topex poseidon a united states france mission. oceanography from space the oceans and climate

8/7/2019 TOPEX Poseidon a United States France Mission. Oceanography From Space the Oceans and Climate

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Earth is the Ocean Planet: ocean waters, vital to all life,cover more than 70 percent of its surface. Stirred andmixed by mighty currents, the oceans distribute heatacross the globe and regulate our climate.

The Earth's climate has changed in the past and itmay soon change again. Rising atmospheric concentra-tions of carbon dioxide and other "greenhouse gases"produced as a result of human activities could generatea global warming, followed by an associated rise in sealevel. But in order to make reliable predictions, we mustfirst gain a quantitative understanding of the role ofocean currents in climate change.This understanding requires corn Drehensive newobservations of the Ocean Planet from space Thechallenge will be met by TOPEX/POSEIDON, a jointmission of the United States and France.

La Terre, plan_te bleue ... Les oceans, indispensables&toute vie, couvrent plus de 70% de sa surface. Brass6spar de puissants courants, les oceans distribuent lachaleur dans tout le globe et 6quilibrent son climat.Par le pass_ celui-ci a dej& 6volue ' toutefois lesactivites humaines entra_nentdes concentrations de gazca[bonique et d'autres gaz & effet de serre. Des 6volutionsrapides sont possibles, comme un rechauffement globalentra_nant une mont6e du niveau des mers. Pour faire desprevisions fiables, nous devons comprendre et mesurerle r61edes courants oceaniques darts leschangements

climatiques.Cette d_marche impliq ue de nouvelles observations_ar_cises effectu_es _ I'_chelle globale. C'est le d_fi querelive la mission TOPEX/POSEIDON, organis6e ish_e-s.........Etats-Unis et la France.

from Space ........................................... 10Mapping the_Ocean Surface ............................................ 12Unprecedented Accuracy ............... I4United States/France Partnershi 16Scient tic Understanding ............... :...,::;; ...... _................ 1-8 "" The Future of the Ocean Planet ................................... 20


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Cl:: _ele,THE TOPEX/POSEIDON MISSION

The United States and France celebrate the Interna-tional Space Year, 1992, with the launch of TOPEX/POSEIDON--the most advanced space mission everdesigned to study ocean currents. The mission issponsored by the U.S. National Aeronautics and SpaceAdministration (NASA) and France's space agency,the Centre National d'Etudes Spatiales (CNES).Sea-Surface Mapping from SpaceTOPEX/POSEIDON will use radar altimetry to measuresea-surface height over 90 percent of the world's ice-free oceans. Circling the Earth every 112 minutes, thesatellite will gather data for three to five years, carryingenough fuel for a full decade of operation. In combinationwith a precise determination of the spacecraft orbit,the altimetry data will yield global maps of ocean topog-raphy-the barely perceptible hills and valleys of thesea surface. From a knowledge of ocean topography,scientists can calculate the speed and direction ofocean currents worldwide.United States/France PartnershipJoint planning was initiated in 1983. NASA, responsiblefor mission operations, is supplying the spacecraft andfour instruments, including the primary radar altimeter.CNES is providing launch aboard an Ariane 42P expend-able launch vehicle as well as two instruments, includingan experimental altimeter. Management is provided byNASA's Jet Propulsion Laboratory and the CNES CentreSpatial de Toulouse.Unprecedented AccuracyFrom an altitude of 1,336 km (830 miles), TOPEX/POSEIDON will measure the distance from the satelliteto the sea surface within 3 cm (1.2 inches). Three inde-pendent tracking systems will determine the position ofthe spacecraft within 10 cm (4 inches). These measure-ments wilt together yield accurate topographic mapsover the dimensions of entire ocean basins--the pri-mary mission objective. These data will permit quantita-tive studies of ocean circulation and its time variabilitythat are crucial to an understanding of climate change.Forecasts of Climate ChangeTPEX/POSEIDON is closely coordinated with theTropical Ocean and Global Atmosphere and WorldOcean Circulation Experiment seagoing measurementprograms sponsored by the World Climate ResearchProgramme. The satellite data, together with TOGA andWOCE measurements, will be analyzed by an interna-tional scientific team. This team will develop and refinecomputer models of the global ocean that can be usedto investigate natural climate variability and assessthe impact of human activities on climate--laying thegroundwork for forecasts of future climate change.


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FROM ANTIQUITY TO THE SPACE AGE

The mystery and power of the seas have always drawnus to voyage across them--to explore, to understand,and to take the measure of the Ocean Planet. Thistimeless and universal need to define ourselves and ourworld is now being reshaped by the Space Age.Early OceanographersNavigational needs prompted the first attempts tomeasure the speed and course of ocean currentsFragmentary records of tidal and current patterns alongthe Red Sea and Mediterranean coasts, compiled byearly sailors and traders as an aid to commerce, havecome down to us from antiquity. Driven by necessity,these intrepid seafarers were the first oceanographers.

Viking expeditions crossed the North Atlantic, reach-ing Vinland (Labrador) around the year 1000. However,tales of a New World across the sea remained locked inNordic lore. But early in the Renaissance, other Europe-ans began to fan out across the world's oceans insearch of riches and new lands. By 1600, Portuguese,Spanish, French, and English expeditions had mappedmost of the oceans and continents upon a globe that,in Europe, had lain half unknown scarcely a centurybefore. These bold adventurers, bridging the Old Worldand the New, initiated the systematic exploration of theseas and revealed the dimensions of the Ocean Planet.

Beginnings of ResearchAs Deputy Postmaster General for the American coloBenjamin Franklin sought to shorten the London-NewYork shipping time by charting Atlantic Ocean curreIn 1775, while sailing from London to Philadelphia,Franklin delineated the edges of the warm Gulf Strethrough water-temperature measurements; his latermap, constructed with the help of Nantucket seacaptain Timothy Folger, is a classic of oceanography.The first office dedicated to ocean mapping had beestablished in France only five years earlier.

Shortly afterward, the English captain James Cocarried out his famous voyage to explore and chartPacific (1776-1779). Marine expeditions quickly muplied. In 1849, drawing upon data collected from slogs, American naval officer M.F. Maury publishedfirst worldwide wind and current charts.

The year 1872 marked the beginning of the firstpurely scientific oceanography expedition: the 42-movoyage of H.M.S. Challenger, sponsored by the BriRoyal Society Covering 113,000 km (70,000 miles)Challenger systematically surveyed the deep oceacollecting data on ocean depth, temperature, curreand other properties. These revelations were organiand compiled over the following 20 years to lay thefoundations of modern oceanography.

4EARLY OCEAN EXPLORATIONS harnessed the powerof globalwinds.Sea-surface winds are the principaldriver of ocean currents,which also helped to determine thecourses ofsuch expeditions.

FRANKLIN-FOLGER MAP OF GULF STREAM, published in 1was based on Franklin's own shipboard measurements. In its gfeatures, it agrees surprisingly well with satellite mapsof the cu


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Oceanography MaturesThe 20th century has seen major advances in oceano-graphic theory and technology, the widespread deploy-ment of instruments in the sea, international scientificcollaborations--and a host of new findings.

The beginning of the century (1900-1930) extendedthe studies of Challenger, particularly through the workof analogous national survey expeditions. HighlightsinclOde invention of the first reliable surface-to-bottomsampling bottle, development of the theory of wind-driven currents, and completion of the first trans-Atlanticphysical, chemical, and biological measurements.

The period 1930-1960 saw rapid technical progress,intensive testing of theories of oceanic processes,and the first global research collaborations. Highlightsinclude use of efficient new instruments for measuringtemperature and tracking deep currents, models oflarge-scale ocean circulation, and recognition of theimportance of eddies in global ocean circulation.

The last three decades (1960-present) have seendramatic strides in ocean-measurement and computingechnology, detailed regional studies, and a growingappreciation of the role of the oceans in climate. High-lights include the Indian Ocean Expedition, mooring ofdeep-ocean current meters, computer models of globaloceanic and atmospheric circulation, the International

Decade of Ocean Exploration, the beginnings of satel-lite oceanography, and the start of TOGA and WOCE.Most importantly, scientists have recognized the neces-sity of studying the Earth as a unified system of coupledcomponents: land, oceans, atmosphere, and biosphere.The Space AgeThe beginning of the Space Age in 1957 heralded atechnological revolution in Earth studies. In 1960, a U.S.weather satellite returned the first images of the Earth'scloud cover. By the 1970s, satellites were routinelygathering information on the physics, chemistry, anddynamics of the atmosphere and features of the landsurface. The 1970s also saw the first use of satellitealtimetry for measurements of sea-surface height.

Measurements carried out by ships and by instru-ments deployed in the sea are essential for research.However, these techniques are limited in both durationand geographic coverage; they cannot provide anunderstanding of global ocean circulation, whichrequires frequent, long-term observations of currentsover ocean-basin scales. The measurement of oceantopography by satellite radar is the only way to obtainthese observations. The promise of two decades ofprogress in radar altimetry from space will be fulfilledby the launch of TOPEX/POSEIDON in 1992.

!INSTRUMENTEDBUOYS permit extended, unattended oceanmeasurements. Thedeployment of instruments in the deep ocean faroffshore was a major 20th-century research innovation.

ARIANE EXPENDABLE LAUNCH VEHICLE developed by theEuropeanSpace Agency is being providedby France for the mid-1992launchof TOPEX/POSEIDON from Kourou, French Guiana.


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THE OCEAN-CLIMATE CONNECTION

What drives ocean currents? What controls their move-ment? How much heat do they distribute around theglobe? Are ocean processes coupled to climate? Thesequestions take us to the heart of the TOPEX/POSEIDONmission and the ocean-climate connection.

Both the oceans and the atmosphere transportroughly equal amounts of heat from the Earth's equato-rial regions--which are intensely heated by the Sun--toward the icy poles, which receive relatively little solarradiation. The atmosphere transports heat through acomplex, worldwide pattern of winds; blowing acrossthe sea surface, these winds drive corresponding pat-terns of ocean currents. But the ocean currents movemore slowly than the winds and have a much higherheat storage capacity. Like a massive flywheel thatregulates the speed of an engine, the vast amount ofheat stored in the oceans regulates the temperature ofthe Earth. The oceans are the thermal memory of theclimate system.Ocean CirculationAtmospheric winds sweep the ocean surface layeralong with them, raising sea-level height downwind.The surface of the tropical Pacific Ocean, for example,is normally piled about 50 cm (20 inches) higher offAsia than off South America because of the steadywestward sweep of the tropical trade winds.The ocean also responds to the Earth's rotation.Ocean currents are deflected to the right (in the North-ern Hemisphere) or to the left (in the Southern Hemi-sphere) by the "Coriolis force," a rotational effect firstexplained by the French mathematician GaspardGustave de Coriolis (1792-1843). Driven by the windsand directed by the Coriolis effect, ocean surfacecurrents circulate in enormous "gyres" around regionsof raised or lowered sea level--the hills and valleysof ocean topography--just as winds blow around theextensive highs and 10ws of atmospheric surfacepressure. Observations of ocean topography, togetherwith our knowledge of the Coriolis force, thus permit thespeed and direction of surface currents to be calculated.A Global Conveyor Belt for HeatAlong the western margins of the oceans, swift andnarrow currents carry warm tropical surface waterstoward the polar seas, where they lose heat to theatmosphere and then sink into the ocean depthsl Thissinking is most pronounced in the North Atlantic Ocean.Migrating back toward the Southern Hemisphere abovethe sea floor, the cold water eventually wells up to thesurface layers of the Indian and Pacific oceans. Thismassive, global circuit takes almost 1,000 years.The steady oceanic transport of tropical heat to thecold polar seas moderates our climate. Without thewarming effect of the Gulf Stream, for example, the

Sea-tHeat

/GLOBAL PATTERNSOF OCEAN CIRCULATION (top) areshschematically on this world map.The pattern is dominated by gthat cover an entire ocean basin; these circulate clockwise in thNorthern Hemisphere, counterclockwise in the Southern Hemis


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climateofEuropewouldesemblehatofnorthernCanada.Conversely,umanactivitiesavehepotentialoinfluencethiscirculationpatternandthusalterourclimate.Climate ChangeThe oceans also regulate cTimate by absorbing carbondioxide and other greenhouse gases. Analysis of oceancarbonate sediments and polar ice-cap deposits showsthat atmospheric composition and ocean circulationhave both varied dramatically in the past: 18,000 yearsago, during the last Ice Age, carbon-dioxide levels were40 percent below those of today and sea level stoodmore than 100 m (330 feet) lower. A better understand-ing of ocean processes is needed to interpret thishistory and to forecast long-term climate trends.

The oceans play a role in short-term climate shifts aswell. At intervals of three to seven years, the weakeningof tropical trade winds allows warm Asian surfacewaters to surge eastward across the Pacific to SouthAmerica. Accompanied by major shifts in atmosphericcirculation and rainfall, these "El Niho" events disturbclimate worldwide. The El Nifio of 1982-83, the worst ofthis century, triggered flooding and landslides thatclaimed 600 lives in Ecuador and Peru and devastatedthe U.S. West Coast; cyclones left 25,000 homeless inTahiti, and severe droughts struck Australia, Indonesia,the Philippines, and South Africa. Improved knowledgeof upper-ocean circulation in the tropical Pacific isessential for the reliable prediction of such events.

GLOBAL CONVEYOR BELT FOR HEAT (bottom) transports warmsurface waters (orange) into the North Atlantic, where they cool, sink,and then move southward (purple) throughout the depths of the globaloceans--upwelling after 1,000 years back tothe surface.THE MIGHTY GULF STREAM, bearing warm water along the U.S.East Coast toward Europe, standsout (red) in this temperature-sensi-tive image from space. Huge eddies, spun off from the stream, carryenergyfar out into the Atlantic; they may lastfor a yearor more.


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THE OCEAN DECADE

How do ocean-atmosphere interactions shape Et Ni_oclimate events? What is the global pattern of oceancirculation? What additional observations from spaceare required? These questions are now being addressedthrough an international, coordinated research effort,including NASA's Mission to Planet Earth. During thedecade of the I990s, oceanography involving seagoingfield experiments and space missions is being intensified.TOPEX/POSEIDON is a core element of this research.TOGAThe international Tropical Ocean and Global Atmo-sphere (TOGA) program was begun by the WorldClimate Research Programme (WCRP)in 1985 to studythe year-to-year variability of the tropical oceans andtheir coupling to the global atmosphere. This decade-long effort involves extensive observations and model-ing studies. The results will help to improve thepredicability of ocean-atmosphere interactions ontimescales ranging from months to years--particularlyEl Ni_o events in the Pacific Ocean.

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DAMAGE FROMEL NIKIOCLIMATE EVENTS is a recurrentcoastalcommunities. The effects of these disturbancesare alinland through floods,droughts, andshifts in rainfall patterns.During the flight of TOPEX/POSEIDN, an inten

TOGA field program will study the warm-water pothe western tropical Pacific--the Earth's largest rvoir of warm surface water--and its role in the Elphenomenon. The altimeter measurements of seaheight will provide a Pacific-wide perspective onfield study. Conversely, the TOGA measurementshelp to validate the observations from space.

..... A.,S,O' L SEOS


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DIRECTMEASUREMENTS OF OCEAN CURRENTS will continue tobeessential. Data from shipboard andmoored instruments are neededfor regional studies and the validation of space observations.WOCEThe World Ocean Circulation Experiment (WOCE) wasbegun by WCRP in 1990 to better describe and under-stand global ocean circulation and its relationship toclimate changes over decades or longer. During the1990s, scientists from 40 nations will carry out anunprecedented series of oceanographic observationsand measurements through a worldwide network of sea-level stations and a fleet of research vessels covering allthe oceans. The WOCE network will permit calibration ofTOPEX/POSEIDON observations--which, in turn, willprovide a global reference fr.amework for the integrationof the WOCE seagoing measurements.

WOCE will provide the data necessary to make majorimprovements in the accuracy of computer models ofocean circulation. As these models become moreprecise, they will be coupled to models of atmosphericcirculation to simulate--and ultimately to help predict--how the ocean and the atmosphere will together deter-mine our future climate.Space MissionsTOPEX/POSEIDON will be complemented by other

important space missions over the next decade.The multipurpose European Remote-sensing

Satellite (ERS- 1 ) satellite, launched in 1991,carries a radar altimeter together with

five other instruments. Although itsaltimetric measurements areless accurate than those

PACIFICTRADE WIND FIELD was calculated from 1978 Seasatscatterometer data. Surface winds aredrivers of heat-transportingocean currents, which playa key role in disruptive El Nitro events.of TOPEX/POSEIDON, its high-latitude coverage allowsERS-1 to study polar ocean and ice topography inac-cessible to the U.S./France mission. The two missionsare complementary: their altimeter data will be mergedto yield a single data set of greater coverage than eithercan provide alone.The NASA Scatterometer (NSCAT), to be flown on theJapanese Advanced Earth Observing Satellite (ADEOS)in 1996, will measure the speed and direction of sea-surface winds with high accuracy over 95 percent of theglobal ocean every two days. The potential overlap ofthe TOPEX/POSEIDON and NSCAT missions providesan exciting opportunity to study the ocean's responseto wind forcing.The ARISTOTELES gravity-measurement satellitebeing considered by the European Space Agency(ESA) for flight in the late 1990s would carry a gravitygradiometer to measure in detail the spatial variationsof the Earth's gravity field. These data would greatlyimprove the accuracy of ocean-circulation calculations.

The Earth Observing System (EOS) is a series ofsatellites that will be a central component of Mission toPlanet Earth beginning in the late 1990s. EOS will providelong-term (15-year) data sets to further our understandingof the interactions among the Earth's land surfaces,oceans, atmosphere, and biosphere, and how these arebeing influenced by human activities. As well as providingnew data on the global hydrologic and biogeochemicalcycles, EOS will extend altimetric measurements ofglobal ocean circulation into the next century


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...how many new beauties do not now begin to presentthemselves in the machinery of the ocean! ... its greatheart not only beating time to the seasons, but palpitatingalso to the winds and the rains, to the clouds and thesunshine, to day and night.M. F. Maury, Physical Geography of the Sea, 1855

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A GLOBAL VIEW FROM SPACE

Oceanography is no longer tethered to the sea. Satellitetechnology has now enabled scientists to place sensi-tive sensors in space, far above the sea surface, to takea new measure of the Ocean Planet.Two Decades of ProgressAlthough an experimental radar altimeter was flown in1973 aboard NASA's Skylab mission, NASA's Geophysi-cal Satellite-3 (Geos-3, 1975-1978) carried the firstinstrument to yield useful measurements of sea leveland its variability with time. The Geos-3 map of GulfStream variability was in good agreement with historicalship observations.NASA's 1978 Seasat carried the first altimeter de-signed for oceanography and returned the first globalsea-surface measurements. Although the missionoperated for only 100 days, it collected more ocean-topography data than the previous 100 years of shipboardresearch. An altimeter on the U.S. Navy's GeodeticSatell ite (Geosat, 1985-1989) gathered the first mult iyearglobal data set on sea-level height, producing the mostaccurate variability measurements yet obtained.

The pioneering Geos-3, Seasat, and Geosat missionsreflect two decades of progress in instrument technol-ogy and scientific analysis. Improvements in altimetricprecision reduced the uncertainty in satellite heightabove the sea surface from Skylab's 60 cm (24 inches)to Geosat's 4 cm (1.6 inches). However, all were limitedby uncertainties in the satellite orbit, which ranged up to10 m (33 feet) for Geos-3 and up to 2 m (6.6 feet) forSeasat and Geosat. Consequently, these missions couldnot provide definitive studies of ocean circulation, whichrequire topographic measurements with an error of14 cm (6 inches) or less across an entire ocean basin.TOPEX/POSEIDON: Meeting the ChallengeTOPEX/POSEIDON is the first mission designed to bringboth high altimetric precision and high orbital accuracyto bear upon the challenge of ocean topographicmapping. It will provide a detailed, global snapshot ofthe ocean surface every I0 days over a period of threeto five years.

A high orbital altitude of 1,336 km (830 miles),chosen to minimize atmospheric drag and the effectof spatial variations in the Earth's gravity, will permitTOPEX/POSEIDON to be tracked with unprecedentedaccuracy. The orbital inclination will carry the space-craft to 66 degrees north and south latitude, allowinga thorough sampling of tidal signals. The satellite'smeasurement tracks across the Earth will be repro-duced within 1 km (0.6 mile) over every 10-day repeatcycle, so that the altimeters can repeatedly measurenearly identical points on the sea surface. The widestspacing between tracks, at the equator, will be only315 km (195 miles).


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MAPPING THE OCEAN SURFACE

What determines the shape of the ocean surface?How is this shape measured from space? How can weminimize the uncertainty of this measurement? Theanswers to these questions draw upon fundamentalphysics, advanced technology, and the inspired perse-verance of a generation of satellite oceanographers.Static Gravity, Dynamic OceansIf the ocean were motionless, the shape of its surfacewould be determined entirely by the gravitationalattraction of the Earth. Even in that case, however, thesea surface would have hills and valleys. Becausematter is distributed unevenly within the Earth's crust--densely packed within mountain ranges, thinned byvalleys--the Earth's gravity field varies substantiallyover both the continents and the seas. This spatialvariation of gravity itself generates sea-surface topogra-phy with a global altitude range of nearly 160 m (525feet) relative to the center of the Earth.

Swept by winds and seething with waves, the oceanis actually in ceaseless motion, flowing in giganticcurrents and gyres directed by the Coriolis effect of theEarth's rotation. The shape of the sea surface is dy-namic, and it therefore departs from the static topography

12INFLUENCE OF GRAVITY ON OCEAN TOPOGRAPHY isschemati-cally illustrated in thiscomparison of sea level with the shape of theocean floor (vertical scales exaggerated). Inthe absence of currents,sea-surfacetopography tends to follow ocean-bottom contours.

DYNAMIC TOPOGRAPHY OF THE GLOBAL OCEAN (top)wasdetermined from two months of Geosat altimetry data. Ocean cucirculate (arrows) around this global system of tropical highs (redpolar lows (blue). TOPEX/POSEIDON will provide much more de


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VARIABILITYFOCEANIRCULATIONbottom) is reveated byrepeatedGeosat altimetric measurementsover the same position onthe ocean surface. Highest variability (red, yellow), associated withswift boundarycurrents, isfound off the east coasts of thecontinents.

determined by gravity alone. This departure, calleddynamic ocean topography, has a global range inaltitude of about 2 m (6.6 feet)--barely one percent ofthe altitude range of the gravitational topography. Butonly the dynamic topography contains information aboutthe speed and direction of ocean currents. The chal-lenge is to measure this slight topographic variabilityover the vast dimensions of the ocean basins.How Altimetry WorksDynamic ocean topography is mapped through a three-step process: (1) The radar altimeter sends short pulsesof microwave energy toward the ocean below; theround-trip travel times of the reflected pulses yield thedistance between the spacecraft and the sea surface.(In addition, the shape of the reflected pulse is used todetermine wave height and sea-surface wind speed.)(2) A precise determination of the satellite orbit thenpermits these distance measurements to be translatedinto a global map of sea level relative to the center ofthe Earth. (3) Finally, the map of sea-surface topographyis compared with a map of gravitational topography,which must be obtained independently; the differencebetween the two is dynamic ocean topography, fromwhich ocean-current velocities can be calculated.

Step (1) requires careful correction for atmosphericeffects. Water vapor absorbs microwave radiation anddelays the radar pulses during their round trip to thesea surface. Following the example of Seasat, TOPEX/POSEIDON therefore carries a microwave radiometerfor concurrent measurements of atmospheric water-vapor concentrations; the water-vapor effect can thenbe calculated and eliminated from the data.

Electrons released by the ionization of gases in theupper atmosphere (ionosphere) by sunlight also intro-duce a pulse-arrival delay. Because this delay dependson the radar frequency, observations at two differentfrequencies permit correction for this effect as well.

The most important uncertainty, however, enters atstep (2): determination of the satellite's orbital altitude.TOPEX/POSEIDON will therefore be tracked withunprecedented accuracy by three independent andcomplementary systems.The TidesRegular sea-level oscillations caused by the tidespresent another challenge to accurate topographicmeasurements. However, the TOPEX/POSEIDON orbithas been chosen to permit separation of the primarylunar and solar tidal signals from the dynamic oceantopography, so that tidal effects can be eliminated fromthe ocean-circulat ion calculat ions. TOPEX/POSEIDONwill in fact provide the observations needed to computedefinitive global tide models, which can then be appliedto data from all future oceanographic missions.


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INSTRUMENTS: UNPRECEDENTED ACCURACY

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The TOPEX/POSEIDON spacecraft, constructed bythe Fairchild Space Company, is a modification of theMultimission Modular Spacecraft (MMS) used forNASA's 1980 Solar Maximum Mission and the laterLandsat-4 and -5 missions. The 2,400-kg (5,300-1b)satellite carries a suite of six instruments, provided bythe United States and France, housed in a specialinstrument module attached to the MMS satellite bus.Power is supplied by a single large solar-cell array.In addition to the dish-shaped antenna used for radaraltimetry, the spacecraft has a variety of communica-tions antennas to link the mission with the NASA Track-ing and Data Relay Satellite System (TDRSS), the CNESDoppler Orbitography and Radiopositioning Integratedby Satellite (DORIS) tracking system, and the U.S.Global Positioning System (GPS) of high-altitude navi-gational satellites.Two AltimetersThe primary instrument is the NASA dual-frequencyTOPEX altimeter, operating at 5.3 and 13.6 GHz, whichdraws upon a long heritage of single-frequency instru-ments extending back to Seasat. Dual-frequencyoperat ion permits correction for the ionospheric-electrondelay effect. This instrument, managed by NASA'sGoddard Space Flight Center (GSFC) and built by TheJohns Hopkins University's Applied Physics Laboratory(JHU/APL), is fully redundant and incorporates wellunderstood, flight-tested technology.The companion TOPEX microwave radiometer,developed by NASA's Jet Propulsion Laboratory (JPL),operates at frequencies of 18, 21, and 37 GHz toprovide estimates of total atmospheric water-vaporcontent. The 21-GHz channel is the primary measure-ment channel; the 18-GHz and 37-GHz channels areused to remove the effects of wind speed and cloudcover, respectively..These data allow reduction of thewater-vapor delay error'to 1 cm (0.4 inch), permitting anoverall altimetric precision of 3 cm (1.2 inches).

The mission also carries an advanced, experimentalsolid-state POSEIDON altimeter, designed by CNESand built by Alcatel Espace, which uses the sameantenna as the NASA altimeter but operates at a singlefrequency of 13.6 GHz. The ionospheric-electroncorrection is provided by a model that makes use ofthe simultaneous dual-frequency measurements of theDORIS tracking system.

Both the operating principles and the expectedperformance of the two altimeters are similar. However,the single-frequency POSEIDON altimeter has only one-fourth the mass, volume, and power consumption of theNASA instrument. Moreover, the telemetry data rate isreduced by a factor of 7 because of more extensiveon-board processing. The CNES instrument is thus aprototype for the satellite altimeters of the future.

Three Tracking SystemsThe NASA tracking system, managed by GSFC, operaby laser ranging to reflectors (built by JHU/APL) arrayearound the altimeter antenna, which permit the satellitebe intermittent ly tracked by a worldwide, ground-basednetwork of 12 laser stations within about 2 cm (less tha1 inch). These data will be used with computer modelsthe Earth's global gravity field (developed at GSFC, atUniversity of Texas at Austin, and in France) for precisiorbit determination and calibration of the altimeters.

The DORIS system determines the satellite's veloby measuring the Doppler shifts of two ultrastablemicrowave frequencies (2,036 MHz and 401 MHz)transmitted by a global network of some 50 ground-based beacons whose positions are known within acentimeters (several inches). Validated by a prototypreceiver launched in 1990 on the French SPT-2 Eobserving mission, DORIS has already provided mothan two million measurements; these have been usto refine data-processing methods and improve gramodels. DORIS is manufactured under CNES managment by Dassault Electronique (receiver), CEIS Espa(beacons), and CEPE and OSA (quartz oscillators).

The mission also carries an experimental GPS receideveloped by Motorola under contract to JPL, in orderdemonstrate GPS capabilities. The GPS system providcontinuous spacecraft tracking with a potential accuracy of 10 cm (4 inches) or better and promises torevolutionize orbit determination for future satellites.

THE TOPEX/POSEIDON SPACECRAFT is a modification of NASAMultimission Modular Spacecraft. A special module houses the sixinstruments contributed by the United States and France.


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GPSTDRS t-- ANTENNACOMMUNICATION

ANTENNA _o i_'_

MMS PROPULSION,POWER. ATTITUDE,CONTROL AND DATA

HANDLING __ SOLAR

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_ ALTIMETERLASER ANTENNARETROREFLECTOR

requency g.

IGlobal Positioning Provides a new tracking data type (range 1227.6 MHz 10 cm 28 kg 29 W i_ceiver ..... -di_Lence"s ion Orbit 1574,4 MHz or better []determination, ---- _ altitude []

NETWORKS OF GROUND-BASED TRACKING STATIONS will determine the posit ion of the satellite within 10 cm (4 inches). The U.S. lasertracking system uses reflect ions of laser beams from the satellite to locate its position; the French DORIS system employs all-weather radio beacons.The U.S. Global Posi tioning System of high-alt itude navigational satel li tes wi ll a lso be used to demonstrate GPS tracking capabil it ies.


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UNITED STATES/FRANCE PARTNERSHIP

The Jet Propulsion Laboratory in Pasadena, California isresponsible for TOPEX/POSEIDON project management,including prelaunch mission planning, development ofthe U.S. sensors, design and development of the U.S.ground data system, and post-launch control andcommunication with the satellite. Within CNES, theCentre Spatial de Toulouse is responsible for participa-tion in mission design and management, developmentof the French sensors and ground data system, andprovision of the Ariane launch-vehicle system andlaunch services.

The mission schedule is divided into five phases:launch, assessment (35 days), initial verification (sixmonths), observation (three years), and extended observa-tion (two more years, with even longer operation possible).Launch and AssessmentTOPEX/POSEIDON will be launched by an Ariane 42Pexpendable launch vehicle from the European SpaceAgency's Guiana Space Center in Kourou, FrenchGuiana. JPL will conduct mission operations, dataacquisition, and data processing; the Centre Spatialde Toulouse will process data from the CNES payload.The critically important assessment phase officiallybegins 18 minutes after launch with the separation ofthe satellite from Ariane and ends 35 days later. Duringthis period, the satellite and sensor systems will bedeployed, activated, and functionally certified, and thesatellite will achieve its operational orbit.

Triple TrackingThe NASA/GSFC laser-ranging system will furnish thbaseline tracking data needed for precise orbit detenation and verification of the altimeter measurements.However, the laser beams cannot penetrate cloudcover. The recently demonstrated CNES DORIS systused in addition to the NASA system, will provide allweather tracking through radio beacons to the onboaDORIS receiver. Since neither system provides conscoverage of the satellite, computer models of the Eagravity field will assist in high-precision orbit determintion; TOPEX/POSEIDON tracking data will, in turn, hto provide still further improvements to these models.The GPS system will supplement the two primarytracking systems as a demonstration of its capabilityfor continuous satellite tracking.Instrument VerificationVerification of the performance of the satellite and itsinstruments is necessary to ensure the validity andintegrity of the scientific data. Although this is a contious task, an intensive verification campaign will beconducted jointly by NASA and CNES during the firsmonths of the mission to calibrate and verify the satedata through comparison with measurements madetwo specially chosen verification sites. This work willbe carried out in parallel with additional, longer-termprograms of data validation conducted through TOGand WOCE.

16DATA FROM TOPEX/POSEIDON will be transmitted to the groundby NASA relay satellites, processed by both NASAand CNES, andarchived in specialdata centers providing international access.

PROCESSING OF TOPEX/POSEIDON DATA from the NASA instments will be carried out at the Jet Propulsion Laboratory, which hprimary mission-management responsibilities for NASA.


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IDEGAUGESillbeusedomakeirectmeasurementsfsea-leveltnumerousemoteslandndcoastalites,ecessaryobservationsrompace.NANSENCEANAMPLINGOTTLESnventedytheNorwegianoceanographerridtjofansenn1900restillanndispensableoolorthesudyfdeep-oceanhysics,hemistry,ndbiology.

NASA Data ValidationPL is instrumenting an oil-drilling platform, providedby Texaco Corporation, 12 km (7 miles) west of Pointonception, California, to obtain data on sea level and

elated parameters. Sea-level measurements will beade by an acoustical device and pressure gaugesounted on the platform. These data, together with data

rom nearby laser tracking sites, will be used to deter-ine the distance from the satellite to the sea surface,hich will then be compared with the altimeter rangingeasurements to calibrate their performance. Othernstruments will include a GPS receiver to measure total

onospheric electron content, together with a surfaceressure gauge and an upward-looking radiometer toheck the altimeter ranging correction.NES Data ValidationNES is instrumenting two islands, Lampione andampedusa, in the Mediterranean Sea. The instrumentonfiguration will include a laser, tidal gauges, deep-ea pressure gauges, a DORIS station, a radiometer,meteorological station, and wind and wave buoys.

hese instruments will verify sea level, atmosphericressure, wind speed, wave height, the water-vapororrection, orbit determination, and other sources ofcean-topography error. Ionospheric corrections will beerified by comparison of the DORIS and GPS measure-ents with those made by the NASA dual-frequencyltimeter, as well as through data furnished indepen-ently by a European ground-based radar system.

Later Mission PhasesThe observational phase begins at the completion ofinitial verification and extends until the nominal end ofthe mission, three years after launch. The prioritiesduring this phase are the recording of observationaldata and maintenance of the scientific capability of thesatellite. If good performance continues and funding isprovided, the TOPEX/POSEIDON mission will be ex-tended for an additional two years or more. The space-craft will carry sufficient fuel for a full decade of operation.Data Distribution and ArchivingThe primary mission product is the Geophysical DataRecord (GDR). During the initial verification phase,activity will focus on data validation; altimetry data willbe processed into interim GDRs and distributed to thescience team for study. By the end of this phase, allthe geophysical measurements will be calibrated andverified, and the parameters necessary for the produc-tion of the final GDR will be approved.The final GDR will be distributed both to the missionscientists and the broader scientific community. As thecomplete record of TOPEX/POSFIDON observations,the GDR will include sea-height measurements and allthe corrections applied to them, as well as data on waveheight, wind speed, and satellite altitude and location.Both interim and final GDRs will be available throughNASA's EOS Physical Oceanography Distributed ActiveArchive Center (PO-DAAC) at JPL and the French datacenter, AVlSO. 17


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SCIENTIFIC UNDERSTANDING

Through the work of an international science team, theresults of TOPEX/POSEIDON will be widely shared,analyzed, and interpreted to increase our knowledge ofglobal ocean circulation and its role in climate change.Selection and RoleWhen TOPEX/POSEIDON was still in its early planningstages, NASA and CNES selected thirty-eight scientiststhrough a competitive Announcement of Opportunity toplay leadership roles in the analysis and interpretationof mission data. They represent nine countries: sixteenare from the United States, thirteen are from France,and the remainder are from Japan, Australia, SouthAfrica, Germany, Norway, the Netherlands, and theUnited Kingdom.

These men and women, together with their more than160 research collaborators, form the TOPEX/POSEIDONscience team, which holds primary responsibility for theachievement of mission science goals. However, theentire TOPEX/POSEIDON data set will be made avail-able to the international scientific community to supportadditional investigations, opening a new spectrum ofresearch efforts directed at an understanding of globalclimate change.Science-Team InvestigationsTOPEX/POSEIDON will return the first accurate topo-graphic data on ocean-basin scales, permitting thoroughinvestigations of circulation in the Pacif ic, At lantic, Indian,

and Southern oceans. When combined with the results oseagoing measurement programs, these data will alspermit global-scale studies of the interaction of activesurface circulation with slow-moving deep waters--the kto understanding long-term climate variability.

Investigation of the Southern Ocean will be of parti-cular interest. The Antarctic Circumpolar Current playa fundamental role in the heat balance of the WestAntarctic Ice Shelf; if this enormous ice mass shouldmelt and break away from the underlying bedrock asa result of global warming, sea level would rise by 4 t5 m (about 15 feet). TOPEX/POSEIDON will greatlyadvance our understanding of this remote region andimprove our ability to predict the fate of the ice shelf.

Tropical-ocean studies will focus on the causes ofPacific sea-level variations and the disastrous El Ni_oclimate events. Other investigations will refine ourknowledge of the Earth's gravity field and its effect onocean topography, permitting more accurate modelsboth gravity and ocean circulation to be developed.

Many mission scientists are also active participantsthe Tropical Ocean and Global Atmosphere programand the World Ocean Circulation Experiment. By mergingthe satellite observations with TOGA and WOCE find-ings, they will establish the extensive data base needefor the quantitative description and computer modelingof ocean circulation. The ocean models will eventuallybe coupled with atmospheric models to lay the foundation for predictions of global climate change.

18SEAGOING EXPEDITIONS conducted by many nations will comple-ment the TOPEX/POSEIDON space observations. Here, U.S. scientistsmeasure the temperature and salinity of Antarctic currents.

COMPUTER TECHNOLOGY has revolutionized oceanographic datanalysis and interpretation. Here, French scientists analyze resultsfrom computer models of global ocean circulation.


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TOPEX/POSEIDON Science Team Leaders. --:u - Name Institution Investigation ,,,,,F. Barlier Groupe de Recherches de G_odesie Spatia e, France-T_G. Born University of Colorado, USA.....C. Doucher Institut Geographique National, France

_e ' Australian Institute of Marine Sc!e_+es, Australiai_:_:__._._zenave Groupe de aecherches de G_od_sie Spatiale, France__o_:_Chel!on ........... Oregon State University, USA"'_"_-Chi.Jrch C0mmor_w-ealthScien{if_Cand_]ndustrial

Research Organization, AustraliaP. De Mey Groupe de Recherches de G(_od_sie Spatiale, FranceY. Desaubies Institut Francais de RecherChep0urI'Exploitation de la Mer, FranceB. Douglas NOAA/National Ocean Service_ USAL.- L. Fu Jet Propulsion Laboratory, USAM. Grundlingh National Research Institute for Oceanography,South AfricaS. Imawaki Kagoshima University, Japan

The Western Mediterranean SeaOcean GyresTerrestrial Reference SystemsThe North-Australian Tropical SeasMarine GeophysicsThe Antarctic Circumpolar CurrentThe South Pacific, the Southern, andthe Indian OceansData Assim lat ion by Ocean ModelsThe Western Equatorial Atlantic OceanOcean Dynamics and GeophysicsGyres of the World OceansThe Oceans around South Africa

The Western North Pacific Ocean .2..;, _i _..i+i..._.i._....i.......... ..Columbia University, usa .......................... The Tropi6&i_;Ctia,qt|cOcea, n ............. -_:i

L;. Le Provost Instiiut _cl:eM_ca6ique de ;(3renoble, France ....... " Global..................cean Tides _:__' ....._,_!'_ii_ + . . Jet Propulsion Laboratory, USA Heat Balance of Global Oceans.Rii_[J'_as _ " " - University of Hawaii, USA ..... ' Tropical Ocean DynamicsY. Mesar_l Groupe de Recherchesc]e Geodesie Spatiale, France Geophysical Validation of Altimetry

"ii_+Merle .. .. .. .. ... .. .. .. ... .. .. . Universit_ Paris VI France_J_Minster GrouDe de Recherches deG6od6s e Spat ale, France Mesoscale and Basin-scale Ocean. .. ... .. .. .. ... .. .. .. ... .. .. .. ... .. .. .. ... .. .. .. ... .. .. .. ... .. .. .. ... .. .. ... .. .. .. ... .. .. .. ... .. .. .. ... .. .. .. ... .. .. .. ... .. .. .. ... .. .. .. ... .. .. .. ... .. .. ... .. ..Variabi lity _-i! --_Se][___i_ _+............................lXlavat_#i__nograp]_ica6d A_t-_(_i_'do - ... ... .. .. .. ... .. .. . The Mia:C_t _sae Westerr_Bou}ida_;_,i_>_'_-_,i!14:. _esear_ll _Eaboratory USA Currentsl iD__156i;s .............. Alfre# _egener Instff(Jte6rl% a-rand ocean Circulation Modeling.":_=_: ...... _.--:--.-........... Manne Research Germany

M. Ollitrault Institut Francais de Rechef#6e pour'/''_'_ ;___ .... The South Atlantic Ocean

i,,_L_Pe,._ersson , * University of Bergen: Ki6rway i__i _ The Nordic Seas _id_Picaut Gr,upe SURTROPAC, ORg_:, #Pance ............ The Tropical Pacific Ocean - ._- _

R. Rapp Ohio b_ate university, USA .... Mean Sea Surface and Gravity:--i_::Sanchez NASA/Goddard Space F:iight (_enter, USA Global Ocean Tides

#,_Segawa University of Tokyo, Japan Marine Geodesy and Geophysics :+: .'[A._'_ou.r.!a_........_ _ . Groupe de Recherches de_od_s_e Spatmle, France Plate Mobons _!

T. Strub Oregon State University, USA - Equatorial and Eastern Boundary +".................. CuffeNsIZ_._C_-:-I_j.T&T- ...... NOAA/Nat_'_onaO--'ceah-se'_i'ce,'-+_'O_A.................................................hePa-cif'iccear_......:_::.._._.Tapiey.. UniversiiYfTexas-&tAOSii_;'O_-A"I___ '_ ............ OceanSUrfaCe Topography "_ P.TEPIis - Institut de Physique du (31obeidelSa[i s, Franc_ AIt metry Analysis with Sea Floor-_-:=- ...................=- -._ . : .... : :_-. ---:-__-:_-.... -: _...-:.._.._::-_........_.......... Electric Datai_r -......................niversitY 0f [Sol0raci0_:LJ_S;.............:_fw:_"f........- OceanicEffe-cison_e'"Ea_h;s'I-n_r__i[ _, Wakker ' Delft University-of rechnoi0gy2 Netherianas ' " Orbit_Comp.utation_ add Sea!Surface : -_..--/--_...._ +-._ .............. Modeling __

P.Woodworth Proudman Oceanographic Lab6i',_io_,nit/e-dingdom MarineResearch......... C?W_msch . Massachusettslnstitut_ of:Te:+h+noi6gy, USA "":. - Global Ocean Circulation "


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Man is challenged by the voice within him, the voice outof the whirlwind of consciousness, to seek and to knowall he can. Knowledge of the air and the sea and the sofidEarth, and of our fellow creatures who share this planet,increases our ability to use the Earth wisely and well.Roger Revetle, 1909-1991


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topex poseidon a united states france mission. oceanography from space the oceans and climate

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