICEL

FLAT-PIATERACNONLETER

summonmamma-Toraft.

AVCSCWERAS

a better understanding of weather processes andlonger forecasts, something better was needed.

The Improved Tiros Operational Satellite (ITOS)represents a major step forward in that it can takecloud-cover pictures at night with a scanning infraredradiometer.* The infrared radiation emitted by theEarth depends upon the temperature and character ofthe radiating surface. Rivers and lakes, for exampleare cooler than farmland. (Fig. 13) Clouds stand outvividly at night in infrared pictures. ITOS, therefore,offers 24-hour coverage of the Earth. In contrast, theTiros/ESSA spacecraft cameras were sensitive toonly the sunlit portions of the Earth. The ITOS craftwill transmit their TV and infrared pictures to usersaround the world either directly from the satellites orthrough the NOAA center at Suitland, Maryland.

The ITOS spacecraft are substantially differentfrom those in the Tiros and ESSA series.. Tiros/ESSAsatellites were relatively small, cylindrical, spin-stabi lized vehicles. In contrast, ITOS is boxlike, withan active attitude control system that keeps itsinstruments continually pointed at the Earth. (Fig. 14)In terms of size, ITOS weighs more than twice thenominal Tiros-ESSA craft; 678 against 300 pounds.The extra spacecraft sophistication pays dividends interms of more complete photographic coverage onboth the day and night sides of the Earth. Otherspacecraft features and the instrument complementare given in Table 5 and on page 21.

ITOS 1, which was called Tiros M before launch,was orbited by a Delta rocket on January 23, 1970.Tiros M was in actuality an ITOS prototype, but itreceived the family name anyway. ITOS A waslaunched December 11, 1970 and designatedNOAA-1.

The nominal ITOS orbit is "Sun-synchronous"with an altitude 790 miles and an inclination of 78°.This type of orbit was found to be extremely useful bythe later Tiros satellites because the lighting of thescene below was consistent for all photographs ateach latitude. In such an orbit, the satellite passesover the Equatorindeed, each latitudeat the sametime each day. Northbound, an ITOS should cross theEquator twelve times a day at 3 P.M., local time, whilethe southbound passage should occur at 3 A.M. Theorbital plane also precesses (rotates) approximately10 each day, keeping pace with the seasons; that is,the satellite's orbital plane always intersects the Sun.

*A radiometer measures radiation within a specific band ofwive lengths. The infrared portion of the spectrum beginsat the tong wavelength end of the visible spectrum. Theinfrared spectrum emitted by a surface depends upon thesurface's temperature and composition.

19

Fig. 15 Global cloud coverpicture made up from swathstaken from an ESSA AdvancedVidicon Camera System.

In addition, the Earth rotates beneath the satellitejust enough so that slightly overlapping bands ofpictures can be snapped during each revolution.(Fig_ 15)

SEEING THE BIG PICTURE (SMS)

The view of the world's weather from a few hundredmiles up is spectacular. In just two hours, an ITOSphotographs a strip of clouds about 1700 miles wideand 25,000 miles long. One sees vividly the greatconvection cells marching across the oceans andcontinents. However, ITOS will not be back to

,

"

Fig. 16 ATS cloud cover picture from about 22,300 miles.

20

photograph the same area for a whole day. This delaymay be critical in the case of rapidly developingstorms. Neither can meteorologists watch lessdangerous atmospheric phenomena develop on acontinuous basis. Since one of the major goals ofspace meteorology is the continuous surveillance ofthe whole globe, it appears that a geostationary orsynchronous satellite poised 22,300 miles out overthe equator might help us achieve this end. (Fig. 16)

Goddard Space Flight Center and its industrialcontractors have been studying geostationaryweather satellites since the mid-1960s. Pictures takenfrom 22,300 miles by TV cameras aboard some of theATSs reinforced meteorologists' desire for geostation-a ry weather satellites. NASA approved the fabrication,test, and launch of the Synchronous MeteorologicalSatellite (SMS) in 1969. The program is still in itsearly stages, and a contractor to build the spacecraftwas selected only in mid-1970. Neverthetess, someof the satellite's major features have been defined inthe Goddard studies, and some of these are firmenough to presenttc!cm.

First, we should examine the requirements leviedon the SMS by the ultimate user, NOAA. Manifestly,the capabilities of the SMS will go far beyond the earlyTiros/ESSA spacecraft.

1. In the visible portion of the spectrum, SMScameras should be able to resolve details towithin two miles. The ultimate goal (not arequirement) is a resolution of 0.5 mile in thevisible and 4.0 miles in the infrared (neededfor nighttime cloud-cover pictures).

2. Satellite electronics should be able to time-stretch picture data; that is, transmit it more

TABLE 5. Design Features and Vital Statistics,ITOS


Communications

Power supply

Attitude control

Thermal control


Structure

Design Featums

250-milliwatt phase-modulatedlink for beacon and telemetry at136.77 MHz. Real-time video linkat 137.5 or 137.62 MHz; 5 wfrequency modulated. Playbackvideo link at 1697.5 MHz, 2 w.Command receiver at 148 MHz.

10,260 n-p 2x2-cm solar cells onthree deployable panels plusnickel-cadmium batteries.Average power level: 70 w.

Three-axis stabilization usinginertia wheel and magnetic coils.Nutation damper. Points to within10 of vertical.

Passive paints and insulation plus.thermal louvers.

Horizon sensors, Sun sensors. Re-sponds to tone commands fromthe Earth.

Rectangular box, 40 x 40 x. 48.6inches. Three solar panels 65 in-ches long. Weight: about A78pounds.

Launch vehicle Delta


Space Tracking and Data Acquisi-tion Network (STADAN). NOAAalso employs several stations toreceive pictures and other data.Pictures can also be picked up byAutomatic Picture Transmission(APT) stations built by anyone us-ing NASA plans.

OPERATIONAL INSTRUMENTS ON ITOS

TV CameraThe ITOS TV camera takes 1700-mile-square

pictures of the Earth with 50% overlap. NOAA usesthese pictures directly in the preparation of weatherforecasts. The camera is a one-inch vidicon with 800scan lines.

The Automatic Picture Transmission (APT) CameraThe cloud-cover pictures taken by the APT camera

can be received by local governments and individualsall over the world. The pictures are transmittedcontinuously and can be recorded with simple,inexpensive equipment. Two one-inch vidicons arecarried by ITOS for purposes of reliability.

Heat Input/Output MeasurementsAnother radiometer measures the heat reflected

and reradiated bythe Earth. By comparingthis withthe total heat received from the Sun, meteorologistscan determine where energy is being added toatmospheric circulation patterns.

SOME SOLAR HEATIS REFLECTEDIMMEDIATELY

SOME SOLARHEAT ISABSORBED ANDTHEN RERADIATED

Nighttime Cloud-Cover PicturesFrom ITOS, a radiometer with a moving mirror

scans the Earth below. Sensitive to both visible andinfrared light, the instrument extends weathersatellite coverage to 24 hours a day.

Solar Proton MonitorITOS also carries six solid-state radiation detectors

to monitor solar protons and electrons near the Earth.

slowly than it generates it to enable "slow"ground stations to pick up the transmissionssuccessfully.

3. Like the Cooperative Applications Satellite(CAS) discussed earlier, the SMS must becapable of collecting data from hundreds ofsmall, remotely located terrestrial stations andtransmit them to a central location.

4. The satellite should also have the abilitytorelay weather maps and other meteorologicaldata of general interest. (Obviously, weather

21

SOLAR PAN EL

RADIALJET.

"S" BAN DANTENNA INSOLAR PANEL

IN FRARED CAMERA

.E1-51/1

AXIAL J ET

APOGEEMOTOR

DELTA SH RO UD

Fig. 17 Conceptual drawing of the SMS shown within Delta launch vehicle shroud.

satellites are also partly communicationsatellites.)

5. Because solar activity is important to weatherforecasting, the SMS must be able to measuresolar protons, solar X-ray flux, and the localmagnetic field.

Goddard Space Flight Center based its SMS designstudies upon the well-proven ATS and Intelo- "!1

designs. The SMS will weigh about 1000 pounds. Itwill probably be a cylinder about 56 inches indiameter and 65 inches long, as dictated by theDelta launch vehicle's aerodynamic shroud. (Fig. 17)

A hydrazine propulsion system will be included to easethe spacecraft into geostationary orbit, keep it there,and move it to new positions over the Equator whennecessary. The technology employed here criginatedin the ATS program. The hydrazine jets will also beused for attitude control. The communicationsubsystem has not been delineated yet. Power willcome from solar cells and batteries. This is a verycrude sketch. The details will be filled in by the newlyselected spacecraft contractor.

The minimum NOAA picture requirements can bemet with the spin-scan camera system already provenin the ATS program. Two of these cameras will berequired on each SMS to meet reliability objectives.

22

The more ambitious goals (0.5 and 4.0 miles visibleand infrared resolutions) will require a newinstrument. The Visible-Infrared Spin-ScanRadiometer (VISSR) has bean proposed as a possiblesolution. This instrument consists of a classicalCassegrain telescope and a scanning mirror thatsweeps the telescopic image in several visible andinfrared wavelength ban& or channels. In this way,pictures of the Earth can be taken in several regionsof the visible-infrared portion of the spectrum.

SMS-A will be launched in late 1972, possibly early1973. Because considerable design and developmentwork remain, the above description should beregarded as preliminary in nature.

NIMBUS, A TEST VEHICLE FOR METEOROLOGICALINSTRUMENTS

Two objectives define the Nimbus program:1. Develop and flight test advanced sensors and

technology basic to meteorological research,the atmospheric sciences, and the orbitalsurvey of Earth resources.

2. Provide for the global collection anddistribution of meteorological data from allsources.

Nimbui is a large, sophisticated spacecraft. It wasconceived in 1959 at Goddard about the same time

that NASA's big observatory-class spacecraft werebeing sketched out. Nimbus was considered to bea generation beyond the Tiros spacecraft. When itbecame apparent that smaller, less expensivespacecraft would meet NOAA's operational require-ments, Nimbus was given the task of testing andproving out sensors used for Earth observation. Thetest-bed idea is essentially the same as the ATSconcept, except that Nimbus' fully stabilized, Earth-pointing capability make it ideal for developingmeteorological and Earth resource sensors.

The Nimbus spacecraft has already proven itselfoperationally. Four Nimbus satellites are now inorbit, and a fifth was lost due to a launch vehicle.failure, as indicated below:

PrelaunchDesignation

Nimbus A

Nimbus C

Nimbus B

Nimbus B-2

Nimbus D

Post launchDesignation Launch Date

Nimbus 1

Nimbus 2

Nimbus 3

Nimbus 4

Aug. 28, 1964

May 15, 1966

May 18, 1968 Thor-Agena failure

Apr. 14, 1969

Apr. 8, 1970

Sensors being tested on Nimbus are mounted inthe sensor ring at the bottom of the spacecraft,which is kept pointed at the Earth by the attitudecontrol subsystem. (Fig. 1) A conical truss structureconnects the sensor ring to the upper housingcontaining the attitude control subsystem, thecommunication subsystem, and other spacecraftequipment. Two wing-like solar panels, which areautomatically pointed toward the Sun, provide powerto the spacecraft and the instruments. Nimbusdetails are presented in Table 6.

PRACTICAL EXPERIMENTS ON NIMBUS E

Atmospheric ProfilesRadiometers can measure how the temperature

and water-vapor content of the atmosphere varieswith altitude by determining how much infraredradiation the atmosphere emits at various wave-lengths. Water-vapor molecules, for example, emitinfrared radiation when they rotate. Verticaltemperature and water-vapor profiles are importantfactors in weather prediction. Nimbus E will carry twoinfrared radiometers for measuring vertical profilesand another to help make worldwide humidity maps,

VERTICALTEMPERATUREPROFILE

TEMPERATURE

A Microwave View of the EarthTwo Nimbus-E instruments will map the Earth in

the microwave portion of the radio spectrum.Microwaves, which are much longer than infraredwaves, are also emitted by water-vapor molecules,vegetation, and the ground itselfin short, just abouteverything. The two Nimbus-E microwave instrumentswill measure the following meteorological factors:vertical profiles of temperature and water-vaporabundance, cloud-cover water content, amounts ofprecipitation, land temperature (where clouds blockinfrared wave-lengths), the intensities of storms andstorm fronts, the water content of the soil, ice cover,and even the quantity of vegetation below.

Tracking and Data Relay ExperimentThe communication part of this experiment will

test the feasibility of relaying data from a low-altitudesatellite (Nimbus) via a geostationary satellite (ATSF/G) to a ground receiving station. An attempt willalso be made to track Nimbus E with high precisionfrom ATS F/G.

Mapping the Composition of the Earth's SurfaceThe infrared radiation emitted by the Earth's

surface depends upon composition as well astemperature. To illustrate, the infrared spectra ofvarious minerals vary markedly, making a satellite-borne infrared spectrometer a potential mineral-prospecting instrument. The Nimbus-E instrumentwill also make thermal maps of the soil and theocean surface.

ROTATION OF WATER MOLECULESGENERATES MICROWAVES

23

TABLE 6. Design Features and Vital Statistics,Nimbus E


Communications

Power supply

Attitude control

Thermal control


Structure

Launch vehicle

Tracking and dataacquisition

Design Features

Wideband stored data, 1707.5and 1702.5 MHz, Pulse-code-modulated telemetry at 136.5MHz. Command receiver at149.52 MHz.

Two movable solar panels plusnickel-cadmium batteries.Provides an average of 260 watts.

Inertia wheels plus cold-gasreaction engines in equipmenthousing. Keeps spacecraft pointedto withio 10 of the local vertical.

Thermal louvers located aroundthe rim of the sensor ring,supplemented by thermal coatingsand insulation.

Sun sensor, horizon sensors,gyroscopes.

Four major elements:sensor ring, providing 18 squarefeet for mounting Earth sensors;equipment housing; solarpaddles, and truss structure.Overall dimensions: 9.5 feet highby 9.5 feet wide with solar panelsextended. Overall weight: about1690 pounds.

Thor-Agena

Space Tracking and DataAcquisition Network (STADAN)

HUSBANDING EARTH'S RESOURCES (ERTS)

Only a few years ago man could modify theconditions on Earth with little thought about theenvironment and the cascading consequences of hisacts. With the Earth more crowded and some ofour easHy garnered non-renewable naturalresources nearly depleted, one hears moreand more the phrase "spaceship Earth," implying theincreasing interdependence of all man's activitieshis farms, his industries, his mines, his wasteproducts, and, as has become obvious, his veryexistence.

Spaceship Earth will support only a few billionhuman beings unless its resources are managedcarefully,. As for all management systems, information

24

must be available to make resource managementwork: abundant information, on a continuous basis,from all parts of the globe. Satellites by virtue of theirfavorable positions high above the Earth are inparticularly advantageous spots to collect some ofthe needed environmental information.

The key to obtaining information on Earthresources from several hundred miles out in spacelies in the analysis of sunlight reflected from theEarth and radiation emitted from the Earth by virtueof its temperature. Flying across the United States byplane, one sees fields, forests, drainage patterns,geological formations, and the works of man, to givea partial list. If the same plane carried an aerialcamera with a telescopic lens, photographic analysiswould reveal soil types, crop identities, major mineraldeposits, etc. Taking the same flight once more butwith a full complement of instruments that "see" inthe infrared, ultraviolet, and microwave regions ofthe spectrum, the panorama enlarges tremendously.(Figs. 18 and 19) By recording the scenes belowand studying them in light outside the visible range,we can discern forest and crop blights, soil moisturecontents, plant species, ice thickness, and the manyother factors summarized on page 25. By collectingand correlating this kind of information, we can,so to speak, take the Earth's pulse continuously,assessing on one hand its suitability as a habitat forman and on the other searching for ways to improvethe environment.

The stakes in this venture are so high thatattaching a dollar sign to Earth resource informationseems superfluous. However, estimates of money thatcould be saved annuaHy in the United States alonerun over a billion dollars. Knowledge about storms,insect infestations and more efficient pollutionsurveys is worth money.

Many other uses of Earth observation satelliteshave been proposed. Some of the more promising aredescribed on page 25.

It is easy to get carried away and promise toomuch. Although photographs taken from aircraft andmanned spacecraft are mast promising, the wholefield is still in an embryonic state. Furthermore, someEarth resources data will be gathered by aircraft andground-based surveys. The most effective mix ofsensor carriers has not been established yet. Onething is certain, though, and that is that informationacquisition, transmission and processing will be amajor part of the undertaking. To ir!ustrate, aninfrared radiometer reading means little to anagriculturist making a crop survey. Data must beconverted into terms understandable to the user and

PRAC11CAL APPLICATIONS OF ERTS INSTRUMENTSAPPLICABLE

INSTRUMENTATIONI--w0

Agriculture and Forestry

Construction of better topographicmaps and farm planning

Estimations of crop types, densities,and expected yields

Calculation of the damage from dis-ease and insect infestationsIdentification of insect infestationand disease patterns and "earlywarnings"

Census of livestock

Estimation of soil moisture contentand irrigation requirements

Census of forest tree types and esti-mation of logging yield

Early warnings of fire, disease, andinsect infestation in forests

OceanographyForecasts of sea state and ice hazardsfor shipping

Location of high biological activityfrom surface temperature for fishingfleets. Large schools of surface-feed-ing fish may also be pinpointed

Location of drifting oil slicksSurvey of coastal geography, includ-ing detailed shoreline topography,identification of stream erosion pat-terns, and mapping of shallow areas

Collection of such scientific data asthe location of areas of biolumines-cence, estimation of plankton density,and the pinpointing of red tides, fishschools, and a!gae concentrations

CC

2 La-

V

V

V

V

V

-v

V

Hydrology and Water ResourcePlanning

Inventory of water in regional basinsby measurement of lake levels, riverflow rates, irrigation patterns, anddrainage patterns

Control and early warning of floodsby monitoring rainfall, weather pre-diction, and drainage basin surveysIdentification of water pollutants andpolluters from maps of thermal dis-charges and the spectral "signa-tures" of specific pollutantsEstimation of water resources throughsnow and frozen water surveys andthe location of seepage and othergroundwater sources

Geology and MiningDetection of minerals (including oil)from topography, drainage patterns,magnetic fields, and direct identifica-tion of minerals

Prediction of earthquakes from slighttemperature differences, soil mois-ture content, and topographical dis-tributionPrediction of volcanic activity fromtemperature changes

Prediction of landslides from soilmoisture and slope of terrain

Location of geothermal power sourcesfrom surface temperature measure-ments

Transportation, Navigation, andUrban PlanningConstruction of detailed maps of ruraland urban areas to help plan trafficarteries, terminalsEstimation of air, road, and sea traffic

Surveys of urban areas, indicatinghousing and population densities,park areas, industrial development,and types of settlement for purposesof planning renewal and new buildingprograms

APPLICABLEINSTRUMENTATION

1--w....1

En 0 0< wre cc 5<W < CC2 trLL. 1..

...16 z m

V V

V

V

V V

V

25

Fig. 18Multispectral cameraphotographs of the SaltonSea area. The same regionappears very different invarious types of light.

t:4

RED BAND

Fig. 19 Three high altitude photographs showing howdifferent crops appear throughout the growing season ininfrared light.

SEQUENTIAL IR COLOR IMAGES OF AGRICULTURETEST SITE DURING GROWING SEASON

SCALE: 1/50,000LEGEND: W=WHEAT

SB=SUGAR BEETSITE: MESA, ARIZONA

AREA: 4 MILES SQUARE

WHEAT FIELD (W): 16" PLANTS, GROWINGSUGAR BEET FIELD (SB): GROWING12 MARCH 1969

,k

26

NEAR-IR BAND

then tagged with geographical and time coord:nates.NASA's experience with weather information systemswill be of great use here.

NASA's Earth Resources Technology Satellite(ERTS) program began at Goddard Space FlightCenter in 1966. The formal objectives of the programare:

1. To define those practical problems where spacetechnology can make beneficial contributions.

2. To conduct research on sensors and establishtheir utilities in Earth observation.

3. To develop and qualify sensors and spacecraft.

MATURINGGROWING

23 APRIL 1969

4.

LA;FE;

R COLOR

GREEN BAND

4. To develop handling and processing techniquesfor Earth resources survey data.

Much of the early ERTS work concentrated or. thesensors because this was the area with the mostunknowns. Flights with manned air- and spacecrafthave already shown the probable usefulness ofvarious types of infrared, ultraviolet, and microwaveinstruments. The next step consists of flying theseinstruments on an unmanned spacecraft.

The ERTS spacecraft (Fig. 20) and the supportingterrestrial data system are still in the design and

HARVESTEDMATURING

21 MAY 1969

Fig. 20 Conceptual drawing of an ERTS basedon Nimbus technology.

et.:j

Mkt

11116._

.I

27

development phase, with the first flight scheduled in1972. Nevertheless, some spacecraft features .

have already been specified. For example, the sensorend of the craft must point at the Earth continuously;and three-axis stabilization will be essential. Solarpower will be used. The attitude control system willincorporate inertia wheels(flywheel,for storingangular momentum) cold-gas jets, horizon sensors,and gyroscopes. Because of the spacecraft sizeabout 2100 poundsan active thermal controlsystem employing louvers was chosen. There will betelemetry channels at 136 and 2287 MHz andcommand links at 148 and 2106 MHz. The technologydeveloped in NASA's Nimbus program will meet theERTS requirements with minimum additionaldevelopment.

Two instruments (rather sophisticated ones) arescheduled to fly on ERTS A in 1972. The first is aspecial TV camera system, consisting of three returnbeam vidicon cameras. These will take pictures ofareas 100 x 100 nautical miles from the nominalsatellite altitude of 492 nautical miles. Each camerais sensitive to a different part of the visible andnear infrared spectrum. The second instrumentis called a multispectral scanner. It will scanswaths of the Earth's surface 100 nautical mileswide as it moves along its orbital track. Again, thepictures will be taken in different parts of thevisible and near infrared spectrum.

The ERTS orbit will be Sun synchronous. Itwill be a polar orbit with a period of about 103minutes and altitude of 492 nautical miles. Orbitprecession will be such that orbital swaths willbe repeated every 18 days.

The ERTS program provides a fitting end to thisbooklet for it uses technology developed in NASA'sapplications programs to extend the value intonew areas. The spacecraft itself will be based onthe Nimbus design, and the ERTS sensors owemuch to the ATS and weather satellite programs.From the communication satellites come thetechnology for handling the flood of data from thesensors and their subsequent conversion into formatsconvenient to the user.

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A GLANCE AT THE FUTURE

In space technology, we have a tool, which, as wehave seen, has already laid the foundations for bettercommunications, better weather forecasting, and thefirst comprehensive assessments of the planet'sresources. The full implications of this tool we do notyet know. Certainly, there will be better spacecraftand better sensors and ways to wring out more of theirmeanings. So, at the very least, we can foresee near-instant communication among men and machineseverywhere on Earth. Weathermen will give uspreviews two weeks ahead of time. But communicationand weather are only parts of a bigger picture.Spaceship Earth is a complex craft and alreadycrowded with humanityand there is no escapehatch. Possibly the ultimate contribution of spacetechnology is in our better understanding of theEarth and how it affects and is affected by its humanpassengers.

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-*17. S. GOVERNMENT PRINT7NG OFFICE : 1971 0 - 444-660

Additional

'. Reading

, For titles of books and teaching aids related to the

subjects discussed in this booklet, see NASA's

educational publication ER48, Aerospace

Bibliography.

Produced by the Office of Public Affairs

National Aeronautics and Space Mministration

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SATELLITES AT WORKSpaceInTheSeventies

National Aeronautics and Space Administration