Introduction
InApril1983,theUnitedStateslaunchedthefirsttrackinganddatarelaysatellitefromthe"Challenger"spaceshuttle,whichisalsothelargestcommunicationssatelliteinmoderntimes.Themainpurposesoftrackinganddatarelaysatellitesare:trackingandmeasuringmid-andlow-orbitsatellites;real-timeforwardingofremotesensingandtelemetrydataforearthobservationsatellites;undertakingcommunicationanddatatransmissionrelayservicesforspaceshuttlesandmannedspacecraftandmeetingmilitaryrequirementsSpecialneedsetc.
Principle
Thecommunicationsatellitethatforwardsthetrackingandremotecontrolinformationoftheearthstationtothemediumandloworbitspacecraftandthedatasentbacktothegroundbythespacecraft(Figure1).Thelinearpropagationcharacteristicsofhigh-frequencyradiowavesandtheinfluenceofthecurvatureoftheearthrestricttheorbitalarcandcommunicationtimeofthetrackinganddatarelaysatellitesofthemediumandlow-orbitspacecraft.Thetrackinganddatarelaysatellitesareequivalenttoraisingthemeasurementandcontrolstationontheground.Attheheightofthegeostationarysatelliteorbit,onesatellitecanobservemostofthespacecraftflyinginthenear-Earthairspace,andthetwosatellitesnetworkcanbasicallycovertheentiremid-andlow-orbitairspace.Therefore,thetrackinganddatarelaysatellitesystemcomposedoftwosatellitesandonemeasurementandcontrolstationcanreplacetheaerospacemeasurementandcontrolnetworkcomposedofmanymeasurementandcontrolstationsallovertheworld.
Mainpurpose
Themainpurposeoftrackinganddatarelaysatellitesis:
①Trackingandmeasuringmid-andlow-orbitsatellites:inordertocoverasmuchaspossibleManysatellitesuseorbitswithlargeinclinationandlowaltitudeonthesurfaceoftheearthandtoobtainhighgroundresolution.Trackinganddatarelaysatellitescanalmostcontinuouslytracksatellitesinlowandmediumorbits,andaccuratelydeterminetheorbitsofthesesatellitesbyforwardingtherangingandDopplershiftinformationbetweenthemandthemeasurementandcontrolstation.
②Real-timeforwardingofremotesensingandtelemetrydataforEarthobservationsatellites:meteorology,ocean,geodesy,andresources,etc.Earthobservationsatellitestemporarilytransferremotesensingandtelemetryinformationwhentheyflyovertheskywithoutanearthstation.Storedintherecorder,andthenforwardedwhenflyingthroughtheearthstation.Thistrackinganddatarelaysatellitecanforwardlargeamountsofremotesensingandtelemetrydatabacktothegroundinrealtime.
③Undertakethecommunicationanddatatransmissionrelaybusinessofthespaceshuttleandmannedspacecraft:thespacemeasurementandcontrolnetworkontheground(seethespacemeasurementandcontrolanddataacquisitionnetwork)canonlycover15%ofthelowearthorbitonaverage.Astronautshavelimitedtimefordirectcommunicationandreal-timedatatransmissionwiththespacecontrolcenterontheground.Twoproperlyconfiguredtrackinganddatarelaysatellitesenablethespaceshuttleandmannedspacecrafttomaintaincontactwiththegroundfor85%ofthetotalflighttime.
④Meetthespecialneedsofthemilitary:Inthepast,thegroundaerospacecontrolcenterofvariousmilitarysatellitessuchascommunications,navigation,meteorology,reconnaissance,surveillance,andearlywarning,oftenhadtotrackthroughaseriesofearthstationsandcivilcommunicationnetworks,Measurementandcontrolanddatatransmission.Trackinganddatarelaysatellitescangetridofthedependenceonmostearthstations,andformanindependentdedicatedsystemtoservethemilitarymoreeffectively.
InApril1983,theUnitedStateslaunchedthefirsttrackinganddatarelaysatellite(TDRS)fromthe"Challenger"spaceshuttle(Figure2).ItisthelargestcommunicationssatelliteinmoderntimesandalsoForthefirsttime,communicationsatelliteswithS,C,andKuthreefrequencybandsareusedonasinglesatellite.Thesatelliteweighsmorethan2tons.Afterthesolarbatterywingsareextended,thewingspanis17.4metersandthelateralspanis13meters.After10yearsofoperationofthesatellite,thesolararraycanstillprovide1,850wattsofpower.Thestaradoptsathree-axisattitudecontrolstabilizationmethod(seespacecraftattitudecontrol).Thesatelliteisequippedwith7differenttypesofantennas.Thetwoparabolicantennaswithadiameterof4.9metersarefoldedintoacylindricalshapeduringthesatellitelaunchprocess.Afterbeinginorbit,theyarecontrolledbyamechanicalscrewtoexpandintoanumbrellashape.EachantennahastwofeedsfortrackinganddataintheSandKubands.relay.Aparabolicantennawithadiameterof2metersisusedfortwo-waycommunicationwiththeKu-bandofthesatellitecommunicationearthstation.Thesethreeantennasareallmountedonaprecisiongimbal,controlledbygroundinstructions,andcanautomaticallytrackotherspacecraftwithapointingaccuracyof0.06°.InthemiddleofthestarisanS-bandphasedarrayantennacomposedof30spiralsformultipleaccesscommunication.ThereisalsoaKu-bandparabolicantennawithadiameterof1.12metersandaC-bandspadeantennafordomesticcommunicationsintheUnitedStates.ThecommunicationcapacitythatKuandS-bandtransponderscanprovideis20S-bandmultipleaccesschannels,2S-bandsingle-accesschannelsand2Ku-bandsingle-accesschannels.Inaddition,12C-bandtransponderscantransmittelephone,TVanddata.