TheTerraSAR-XSatellite
WolfgangPitzandDavidMiller
Abstract—TerraSAR-Xisaversatilesyntheticapertureradar(SAR)satellitewithactivephasedarrayantennatechnologyandrepresentsthebackboneoftheGermannationalradarEarthob-servationmission.WithitslargevarietyofdifferentSARimagingmodesanditshighoperationalflexibility,TerraSAR-Xideallyservesthescientificcommunityandusersfromtheindustrialsectorandgovernmentalinstitutions.Theinnovativesatellitesys-temdesigncombinestherichexperiencefrompastGermanandEuropeanSARspacemissionslikeX-SAR,SRTM,ERS1and2,andEnvisatcombinedwithstate-of-the-artEarthobservationbustechnologyasused,e.g.,ontheCHAMPandGRACEsatellites.IndexTerms—SARsatellite,syntheticapertureradar(SAR).
I.INTRODUCTION
EMOTEsensingoftheEarth’ssurfacefromspacebyanactivemicrowaveinstrumentallowsobservationindepen-dentoftimeofdayorcloudcover.Thesyntheticapertureradar(SAR)principleanddevelopmentofkeytechnologiesinspacequalityhaveallowedtosteadilyimprovegeometricresolutionandradiometrysensitivity.
EuropeanspaceborneSARprogramsstartedinthe1980swiththeERS-1satellitecarryingapassiveC-bandantenna.TheX-SARinstrumentastheGermancontributiontotheNationalAeronauticsandSpaceAdministration’sSpaceShuttlemissionsSIR-C/X-SARandtheShuttleRadarTopographicMission(SRTM)followed.Improvedoperationalflexibility,increasedimageresolution,andthetopographicinformationcontentprovidedbythisfirstspaceborneinterferometerim-pressivelydemonstratedthecapabilitiesofX-bandspaceborneradar.ThisexperimentestablishedthebasisforasuccessfulGermanX-bandSARprogramtodevelopkeytechnologiesformodernphasedarrayantennas.ThetechnologydemonstratorofanactivephasedarraySARantenna(DESA),developedintheearly1990s,becamethekeyelementoftheactiveantennaofTerraSAR-X(Fig.1).
TheTerraSAR-Xsatelliteisafurthersteponthepathofincreasingperformanceandalsoprovidesauniqueflexibleser-vicetousers,allowingthedevelopmentofnewSARmethodsandawiderangeofapplications.TheTerraSAR-Xmissionprovides2-DEarthsurfaceimagingwithgeometricresolutiondownto1mandnoiseequivalentsigmazerobelow−19dB.InaformationflighttogetherwithitsfuturepartnerTanDEM-X,
R
Fig.1.TerraSAR-XS/C.
3-DimagingofthecompletelandmassoftheEarthwillbeperformedwithaheightresolutionof2m.
ThispapertreatsthedesignandperformanceofthebusandtheSAR(SectionII),theadditionalpayloads(SectionIII),andsatelliteprelaunchtesting(SectionIV).SectionVdescribesthecurrentin-orbitstatusofTerraSAR-X.Then,SectionVIgivesanoutlookontheTanDEM-Xsatelliteandmission.
II.DESIGNANDPERFORMANCE
A.SpaceSegment
ThetechnologicalfoundationsoftheTerraSAR-Xspaceseg-mentdesignwerelaidinthefinaldecadeofthelastmillenniumwiththesuccessfulSpaceShuttlemissionsX-SAR[1]andSRTM[2],[3]regardingtheradarinstrumentandwiththelowEarthorbit(LEO)sciencemissionsCHAMP[4]andGRACE[5]forthesatellitebus.
Furthermore,theGermanAerospaceCenter(DLR)fundedtechnologystudieswhichplayedanindispensableroleinthereductionofriskconnectedwiththeintroductionofnewSARtechnologiesappliedonTerraSAR-X.OfkeyimportancewastheDESA(X-bandSARAntennaDemonstrator)study,whereaprequalificationmodelofoneofthe12TerraSAR-Xantennapanelswassubjectedtorigorousfunctionalandperformancetesting,andtheTOPAS(TechnologyDevelopmentforOnboardSARProcessingandStorageDemonstrator)study,wherenovelSARdataonboardprocessingandstoringtechniqueswereinvestigatedandprequalified.Onthebasisofthisrichheritage,itwaspossibletorealizeanefficientdesignwhichoffersanexcellentperformance-to-costratio.
TheTerraSAR-Xspacesegmentwasoptimizedforopera-tioninLEOinkeepingwiththerequiredradarperformanceandtechnologycapability(themissionorbitcharacteristicsarelistedinTableI)andiscompatiblewithawiderangeoflaunchersduetoitscompactshapeandlowmass.
ManuscriptreceivedMarch18,2009;revisedJuly21,2009andOctober19,2009.CurrentversionpublishedJanuary20,2010.
TheauthorsarewithAstriumSatellites,88039Friedichshafen,Germany(e-mail:wolfgang.pitz@astrium.eads.net;david.miller@astrium.eads.net).Colorversionsofoneormoreofthefiguresinthispaperareavailableonlineathttp://ieeexplore.ieee.org.
DigitalObjectIdentifier10.1109/TGRS.2009.2037432
0196-2892/$26.00©2010IEEE
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TABLEI
TerraSAR-XMISSIONORBIT
Fig.2.ZenithsideoftheS/Cshowingthesolararray.
Fig.3.RightsideoftheS/C.
Thehexagonaloutershapeofthespacecraft(S/C)efficientlyfitstheshapeofatypicallauncherfairing.Thebody-mountedsolararrayisseenontheleftinFig.2.TheradarantennaisseenontherightinFig.3.Themajorityoftheelectronicunitsare
accommodatedontheoutsideofthebasicstructuralhexagon,whichallowseasyaccessduringintegrationandtest.Themaindimensionsare4880mminlengthand2400-mmmaximumwidth.Innominalflightattitude,theSARantennaoff-nadirangleamountsto33.8◦.Theextensiveuseoflightweightcarbonfiber-reinforcedplastic(CFRP)technologyinasatellitestruc-tureandaSARantennaleadstoalowoverallwetmass(i.e.,includingfuel)ofabout1350kg,includingadditionalpayloads.AllsystemsonTerraSAR-Xaresinglefailuretolerant,andthereliabilityfigure(i.e.,predictedprobabilityofmaintainingtherequiredfunctionality/performance)is0.8after5.5yearsofnominalmissionlifetime.B.SatelliteBus
ThemainsystemperformancecriticaltasksoftheTerraSAR-XsatellitebusaretoprovidetheSARinstrumentwithsufficientenergyandprecisepointingandalsotomaintainastableorbitandadequatethermalconditions.
TheonboardcomputerincludesaprocessormodulebasedontheERC32-MCM(thehighlyintegratedmultichip-moduledevelopmentoftheradiation-tolerant32-bitSPARCV7micro-processor)whichprovides>10millioninstructions/scomput-ingspeed.
Electricalpowerisgeneratedbymeansofa5.25-m2largeGaAstriple-junctionsolararray,storedina108-Ah(begin-of-life)Li-ionbatteryanddistributedovera50-Vdcmainbussys-tem.Underworstcaseseasonalandend-of-lifecircumstancesandassumingasolararraystringfailure,theavailablepowerisstillsufficientforaminimumSARinstrumentoperationof170s/orbitinaverage,with10%margin.However,apartfromasmallregionaroundsummersolstice,whereeclipseswithamaximumdurationof22.4minareexperienced,electricalenergyisabundantsothatSARimagingofupto10min/orbitistypicallypossible.
High-precisionsatellitepointingcontrolanddeterminationareachievedonthebasisofstartrackerslocatedclosetotheSARantennaplanesothatanantennaboresightpointingaccuracyof65arcsec(3σ)isreached.NominalattitudecontrolfollowsanoveltotalzeroDopplersteeringlawdevelopedbyDLR[6].Preciseorbitdetermination(POD)isperformedwithatwo-frequencyGPSreceiverandrawdatapostprocessingonground,whichallowsfororbitrestitutionaccuraciesinthecentimeterrange[7].Asetofhigh-torquereactionwheelsenablesrapidrotationintotheso-calledsunside-looking(SSL)orientation(wherebytheSARlookstotheleftsideofthesubtrack)whichisusedtoimproveaccessofhigh-priorityimagingtargets.InordertopointtheSARantennaintotheSSLdirection,arollmovementof67.6◦isnecessary,whichisachievedinlessthan180s.
Thesatellitebusisequippedwithamonopropellanthy-drazineblowdown-modepropulsionsystemfororbitmain-tenanceandsafe-modeattitudecontrol.Atotalof78-kgpropellantloading,providingadeltav(totalthrust)of105m/s,issufficientforalmosttenyearsofoperation,enablingthesatellitetobekeptwithina±250-morbittubeduringtheentirelifetoallowforrepeat-passSARinterferometry.Theattitudeandorbitcontrolsystemiscompletedbysensorsto
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BASICSARMODEPERFORMANCE
determineattitude(coarseEarthandsunsensor)andattituderates(inertialmeasurementunitandmagnetometer)andalsoamagnet-torqueractuatorforattitudecontrol.
Regardingthermalcontrol,acombinationofmultilayerinsu-lation,radiatorfoils,andsoftware-controlledheatersguaranteesalow-variationtemperatureenvironmentoverallseasons,satel-litemodes,andorientations.Allperformance-sensitiveradarelementsaccommodatedwithinthesatellitebusarecloselythermallyconditionedusingfoilheaterswithinarangeoflessthan1K.
CommandandcontrolofthesatelliteareperformedinanencryptedwaysupportedbyanS-bandtelemetry,tracking,andcommandsystematdataratesof4kb/sforuplink,32kb/sforreal-timehousekeeping(HK)downlink,and1Mb/sforHKdumpdownlink.SARdataaretransmittedtothegroundwithanX-bandsystematanetdatarateof270Mb/s.TheX-banddownlinkantennaismountedona3.3-m-longdeploy-ableboom—theonlymechanismonthesatellite—inordertopreventinterferencewiththeX-bandSARinstrument.ThisconceptenablesforsimultaneousSARdataacquisitionanddatadownlink.
Thebusunitsaregenerallyredundanttoprovidetolerancetoanysinglefailure.Somehardwareis“hot”redundant(i.e.,constantlyactivated)forfastreactionorotherreasons.C.SARInstrument
ThekeycharacteristicfeatureofTerraSAR-Xistheadvancedhigh-resolutionX-bandSARbasedonactivephasedarraytechnology.ThespecifiedperformanceparametersforeachofthethreebasicSARmodes,namely,spotlight,stripmap,andScanSAR,arepresentedinTableII.
1)SARAntenna:TheSARcenterfrequencyis9.65GHz,andthebandwidthisselectableuptoamaximumof300MHz.TheSARantennaiscapableofoperationintwopolarizations,namely,HandV,andconsistsof12panelseachwith32slottedwaveguidesubarrays.Everysubarrayisequippedwith
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Fig.4.TerraSAR-XsubarraywithTRM.
Fig.5.TerraSAR-Xantennapanel.
atransmit/receivemodule(TRM),whichtotalsto384TRMsforthecompleteSARantenna.Fig.4showsaviewofasingleactivesubarray(gluedpairofHandVslottedwaveguideswithitsTRM).Fig.5showsanintegratedantennapanel(arrayof32activesubarrayswithsignaldistributionnetworksandpowerconditioners).
Theapproximatedimensionsoftheantennaare4800mminlength,800mminwidth,and150mmindepth.Thebeamsteeringcapabilityis±0.75◦inazimuthand±20◦inelevation.Ahighlyefficientthermalconditioningsystemguaranteeslowtemperaturegradientsovertheantennasurfaceandalowtem-poralheat-upgradient.Duetothisfeature,continuousimagingtimesofupto10minarepossiblewithoutoverheatingtheantenna.AdvancedCFRPtechnologyisusedforthesubarrays,whichalmostcompletelypreventsthermomechanicaldistor-tionsontheantenna.(See[8]fordetails.)TheTRMsarepoweredbyanacbussystemwhichupconvertsthe50-Vdcofthemainbusinto115V/30kHzac.Theacvoltageisthendistributedtoeachantennapanel,where,onpanellevel,itisdownconvertedandrectifiedtosupplytheTRMswiththenecessaryvoltages.
2)InstrumentUnits:TheantennabeamisdeterminedbythecentralantennacontrolelectronicsunitwhichprovideseachTRMviaadatabussystemwithamplitude,phase,andpolarizationsettinginformationforeachradarpulse.Alargevarietyofpulseformsandbandwidthscanbecreatedduetothefreelyprogrammabledigitalchirpgenerator.Anextensive
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calibrationnetworkcoveringvirtuallyallradio-frequency(RF)pathswithintheinstrumentuptotheTRMsservesasessentialbasisforin-flightsignalcalibration.TheSARdatageneratedbytheinstrumentcanbecompressedfollowinganonlineblockadaptivequantizerschemewithselectableratiosof8:6,8:4,8:3,or8:2beforetheyarestoredinthesolid-statemassmemoryunitwith384-Gbbegin-of-lifeand256-Gbend-of-lifecapacity.Receive,transmit,andcalibrationsignalpathsoftheradarelectronicsandtheantennaarecross-strappedbymeansofare-dundancynetwork.Theinstrumentelectronicsandredundancynetworklayoutalsoallowsforoperationinthedual-receiveantennamode,wheretheechoesreceivedbyindividualazimuthantennahalvescanbeseparatedduringgroundprocessing.Thisfacilitatesalong-trackinterferometryandfull-polarimetricdataacquisition[9],[10].
3)InternalCalibration:TheSARdesignincludesanin-ternalcalibrationschemewherebytransmitsignalsareroutedintothereceivertoallowmonitoringofamplitude/phaseduringoperation(eitherduringon-groundtestingorinorbit).Thisschemeproducesdatawhichcanbeusedbygroundprocessingforradiometricand/orphasecorrectionoftheimagedata.
Theinternalcalibrationrouteisnotexactlythesameasthenominalradartransmit/receivepath.Ontheonehand,someelementsofthetransmit/receivepatharenotfullycovered,e.g.,theinternalcalibrationsignalsarecoupledoutatthefront-endTRMs(transmitsignaltowardsubarrays),henceundergoadifferentpaththantheradarsignalitself.Ontheotherhand,someelementsoftheinternalcalibrationpatharenotpartofthenominalradarpath,e.g.,theaforementionedinternalcalibrationsignalsareroutedbacktotheRFelectronicsviadedicatedcoaxialcables.
Temperaturevariationsofthoseelementswhicharenotfullycoveredbyorspecifictotheinternalcalibrationloopsresultininstabilitieswhichcannotbecorrectedbyusingtheinternalcalibrationdata.On-groundmeasurements(i.e.,characteriza-tion)oftheseelements(viz.,amplitudeandphasevariationwithfrequencyandtemperature)allowasecondlevelofcorrectioninthegroundprocessing.
Fig.6showstheresultsofon-groundverificationoftheinternalcalibrationscheme:blueforpower–gainproductde-terminedbyinternalcalibration,redforpower-gainproductdeterminedusinganexternalstimulus,andtheirdifference(inblack)foroperationusinginternalcalibrationevery60sanddeactivationoftheTRMautomatictemperaturecompensationinbetween.(See[11]fordetails.)Thesmoothdriftallowstheaccurateinterpolationoftheinternalcalibrationmeasure-ments.Thisverificationprovestheperformanceoftheinternalcalibrationschemewithasingleantennapanel.Thisschemethencanbeused(withoutexternalequipment)forperformanceverificationduringthermal-vacuumtestingwiththecompleteantennaand,basedonthissuccessfulverification,duringin-orbitoperationtoensureradiometricstability.
III.ADDITIONALPAYLOADS
A.LCT
TheLaserCommunicationTerminal(LCT)isanexper-imentalpayloaddevelopedandbuiltwithfundingfrom
Fig.6.TerraSAR-Xtestresultforradaramplitudestability.
DLR/BundesministeriumfürBildungundForschung(BMBF)underleadershipoftheindustrialprimecontractorTesat-SpacecomwithmajorcontributionsfromEADSAstriumandZeissOptronik[12].ThemainaimoftheLCTexperimentistodemonstratethefeasibilityoflaser-basedcommunicationbetweensatellitesindifferentorbitsathighdatarates.Forthispurpose,theLCTonTerraSAR-Xwasusedinafirstverifica-tionsteptotransmitSARdatatoaground-basedterminalataspeedof5.625Gb/susinga1064-nmdiode-pumpedsolid-statelaser.
Inafurtherverificationstep,anintersatellitelink(ISL)wasestablishedwiththeU.S.MissileDefenseAgency’sNearFieldInfraredExperimentsatellite.TheLCTwasthefirstlasercommunicationsysteminspaceoperatingonacoherentbasiswithahomodynedigitalreceiverandbinaryphase-shiftkeyingmodulation.Theopticaloutputpower−ranges8between0.5and1.0W,andthebiterrorrateisbelow10foradistanceofupto8000kmandbelow10−4forthelinkthroughtheatmosphere.TheLCTisaccommodatedontheantisunsideofTerraSAR-XtoguaranteeafreehemisphericalfieldofviewwhichalsoincludestheEarthsurface.Theterminalconsistsofonephysicalunitwithamassofapproximately33kgandanaveragepowerconsumptionof136W,functionallydividedintoanopticsunitincludingacoarsepointingmechanism,atelescope,afinepointerandareceiver,andaframeunitcontainingtheelectronics.Forthermalcontrolreasons,theLCTissupportedbyaheatpiperadiatorwithanareaof0.57m2andaweightofapproximately10kg.DuetotheabundantTerraSAR-Xenergymargin,theLCTcanbeoperatedsimul-taneouslywithSARimagingduringmostoftheyear.B.TOREquipment
TheTracking,Occultation,andRanging(TOR)equipmentisfurnishedbyGeoForschungsZentrum,Potsdam,Germany,
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VIBRATIONTESTLOADS
andtheCenterforSpaceResearchoftheUniversityofTexasinAustin,USA,withcofundingfromDLR/BMBF.TheTORinstrumentpackageconsistsoftheredundantdual-frequencyGPStrackingreceiversystemIGOR(integratedglobalnaviga-tionsolutionsystemandoccultationreceiver)andalaserretroreflectorset(LRR)[13].TheIGORisanenhancementoftheBlackJackreceiver,incorporating,e.g.,additionalreceivefilter-ingtoavoidSARinterferenceandhi-relpartswherepossible.TheGPStrackingdataacquiredbyIGORarethebasisforPODprocessingonground.Orbitproductswithaccuraciesof<2minposition,tobeusedforSARimageprocessingandgeolo-cation,aremadeavailablewithinashorttimeframe,whichistypicallylessthan24h,afterreceptionofIGORdataonground.AfurtherTORmissionobjectiveistocollectatmosphericandionosphericradiooccultationdatawithIGOR,whichareusedfortheimprovementofweatherforecastandforclimaticstudies.TheLRRisapassiveopticaldeviceenablingforpreciselasertrackingfromground,supportedbytheglobalnetworkoflasertrackingstationsoftheInternationalLaserRangingSer-vice.TheLRRonTerraSAR-XallowsfororbitdeterminationaccuraciesinthecentimeterrangeandhasmainlybeenusedtoindependentlyverifythequalityoftheonboardGPStrackingdata.ThetotalweightoftheTORpackageislessthan10kg,anditspowerconsumptionisbelow17Winaverage.IV.TERRASAR-XPRELAUNCHSATELLITETESTINGTheTerraSAR-Xsatellitewassubjectedtoaclassicalsystemenvironmentaltestcampaignofthefollowing(insequence):1)massproperties;2)separationshock;3)vibration;4)boomrelease;5)thermalbalance/thermalvacuum(TB/TV);6)elec-tromagneticcompatibility;and7)acousticnoisetesting.Alltestswereperformedoveraperiodofaboutthreemonthsinsummer2006attheIndustrieanlagen-BetriebsgesellschaftmbHfacilityinMunich.A.VibrationTesting
Thevibrationtestdemonstratedthesatellite’scapabilitytowithstandthedynamicloadenvironmentduringthelaunchandverifiedthestiffnessrequirementsimposedbythelauncher.Thetestprogramconsistedforeachofthethreemajorgeometricalsatelliteaxesofalow-levelsinesweeptodeterminethefirsteigenfrequency,followedbyanacceptancelevelrunwithloadsasshowninTableIII.Itwasconcludedbyafurtherlow-level
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Fig.7.TerraSAR-Xvibrationtest.
sinesweeptoverifytheintegrityofthestructure.Fig.7showsthesatelliteonthevibrationshaker.B.TB/TVTesting
TheTB/TVtest,whichincludedthesimulationofsolarandEarthradiation,demonstratedthesuitabilityofthethermaldesignandtheproperfunctionoftheoverallsysteminvacuumunderextremetemperatureconditions.TheoveralldurationoftheTB/TVtestwastendays,subdividedintoahottestingperiodwithtypicalsatelliteinternaltemperaturesabove30◦Candacoldtestingperiodwithtypicalinternaltemperaturesbelow−10◦C.Formostofthetestduration,thevacuumqualitywasbetterthan10−6mbar.TheTB/TVtestalsoincorporatedSARinstrumenttestswithradiatingSARantenna,whichwereveryusefultoverifythepredictedheat-upbehaviorofthecompleteantennaduringatypicalSARdatatakeinspace.Forthispurpose,theTVchamberhadbeenequippedwithavacuumproofRFdampingmaterial.Fig.8showsthesatelliteinsidethevacuumchamberbeforeclosure.Earthradiationissimulatedbytemperature-controlledEarthsimulatorpanelsandthethermalspaceenvironmentbyLN2-cooledshrouds,placedaroundthetestchamberwalls.Thesunsimulatorbeamentersthechamberfrombelow,isreflectedbyamirroratthefarendofthechamber,andilluminatesthesatellitesolararray.
V.TERRASAR-XIN-ORBITSTATUS
TheTerraSAR-XS/CwaslaunchedonaDneprrocketfromBaikonurinKazakhstanonJune15,2007.TheS/Cisworkingnominally,andexcellentimagesarebeingcontinuouslypro-ducedongroundfromthedownlinkedradardata.Allredundanthardwareisavailableexceptforone(of384)antennaTRM
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Fig.8.TerraSAR-XTB/TVtest.
Fig.9.TerraSAR-Xfuelconsumption.
whichwasdeactivatedbeforethelaunchfollowinganomaloustelemetrydata.
ThereisagoodstatusofconsumablesandpredictionsthatshowthatthisfitstheplanningfortheTanDEM-Xmission.Fig.9showsthefuelconsumptionsincelaunchcomparedtoanassumedlineardecreaseover6.5years:Green/redindicatesafuelstatushavingmore/lessthanthelinearassumption,respec-tively.AradiationupsetinApril2008didleadtounexpecteduseoffuel,butasoftwareimprovementsolvedtheproblem.Inthemeantime,thereisapositivefuelbudget.
Theconditionofthebatteryisconstantlymonitoredbycheckingthatitsvoltageduringoperationdoesnotdroptoolowandapproachthelevelwhereanautomaticonboardreactionwouldoccur.Everyyear,adedicateddatatakeisperformedtoprovidedatatoallowpredictionoftheremainingcapacity.Fig.10showstheresults.Thesolidcurveistheoriginal(atlaunch)prediction;thedashedcurveistheupdatedprediction;today,about84%oftheoriginalcapacityisstillavailable.
VI.TANDEM-XOUTLOOK
TheTanDEM-Xmissionbecameavisionduringthedevel-opmentoftheTerraSAR-Xsatellite.TheTanDEM-Xmission
Fig.10.TerraSAR-Xbatterycapacity.
istheparalleloperationofasecondsatelliteincloseformationtocollectradarinterferometricdatatoderiveaglobaldigi-talelevationmodel(DEM)ofthecompleteEarthlandmass.ThisneededaminimumextensionoftheSARdesignonTerraSAR-Xtosupportthesynchronizedoperationofbothradars.Forthebus,theapproachwasconstrainedbyonlyallowingsoftwarechangesonTerraSAR-X.ThebusdesignonTanDEM-Xwasextendedtoallowformationflightofbothsatellites.ThesoftwarechangesaretobeverifiedduringtheTanDEM-Xon-groundtestsanduplinkedtoTerraSAR-Xinpreparationfortheformationflight.TanDEM-Xiscurrentlyundertest,anditslaunchisplannedattheendof2009toallowthree-yearlifetimeoverlapwithTerraSAR-X.Formoredetails,see[14]–[17].
Anadditionalpropulsionsystembasedonhigh-pressurenitrogengasisaccommodatedonTanDEM-X.Thiscoldgassystemprovidessmallerimpulsesthanthehydrazinesys-temonbothsatellites(whichisusedfororbitmaintenance)andsupportsformationflyingbyfineorbitcontroloftheTanDEM-Xsatellite.
TheTerraSAR-Xinstrumentwasequippedwithfeaturesre-quiredforpulserepetitionfrequencyandphasesynchronizationbetweenthetwosatellites.PhasesynchronizationisfacilitatedbyexchangeofradarpulsesbetweenbothSARs—theso-called“sync”pulses.Sixhornantennasoneachsatelliteprovidequasi-omnidirectionalcoverage,asshowninFig.11.Theredconessymbolizethepointingdirectionsoftherespectivehornantennas.Theiraccommodationwasheavilyconstrainedbythelauncherenvelope.Measurementswithsimulatedrepresenta-tiveblockageweremadetoconfirmsatisfactoryantennafieldofview.
ForDEMdatacollection,formationflightisnecessarywithseparationsdowntoabout300m.Thisinfersriskofcollisionofthesatellitesandriskofmutualilluminationbythemainbeamsoftheradarantennas.Anumberofmechanismshavebeenintroducedonbothsatellitestosafeguardagainsttheserisks.1)Anadditionalsafemodeusingmagnettorquerstoavoidchangeinaltitudeinthecaseofaradiationupsetwhichcouldleadtoriskofcollision.
2)AnexclusionzonelogictosuppressSARtransmissioninthecaseofattemptedoperationinaforbiddenorbit
PITZANDMILLER:TERRASAR-XSATELLITEFig.11.TerraSAR-XandTanDEM-Xsynchorns.
segmentwherereceive-onlyoperationisplanned.Receive-onlyoperationisusedduringbistaticoperationoftheradarsforDEMdatacollection:Oneradartrans-mits,andbothradarsreceive.
3)AsyncwarninglogictosuppressSARtransmissioninthecasethatlowpowerlevelsofsyncpulsesinferapossiblyincorrectorbitpositionofthepartnersatellite.
4)Furthermore,anISLreceiverandadecoderonTanDEM-Xallow“listening”totheTerraSAR-Xlow-rateS-bandtelemetry.InthecasethatTerraSAR-Xisinsafemode,TanDEM-Xsuppressesradartransmissionconsid-eringtheconcernofradarilluminationofTerraSAR-X.
VII.CONCLUSIONANDOUTLOOK
Thedesignandperformanceofthebus,SAR,andaddi-tionalpayloadshavebeendescribedfortheTerraSAR-XandTanDEM-Xsatellites.Thiscoversthearchitectureandtechnol-ogyoftheSARactiveantennaandalsotheinternalcalibrationschemetoexplainhowtheSARflexibilityandstabilityareachieved.Anoutlineoftheprelaunchtestprogramunderlinestheapproachtosecurethefunctionalityandperformanceinorbit.AsummaryofthecurrentTerraSAR-Xin-orbithealthstatusconfirmsthedesignandtestapproachandprovidesthebestfoundationforthesuccessofTanDEM-X.Safeguardsforcloseformationflighthavebeendevelopedandwillbetestedinorbitbeforethebeginofthisexcitingandnovelphase.
Inthemeantime,thenextgenerationSARinstrumenthasalreadyentereditsdefinitionphase.Acleartrendtowarddigitalbeamformingcanbeobserved,providinghighgeometricreso-lutionincombinationwithaverywideswath[18].Toachievethis,newtechniquessuchas,forexample,scanonreceiveor
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multiapertureprocessingarerequired.Asaconsequence,theanalog-to-digitalconversionoftheradarechowillbeperformedonanantennasubaperturelevel,providingdigitalinputtothesubsequentbeamformingunits.
Thedemonstrationofthefeasibilityandperformanceofadigitalbeamformingantennaissubjectofthe“HighResolutionWideSwath”SARDemonstratorthatiscurrentlyunderdevel-opmentatAstriuminFriedrichshafen[19].
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622IEEETRANSACTIONSONGEOSCIENCEANDREMOTESENSING,VOL.48,NO.2,FEBRUARY2010
WolfgangPitzwasbornin1961inLudwigsburg,Germany.HereceivedtheMasterofAerospaceEn-gineeringatStuttgartUniversity,Stuttgart,Germany,in1988.
Hejoinedthespaceindustryinthesameyear.Stayingwithhisfirstemployer,formerlyDornierSystemsandnowAstriumSatellites,hefirstworkedasThermalControlEngineeronvarioussatelliteprojectsandbecameProjectManageroftheGermangravitymissionCHAMPin1996.InparalleltotheworkonCHAMPheledaseriesofnationaland
internationalsatelliteearly-definitionphases,mostnotablyfortheDIVAandROCSAT-2missions.In2000hebecameProjectManageroftheGermanDLRSARsatelliteTerraSAR-X.Startingfrom2005heplayedamajorroleinthedefinitionoftheGermanDLRSARsatelliteTanDEM-X.AfterthelaunchofTerraSAR-Xin2007hebecameProjectManageroftheSARinstrumentfortheESAGMESmissionSentinel-1.Since2009hehasbeenHeadofIndustrialisa-tionoftheAstriumSatellitesEarthObservationandScienceDivision.
DavidMillerwasbornin1955inLiverpool,U.K.HereceivedtheB.S.degreeinphysicsfromBristolUniversity,Bristol,U.K.
HejoinedBritishAerospacein1977andgainedfirstexperienceonavionicsandradarsystems.In1981,hemovedtoGermanyandjoinedDornierSystemswhichlaterbecameAstrium.Afterper-formanceanalysisoftheERS-1ActiveMicrowaveInstrumentandtheShuttleX-SARexperiment,hebecameProjectManagerfortheAdvancedScat-terometeronMetop.Hebecameresponsibleforthe
radaronTerraSAR-Xin2002andbecameManageroftheTanDEM-Xsatellitein2006.
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