grace gravity recovery and climate experiment

8/7/2019 GRACE Gravity Recovery and Climate Experiment

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ACKNOWLEDGEMENTSContent Developed By: Alan Ward

Designed By: Winnie Humberson

Edited By: Margaret Baguio,Bill Bandeen, Charlotte Griner, Michael King, Margaret Srinivasan.

Technical Input Provided By: Christoph Reigber, Rick Fitzgerald, Frank Flechtner, Bob Garnett, Marc Imhoff, Byron Tapley,Srinivas Bettadpur, John Ries, Don Chambers, Ab Davis,Mike Watkins.

Front Cover Foreground: These images show three different views of the Earth's geoid-a surface of equalgravitational potential that, over the ocean, closely follows the sea surface.Thegeoid was determined from data collected by previous satellite missions.Amongthese satellites is the Challenging Minisatellite Payload (CHAMP), the forerunner toGRACE, launched by the German Georesearch Center GeoForschungsZentrum Potsdam(GFZ). Similar products will be developed using data from GRACE with an expectedimprovement in accuracy of several orders of magnitude. These imageswere providedcourtesy of GFZ. The illustration of the GRACE satellites was provided by Chris Meaney.

Front Cover Background: This day/night image of the Earth was produced using a combination of data fromthe Moderate Resolution Imaging Spectrometer (MODIS) sensor aboard NASA's Terrasatellite (daytime portion), and the city lights image produced using Operational line Scan(OLS) data from the Defense Meteorological Satellite. The daytime side of the Earth in thisimage was made using surface reflectance data from MODIS. The data was compositedover a period of several months during the Northern Hemisphere of summer 2001.(Image courtesy of Robert Simmon and the MODIS ScienceTeam.)

Back Cover: This isa photograph of the two identical spacecraft in the environmental testfacility at Industrieanlagen-Betriebsgesellschaft GmbH (IABG) in Ottobrun, Germany. Whendeployed into space, the two craft will be upright inside the launch vehicle. After launch,they will fly 500 km above the Earth and will be separated by 220 km.The photograph wasprovided courtesy of Astrium GmbH.


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WHAT IS IT?You CAN'T SEE IT!

You CAN'T SMELL IT!You CAN'T TOUCH IT!

BUT IT'S THERE. IN FACT, IT'S EVERYWHERE.

While gravity is much weaker than other basicforces in nature, such asmagnetism andelectricity, its effects are ubiquitous anddramatic. Gravity controls everything from themotion of the ocean tides to the expansion of the entire Universe.To learn more aboutthe mysteries of gravity, twin satellites named GRACE-short for the Gravity Recoveryand Climate Experiment-are being launched to make detailed measurements of Earth'sgravity field.This experiment could lead to discoveries about gravity and Earth's naturalsystems,which could havesubstantial benefits for society and the world's population.

The GRACE mission will be the inaugural flight of NASA's Earth SystemSciencePathfinder Program (ESSP).A component of NASA's Earth ScienceEnterprise (ESE),theESSPmissions are intended to address unique, specific, highly focused scientific issuesand provide measurements required to support Earth science research.

The ESSPmissions are an integral part of a dynamic and versatile program consisting ofmultiple Earth system science space flights.The ESSPprogram is characterized by rela-tively low- to moderate-cost, small-to medium-sized missions that are capableof beingbuilt, tested and launched in short-time intervals. Eachmission is led by a PrincipalInvestigator (PI),who is responsible for all elements of the mission,from ensuring thescience accuracy to making sure the mission stayson budget and on time. ESSPmis-sions are capableof supporting a variety of scientific objectives related to Earth sci-ence, including the atmosphere, oceans, land surface,polar ice regions and solid Earth.Investigations include development and operation of remote sensing instruments andconducting research using data returned from these missions.Subsequent satellitelaunchesare planned over the next few years,all of them focusing on the atmosphericsciences.


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. . .. . .~t-tThis ~6\is a three ,,~dimension- ~ 1 J ul renderingof the Earth's m p l V 1.7geoid magnified a J rr0./' .1"thousand times, with the por- .( ~ "/ .tion due to the Earth' oblateness Iremoved. The magnification highlights smaller scale variations in gravitational potential that are caused by the uneven distribution of massover the surface of the Earth.The GRACE mission will measure these fluctuationswith unprecedented precision. (Image courtesy of Jet Propusion Laboratory.)

W H YA M iss ion To S tudy G rav ity?I f Earth were a smooth sphere composed of similar elements or ingredients, therewould be no need for a GRACE mission; the assumption made in most introductoryphysics courses that the acceleration due to Earth's gravitational field has a constantvalue would indeed be correct-end of story. However, previous observations haveclearly demonstrated that our Earth isn't smooth and homogeneous and it really isn'teven a sphere! What's more, the images shown above are just instantaneous snapshotsfrom one moment in time. The reality is that the gravity field is continually changing,mostly due to variations in water content as it cycles between the atmosphere, oceans,continents, glaciers, and polar ice caps. By far the largest "lump" is the flatteningobserved at the poles-the Earth's oblateness. The above profiles have removed theportion of the response due to oblateness in order to focus on the smaller anomaliesthat exist. GRACE will reveal the broad features of the Earth's gravitational field overland and sea; it will also allow for these smaller scale features to be identified and stud-ied with unprecedented accuracy, and it will show how the Earth's gravity field varieswith time.


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G R A C E 2 0 0 2 :A S cien tific G eo de syGravity is the invisible force that pulls two masses together. The branch of science deal-ing with obtaining precise measurements of the Earth, mapping points on the surface,and studying its gravitational field is known as geodesy. Producing a precise model ofthe fluctuations in gravity over the Earth's surface has proven to be a formidable task.Currently, data from several dozen satellites must be combined to produce a model ofEarth's gravitational field. These models do a good job at replicating the large-scale fea-tures of Earth's gravitational field but cannot resolve finer-scale features or accuratelydescribe the small month-to-month variations associated with the hydrologic cycle. Theunique design of the GRACE mission (twin satellites flying in formation) is expected tolead to an improvement of several orders of magnitude in these gravity measurementsand allow much improved resolution of the broad-to finer-scale features of Earth'sgravitational field over both land and sea.

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-100 -50 o 50 100This is a plot of the Earth's geoid (surface of equal gravitational potential) produced by the Earth GravitationalModel (EGM96), one of many models used for gravity studies. The SpaceGeodesy Branch of NASA GoddardSpace Flight Center worked in collaboration with the National Imagery and Mapping Agency (NIMA) andTheOhio State University (OSU) to develop this model. Data from GRACE isexpected to allow for a quantumleap of several orders of magnitude in the precision and resolution of the geoid. The improved rendering ofthe geoid will have benefits for many disciplines that rely directly or indirectly on precise measurements of thegravity field, including disciplines that study the Earth's climate.


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The distribution of massover the Earth is non-uniform. GRACE will determine thisunevenmassdistribution by measuring changes in Earth's gravity field.The term massrefers to the amount of a substance in a given space,and is directly correlated to thedensity of that substance.For example, a container filled with a more dense material,like granite, hasmore massthan that samecontainer filled with water. Becausemassand density are directly related, there is also a direct relationship between density andgravity.An increase in density results in an increase in mass,and an increase in massresults in an increase in the gravitational force exerted by an object. Density fluctua-tions on the surface of the Earth and in the underlying mantle are thus reflected invariations in the gravity field.As the twin GRACE satellites orbit the Earth together,these gravity field variations cause infinitesimal changes in the distance between thetwo.These changeswill be measured with unprecedented accuracy by the instrumentsaboard GRACE leading to a more precise rendering of the gravitational field than hasever been possible to date.

This schematic diagram illustrates thedifferent forces that redistribute masson the surface of the Earth. Sincemasschanges and gravity changes arecorrelated, the data retrieved by theGRACE satellite should help scientistsbetter quantify these movements andtheir impact on the Earth's climate.(Image provided by GFZ.)

GRACE will do more than just produce a more accurate gravitational field plot, howev-er.The measurements from GRACE haveimportant implications for improving theaccuracy of many scientific measurements related to climate change.Substantiveadvancesin the interpretation of satellite altimetry, synthetic aperture radar interfer-ometry, anddigital terrain models, covering large land and ice areas used in remotesensingapplications and cartography, will result from the improved gravitational fieldmeasurements provided by GRACE.These techniques provide critical input to manyscientific models used in oceanography,hydrology, geology and related disciplines,and,for this reason, the Earth Sciencecommunity eagerly anticipates the GRACE launch.The next few pagespresent some of the expected scientific applications.


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TRACKINGWATERMOVEMENTONAND------BENEATH EARTH'S SURFACE

Perhaps the most interesting and least well-measured cause of fluctuations in theEarth's gravitational field is movement of water over the surface of the Earth. The gravi-tational data collected by GRACE will be combined with data from other NASA satel-lites, aircraft and ground-based measurements to study the movement of liquid waterover our home planet with a level of detail never before possible. Water moves in sig-nificant quantities throughout the Earth's hydrologic cycle (see diagram), and at a rapidrate relative to other processes that redistribute mass over Earth's surface.

The gravitational variations observed by GRACE are primarily attributable to the sea-sonal and interannual movement of water throughout the hydrologic cycle. This meansthat by combining measurements from GRACE with measurements taken on theground, scientists will be able to improve their models of water exchange between theocean and land surfaces-through rainfall, deep soil moisture, and runoff. This can bedone from continental scales down to regional scales of a few hundred kilometers.

This diagram illustrates the hydrologic cycle and shows how water in solid, liquid and vapor forms,circulates over, under and above the surface of the Earth. Gravity fluctuations correlate with varia-tions in the density of the land surface below. These illustrations can be used to track water move-ment. In effect, gravity becomes a mechanism to track water movement our eyes cannot see.GRACE data may lead to the identification of new sources of fresh water-which isof particularinterest to populations located in arid regions of the Earth.


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TRACKING CHANGES IN ICE SHEETS_____ AND CHANGES IN GLOBAL SEA LEVEL

The question of whether ice caps are shrinking or growing is important for climatechange studies. As the area covered by ice sheets decreases, the surface area of theEarth's oceans increases, leading to increased heat absorption and rising temperatures.This, in turn, melts more ice and contributes to global sea level rise. Data provided byGRACE will help scientists better understand how ice sheet mass is changing and theimpact the changes are having on global sea level. GRACE data, when combined withheight variations measured with ground-based, aircraft, and satellite instruments, likethe Geoscience Laser Altimeter System (GLAS) on the Ice, Clouds, and Land ElevationSatellite (lCESat), will allow for improved computations of ice sheet mass balance. Alsoby combining the GRACE gravity field measurements with surface elevation measure-ments (such as from radar altimeters), scientists will be better equipped to distinguishbetween the mean ocean level changes due to thermal expansion and those due toactual redistribution of water.

The issue of long-term changes in ice sheet mass isan important one for cl imate studies, and data fromGRACE should help scientists quantify these changes.The diagram il lustrates the idea that as the ice sheetmelts, the increased surface area of open waterabsorbs more heat, raising temperatures, meltingmore ice, and also contributing to sea level rise.

GRACE will be able to precisely measure very smallchanges in Earth's gravitational field that result fromchanges in the mass of ice sheets. Shown here iscomputer generated image of Greenland that showshow the ice mass is changing. Blues indicate areaswhere the loss of ice isgreatest, and yellows indicateregions that are apparently thickening. Gray areasindicate no significant change in ice thickness.


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Satellite altimeters such asthose onTOPEX/Poseidon and Jason-l can meas-ure the total mean sea level change, asillustrated here with TOPEX/Poseidondata from 1993 to the middle of 2001.However, the altimeter alone cannot dis-tinguish between what portion of theobserved anomaly can be attributed towarming of the ocean and what part isdue to water being added from meltingice sheets. Data provided by GRACE,used in conjunction with altimeter meas-urements like those shown here, shouldhelp scientists better distinguishbetween these two phenomena. (Imagecourtesy of Don Chambers, Universityof Texas,and Steve Nerem of theUniversity of Colorado.)

For example, data from GRACE is expected to greatly improve scientists' understanding ofhow much global sea level is being impacted by a phenomenon known as postglacialrebound. Postglacial rebound refers to the slow rebounding of the Earth's crust that isoccurring now that the weight of the ice from the last Ice Age is no longer present. Therebounding of the crust can affect relative sea level at the coastline in a manner that variesfrom place to place. This can confound efforts to determine how much of the overallobserved change in sea level is actually caused by thermal expansion of the oceans result-ing from global warming.

Postglacial rebound accounts for the verticalmovement of land in many parts of theworld. These shifts, which have been continu-ing since the last IceAge ended, affect relativesea level at the coastline in a manner thatvaries from place to place.Such movementscan confound tide gauge records obtainedfrom coastal sites and thus complicate effortsto track the overall change in global sea level.The data retrieved by GRACE will be com-bined with the previously mentioned altime-ter readings to get a better handle on howmuch of the perceived change in sea level isattributable to the phenomenon of postglacialrebound and how much might be attributedto global warming. This particular imagedepicts the predicted time rate of change ingeoid height expected to result from post-glacial rebound as predicted by the ICE-4G(VM2) Toronto model (Image provided byW.R. Peltier, University of Toronto, Canada.) Change in Geoid Height Over Time (mm/yr)

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STUDYING OCEAN CURRENTS BOTH NEAR--THE SURFACE AND FAR BENEATH THE WAVESI f the ocean were motionless, the sea surface would still have hills and valleys. Thiss ta ti c t opography of the ocean is caused by the fluctuations in the gravity field caused byvariations in density over the surface of the ocean. Because the oceans are always inmotion-swept by winds, seething with waves, and circulating in gigantic currents andgyres driven by surface winds and by the rotation of the Earth-the shape of theocean's topography cannot be determined by static gravity measurements alone. Anadditional component, known as the d yn am ic to po gr ap hy must be accounted for to pre-cisely measure the shape of the ocean surface at any given time. Although dynamictopography only accounts for about one percent of the total variation of the ocean'stopography, it is an extremely important component for it contains important informa-tion about the speed and direction of ocean currents. Consequently, scientists wouldvery much like to improve their ability to measure and track changes in dynamic topog-raphy.

This image shows sea surface topography data from TOPEX/Poseidon. When suchdata are combined with the precise geoid measurements expected from GRACE,scientists should be able to study currents from satellites using a technique knownasdynamic topography. This image was acquired in March 2001 in the aftermathof earlier EI Nino/La Nina conditions in the tropical Pacific. Areas toward theblue end of the spectrum represent anomalously low sea surface heights forMarch and areas toward the red end represent anomalously high values forMarch.

GRACE data should help scientistsunderstand the role ocean currentsplay in regulating Earth's climate. Thisimage from the MODIS sensoronboard the Terra satellite shows theGulf Stream (red), a surface oceancurrent that plays a critical role inredistributing heat from the tropics tothe poles. It has a moderating influ-ence on the climate of NorthwestEurope, which would otherwise bemuch colder-similar to other loca-tions at the same latitude, such asGreenland. This particular image wasobtained on May 2, 2001 and wasprovided by l.larn Gumley of theMODIS Atmosphere Team,Universityof Wisconsin-Madison CooperativeInstitute for Meteorological SatelliteStudies.


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Two pieces of information are needed to make a dynamic topography measurement,sea surface height and a model for the Earth's geoid. Since the impact of dynamictopography on the shape of the ocean surface is quite small compared to the impact ofthe static gravity field, the measurements must be extremely precise and of high resolu-tion. Satellite altimeter measurements from TOPEX/Poseidon and Jason-1 can providethis level of detail for sea surface height measurements, but to date, there are no geoidmeasurements with sufficient precision and resolution to allow satellite-derived meas-urements of dynamic topography. Scientists expect the data returned by GRACE toprovide the precise, high resolution geoid measurements required to measure dynamictopography affects. They hope to use these data to better understand the role thatocean currents play in regulating the Earth's climate.

Ocean currents regulate Earth's climate by transporting heat from equatorial regions tothe poles. The Earth's oceans essentially serve as huge heat reservoirs and currentsboth at the ocean surface and far beneath the waves-known as deep ocean cur-rents-act as massive "conveyor belts" that distribute this heat over the surface of theEarth. Consequently, any changes in these equator-to-pole "conveyor belts" could signif-icantly change the weather all around the globe. Scientists are particularly hopeful thatGRACE will help them better understand the role that deep ocean currents play inregulating climate. Until now, deep ocean currents have remained shrouded in mysterysince they have been very difficult to measure. However, the design of the GRACEsatellites makes them ideally suited to examine deep ocean currents and should lead toimproved ability to track them and study their impact on global climate.

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This map illustrates the heat exchange flux over Earth's oceans for a five year period. A negative value indicatesan area where the ocean is losing heat while a positive value indicates an area where the ocean isabsorbing heat.Ocean heat exchange flux plays a crucial role in regulating the Earth's climate and data from GRACE should helpimprove the accuracy of these measurements


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____ TRACKING CHANGES IN THE SOLID EARTH

GRACE data will also improve our understanding of Earth's lithosphere-solid Earth.More precise measurements of the Earth's gravity field, such asthose provided byGRACE, should lead to an improved understanding of the dynamics of the inner struc-ture of the Earth.The characteristics of the continental lithosphere are the subject ofvarying opinions. Questions remain regarding its thermal and compositional nature,thickness, and mechanical properties. GRACE can track variations in the density of thelithosphere by looking at its impact on the Earth's gravitational field.Thus,data fromGRACE will be used to compare competing models of the lower mantle viscosity andcontribute to the improved understanding of mantle convection.

USING GPS RECEIVERS AS ATMOSPHERIC___ LIMB SOUNDERS

In addition to the gravity meas-urement, the GRACE mission willalso employ an innovative newtechnique that usesits extremelyprecise Global Positioning System(GPS)receivers asatmosphericlimb sounders. limb soundingmeasuresthe excessdelay ofradio wavesasthey passthroughand are bent by the Earth'satmosphere. Justas light isrefracted or bends as it enterswater becauseof the slowerspeed of light in the water, GPSsignalsare refracted asthey passthrough the atmosphere.The pre-cisedegree of bending (and hencethe precise excessdelay) is highlydependent on atmospheric pres-sure,temperature, and moisturecontent. These can all be estimat-ed from precise observations ofthe changingsignaldelay as the

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This diagram,originally provid-ed by GFZ, illus-trates how theoccultationprocess works.The GRACEsatell ite tracks aG PS satellite as it"rises" and "sets"through the limbof the atmos-phere, while asecond G P Sserves asa refer-ence point.

GPSSatellite rises or sets.The ionosphere, which is the name applied to the ionizedupper atmosphere above 100 km altitude, also affects the speed of radio wavesaccording to the density of electrons in the ionosphere. It turns out that GPSsignaldistortion caused by the ionosphere varies as the radio frequency of the signalchangesbut that atmospheric distortions remain constant with changingradio fre-quency.Thus, this technique not only measuresthe distortion of the GPSsignalbutit can also distinguish how much of the observed distortion iscaused by the atmos-phere and how much is caused by the ionosphere.

As GRACE orbits the Earth, it will use forward and aft looking antennas to meas-ure the signalsfrom the slower orbiting GPSsatellites as they appear to rise or setbehind the Earth's limb. GRACEwill also measureGPSsignalsfrom higher elevationGPSsatellites that are not affected by the atmosphere (illustrated in the diagram)asa standard of comparison.

By observing how the GPSpath delaysthrough the atmosphere change,GRACEwill create profiles of pressure,temperature, and humidity for the atmosphere andmeasure the variability of the ionosphere down to 100 km altitude. It is anticipatedthat the limb sounding technology aboard GRACE will extend and complementother spaceborne atmospheric sensors.GRACE will provide about 500 limb sound-ing measurements per day,unaffected by weather conditions (such asclouds andstorms) that hinder or block other satellite sensors.The GRACE limb soundingmeasurements will be used to determine the effectivenessof limb sounding inimproving numerical weather prediction in a cost effective manner.

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GPSsounding of the continuously changingionosphere aims to advance our under-standing of the Sun'sinfluence upon the Earth's environment, including its effects on cli-mate, weather, radar and radio communications. Limb sounding will also be studied as ameansof detecting rapid vertical changesof the Earth's surface suchasvolcanic explo-sions,earthquakes, tsunamis and other such phenomena,which are thought to causedisturbances in the ionosphere.

D A T AP ro c es sin g and A rch iv in gSystemdevelopment, data processing and archiving are performed in a shared ScienceData System (SDS) between the Jet Propulsion Laboratory (JPL),the University of TexasCenter for SpaceResearch(UTCSR), and the GeoForschungsZentrum Potsdam (GFZ).Telemetry data are received by the GRACE Raw Data Center (ROC) at DeutschesZentrum fur Luft und Raumfahrt (DLR) in Neustrelitz, Germany-illustrated in theInstrumentation section.

The first level of data processing is performed at J P L , where sensor calibration factorsare applied, the data are correctly time-tagged, quality control flagsare added,and thedata sample rate is reduced from the high rate data of previous levels.Data are thensent to UTCSR and GFZ, where the mean and time variable gravity field isderivedfrom the calibrated and validated data. Data are archived for distribution at JPL'sPhysicalOceanography Distributed Active Archive Center (PO.DAAC) and at GFZ'sInformation Systemand Data Center (lSDC).15 GRACE data include 3D-dayestimatesof gravity fields, aswell asprofiles of air mass,density,pressure, temperature, watervapor,and ionospheric electron content.

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I N S T R U M E N T A T I O Na nd M e a su rem e nts

A sneak peak at the innards of a spacecraft! For the GRACE mission, the spacecraft itself is the main instru-ment. This picture shows the GRACE spacecraft with the solar panels removed and gives a clear view of thevarious components. A diagram of the interior components of the space craft appears on the inside backcover.

a schematicdiagram of the twoGRACE satellites inorbit with the K-band microwavebeam connectingthem and preciselytracking fluctuationsin the distancebetween the twosatellites. The fluc-tuations are used toinfer changes in thegravitational field onthe surface of theEarth below.

GRACE isdifferent from most Earth observing satellite missions-Terra andAquafor example-because it will not carry a suite of independent scientific instruments onboard.The two GRACE satellites themselves act in unison asthe primary instrument.Instantaneous changesin the distance between the twin satellites are used to make anextremely precise gravitational field measurement.

To measuregravity from space,the two identical GRACE satellites fly in the same orbit-one 220 km (137 miles) aheadof the other. As the pair circles the Earth, areas of slightlystronger gravity will affect the lead satellite first, pulling it awayfrom the trailing satellite.The uniquely designedSuperstar Accelerometer is used to distinguish gravity influencesfrom those of air drag.The K-band ranging instrument is capableof measuringthe distancebetween the satellites with a precision better than the width of a human hair.By monitor-ing this distance, GRACE will be able to detect fluctuations in the gravitational field and,therefore, differences in the density of the Earth's surface beneath the satellites.The datawill be combined with GPSdata to produce a map of the gravity field approximately oncea month.

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Th is diagram illustrates the flight con-f igu ration and ground support fo r theGRACE m ission . Fluc tuations in densi-ty of the Earth 's surface result in verysm all changes in the d istance betweenthe two satellites, wh ich are m easuredw ith very h igh precis ion by the K-band rang ing sys tem . The S-band relay(shown pro trud ing from the bottomof each satellite) allows for com muni-cation w ith surface tracking stat ions.The GPS satellites are used as refer-ences to determ ine the prec ise loca-t ion of the tw o satellites in orb it andallow for the creation o f gravitym aps-approxim ately once a m onth.

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E D U C A T I O Nan d P ub lic O utreachThe GRACE mission benefits fromand will build upon the interest inoceanography that hasbeen grow-ing in recent years with the recog-nition of the E I Nino/La Nina phe-nomenon. GRACE provides astrong basisto draw public andstudent interest since new,extremely accurate models of theEarth's gravity field will be generated.

The TexasSpaceGrant Consortium (TSGC)works in partnership with the mission team todevelop and implement GRACE education and outreach.GRACE master teachers were selected from across the nation through an applicationand review process.These master teachers havecreated interdisciplinary K-12 educa-tional materials that meet the National Educational Standardsto support the GRACEmission in the areas of Satellites,Gravity,Weather/Climate/Atmosphere, Oceans,andGRACEgeneral mission facts and objectives.These master teachers havetrained hun-dreds of teachers about the GRACE mission impacting tens of thousands of students,and the activities havebeen tested in classrooms across the nation.When GRACElaunches,the activities will be availableto all educators on the TSGC web site,http://www.csr.utexas.edu/grace/education/. In addition to classroom activities, an on-line GRACE activity guide,coloring book, and on-line quiz will be availablefor all inter-ested.

Animations, photos and interviews with mission personnel will be availableto all tradi-tional media outlets-television, print, and radio. Computer animation, showing thespacecraft deployments, orbit ground tracks, data collection, and real world applicationsof the data, is available.The GRACE web site will also document, publicize,and expandthe mission's sciencedata including computer animations, downloadable educationalmaterials, a frequently askedquestions (FAQ) archive,and evaluation/ suggestion formsfor feedback.This information will be updated regularly.

GRACE will revolutionize the way we look at Earth, providing new benefits for six bil-lion people living on this beautiful, blue planet.Through youth development and educa-tion we will enhance our understanding of this dynamic and living world we call home.

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S p ac e C en te rO ff ic e N at io na l d 'E tu des e t d e R ec he rc he s

Aerospat ia leT ec hn ic al U n iv ers it y o f M u nic hU niv ers ity o f B on nU niv ers ity o f L yo n

H am pto n, V AB ou ld er, C OG re en b elt , M DP as ad en a, C AL au rel, M DC oco a B each , V AH am pto n, V AC am b rid ge, M AC olu mb us , O HP alo A lto , C AA us tin , T X

F ri ed ri ch s haf en , Ge rman yB rem erh av en Germ a n yB o nn , G erm a n yO b erp fa ff en h of en , G erm a n yC o pe nh ag en , D e nm a rkB rem en , G erm a nyP ots da m , G erm a nyT o ulo u se , F ra nc eO t to b ru n n, G erm a nyC o pe nh ag en , D e nm a rkM os co w, R us siaPa ri s, F ra n ceM u nic h, G erm a nyB o nn , G erm a n yL yo n , F ra nc e

Key Par ti c ipantsB . Tapley UTCSR , P rinc ipal Inves tigato r (P I)C . R eigber GFZ, C o -P IA . D av is J PL , P ro jec t M anagerM . Im hoff GSFC , ESSP Pro jec t Sc ien tis tE . Koenem ann DLR , GRACE ManagerR . F itzgerald GSFC , GRACE M ission ManagerM .M . W atkins JPL , P ro jec t S cien tis tu . s . S cie nc e T ea mB . Tapley UTCSR , LeadS . B ettadpu r UTCSRR . Gasparov ic JHUAPLR .S . G ross JPLw .G . Melbou rne JPLD . Po rter JHUAPLC .K . Shum O SUJ. Wahr CUM .M . W atkins JPLC . W unsch M ITE uro pe an S cie nc e T ea mC. R eigber GFZ, L eadG. Balm ino GRGSF. Flech tner GFZK .H . Ilk UBN . Jakowski D LRP . Knudsen KSGY . R icard ULR . Rum m el TUMJ. Schroeter AW lP . Schw in tzer GFZ

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T E C H N I C A LSpeci f icat ionsG e ne ra l S pa ce cr aft In fo rm a tio n (T w in S ate llite s).) Width : 1942 m m.) Leng th : 3123 m m.) Heigh t : 720 m m.) Mass : 4 87 kg p er s ate llite.) D e si gn L ife : 5 y ea rsOrb it Cha ra c te r is t ic s.) Tw o satellites , co -orb iting at near po lar inc linat ions at 300-500 km altitude .) Separated along track by 220 km .K e y C omp on e nts

K -b an d R an gin g S ys te m . P ro vid es p rec ise (w ith in 10 11m ) m easu rem en ts o f th e d is tan ce ch an ge b etw eenth e tw o satellites an d h en ce m easu res th e flu ctu atio ns in g rav ity .S -b an d B oo m. U sed to sen d d ata fro m th e satellites b ack to E arth fo r p ro cess in g.S up e rS TAR Acc e lo r ome t e rs . P re cis ely m e as ures th e n on -g rav ita tio nal ac ce leratio ns a ctin g o n th e s atellite .S ta r C am e ra A ss em b ly . P rec is ely d eterm in es s atellite o rien tatio n b y tra ckin g th em re lativ e t o th e p os itio no f th e s tars .U lt ra S ta ble O s c il la to r. P ro vid es freq uen cy g en eratio n fo r th e K -b an d R an gin g system .C oarse E arth an d S un S en so r. P ro vid es o m ni-d irec tio nal. re liab le an d ro bu st. b ut fairly co arse E arth an dS un tracking . To be used d urin g in itia l acq uis it io n an d w hen GR AC E is o perating in "safe m o de."C en te r o f M ass T rim A sse mbly . P rec isely m easu res o ffs et b etw een th e satellite's c en ter o f m ass an d th e"acc elerat io n-p ro of" m ass , an d ad ju sts cen ter o f m ass as n eed ed d urin g f lig ht .B lac k-Jac k G PS R ec eive r an d In stru me nt P ro ce ss in g U nit. P ro vid es d ig ita l s ig na l p ro ces sin g; m e as ure sth e d is ta nc e c han ge rela tiv e to t he G PS s at ellite c on stella tio n; an d p ro vid es s ec on da ry atm o sp he rico c cu la ti on e xpe riment s.L as er R e tr o-R e fle ctiv e A ss em b ly . P ro vid es m eas urem en ts o f th e G RA CE satellite o rb its relat ive tot er re st ri al t ra ck in g ne two rk s.G lo ba ls ta r S ilic on S ola r C ell A rra ys . C overs th e o uter sh ell o f th e sp acecraft an d g en erates p ow er.T hre e-a xis S ta bilize d A ttitu de C on tro l S ys te m: U ses star cam era an d g yro sen so rs and a co ld -q asn itro gen th ru ster sys tem , w ith m ag neto rq uers fo r f in e co rrectio ns o f sp acec raft p os itio n.1 75 0-A M ic ro pro ce ss or fo r F lig ht C om pu te r. P erfo rm s c alc ulatio ns fo r at titu de a dju stm e nt an d tele m etryprocess ing.* S ee pho tos an d d iagram s on page 14 and 15 fo r deta iled layo ut o f spacecraft .


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S C I E N C EMea su rement Ob je ctiv esThe fo llow ing tab le p resen ts a conc ise sum m ary o f exac t ly w hat m easu rem en ts GRACEw ill o b ta in and the in s trum en ts that are used to ob ta in them , as w ell as how m any space-c raft are in vo lved in each m easu rem en t.

Sc ie nce A pp lica tion M easu rem en t Ins trum en t # o f sp ac ec ra ft n ee de df or m e a s ur em e n tsE arth G rav ity F ie ld In ter-S ate ll ite K -B and M ic row ave 2Ra ng e C h an g e L in kE arth G rav ity F ie ld N on -G rav ita t io nal A ccelerom eter 2Accelerat ionsE arth G rav ity F ie ld Sate llite O rb its B lack Jack GPS Receiver 2

A tm ospheric GPS -to -GRACE B lack Jack G PS R eceiver 1O ccu lta t io n P hase C hange

Th is sc ience data in con junc t io n w ith anc illary data, w ill th en be used to gain es t im ates o fspherica l harm on ic coeff ic ien ts o f the E arth 's g rav ita t io nal po ten t ia l. T hese coeff ic ien ts arenecessary to cons tru c t the 3D -day m aps o f g rav ita t io nal po ten t ia l that w ill b e the end -p roduc t from the p rim ary GRACE m easu rem en ts .

GRAVITY MEASUREMENT ERRORI n te r- s at el l i te R a n ge Mea sur emen t : < 4 u rn @ tw ice/rev

< 1 0 u rn t ot al @ 0.1 to 0.0001 H zNon -G ra v it at io n al A c c el er at io n Mea sur emen t : < 1 0 -1 0 m / s? n ois e (+1 I f )

< 4 x 1 0 -1 2 m / s 2 t on es .P rec is io n O rb it M easu rem en t: 5 cm abso lu te

< 0.2 mm , re la t iv e in p lan e

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S C I E N C EApp lic a tio n s Summa ryThe estimates of the Earth gravity field from GRACE, in conjunction with other space-basedmeasurements, in situ data and geophysical models will be used to determine the time varyingchanges in the massof the Earth's dynamical system due to different geophysical processes asdescribed in detail in this brochure.

S ta tic G r av ity F ie ld M e a s ur em en tsAPP LI CA TION RESO LUT IONO ceanic H eat F lux >1000 kmO cean Curren ts >1000 kmSolid E arth Sciences >300 km

T im e V a ria ble G r av ity F ie ld M e a s ur em en tsA PP LIC AT IO N R ES OLU TIO N T IM E S CA LE C OM ME NT SO ceanic H eat Flux >1000 km Seasonal 30 day estim ateO cean B o ttom Pressure >500 km Seasonal 30 day estim ateDeep O cean Currents >500 km Seasonal 30 day estim ateSea Level R ise >700 km Secu lar 5 year estim ateEvapo - Transpiration >300 km Seasonal 30 day estim ateGreen land/An tarctic Ice Secu lar 5 year estim ate

Seasonal 1 year e s tim ate

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grace gravity recovery and climate experiment

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