卫星通信第7章改

2020/11/251Chapter7TheSpaceSegment7.1IntroductionSpacesegment:satellite,TT&Cequipmentcarriedaboardthesatellite(payload,bus)Bus:vehiclecarryingthepayload,power,attitudecontrol,orbitalcontrol,thermalcontrol,commandandtelemetry.Payload:transponder(connectingthetransmitandreceiveantenna),antennasubsystem.Satellitecommunicationsystem:groundsegmentspacesegment2020/11/252Chapter7TheSpaceSegment7.1Introduction2020/11/253Chapter7TheSpaceSegment7.2PowerSupplyTheprimaryelectricalpowerforoperatingtheelectronicequipmentisobtainedfromsolarcells.Solarcellpanel(Spin-Stabilized760W-940W;Three-AxisStabilized2kW-6kW)Duringthelaunchsequence,outercylinderistelescopedovertheinnerone.Rectangularsolarsailsarefolded.2020/11/254Chapter7TheSpaceSegment7.3AttitudeControlTheattitudeofasatellitereferstoitsorientationinthespace.Attitudecontrol:ensurethatdirectionalantennaspointintheproperdirections,isdifferentfromstationkeeping(orbitposition)Attitudecontrolinstruments:infraredsensors(horizondetector)Attitudemaneuver(机动):SatelliteAttitudeAdjustmentControllingtorques:passiveattitudecontrol,Activecontrolpassiveattitudecontrol:spinstabilizationandgravitygradientstabilization(interactionofsatellitewiththegravitationalfieldofcentralbody).Activecontrol:momentumwheels(飞轮),electromagneticcoilsandmassexpulsion(gasjetsandionthrusters(离子推进器))2020/11/255Chapter7TheSpaceSegment7.3AttitudeControl2020/11/256Chapter7TheSpaceSegment7.3AttitudeControl•Thethreeaxes:roll(滚动),pitch(俯仰)andyaw(偏航)(RPY)axes.•Allthreeaxespassthroughthecenterofgravityofthesatellite.•Foranequatorialorbitmovementabouttherollaxismovestheantennafootprintnorthandsouth;movementaboutthepitchaxismovestheantennafootprinteastandwest;movementabouttheyawaxisrotatestheantennafootprint.2020/11/257Chapter7TheSpaceSegment7.3AttitudeControl7.3.1SpinningsatellitestabilizationBeachievedwithcylindersatellites.ForGEOsatellites,thespinaxisisparalleltotheN-Saxisofearth.ImpulsetypethrustersorjetscanbeusedtoincreasethespinrateagainandtoshifttheaxisbacktoitscorrectN-Sorientation.Foromnidirectionalantenna,theantennawhichpointsalongthepitchaxisrotateswiththesatellite.Fordirectionalantenna,theantennamustbedespun,givingrisetodual-spinconstruction.2020/11/258Chapter7TheSpaceSegment7.3AttitudeControl7.3.2Momentum(动量)wheelstabilizationStabilityalsocanbeachievedbyutilizingthegyroscopic(陀螺)effectofaspinningflywheel.Thismethodisusedinthenoncylindricalsatellites.Momentumwheel:flywheel,bearingassembly,thecasinganelectricdrivemotorwithassociatedelectroniccontrolcircuit.Whenamomentumwheelisoperatedwithzeromomentumbiasasreactionwheel(反力轮).Reactionwheelsareusedinthree-axisstabilizedsystem.2020/11/259Chapter7TheSpaceSegment7.4StationkeepingItisimportantthataGSOsatellitebekeptinitscorrectorbitalslot.east-weststationkeepingmaneuvers:TheequatorialellipticityofearthcausesGSOsatellitetodriftslowlyalonetheorbit,tooneoftwostablepointsat750Eand1050W.Tocounterthisdriftanoppositelydirectedvelocitycomponentisimparted(给予)tothesatellitebymeansofjets.Bepulsedonceevery2or3weeks.Thesemaneuversaretermedeast-weststationkeepingmaneuvers.Satelliteinthe6/4GHzmustbekeptwithin+-0.10,andinthe14/12GHzwithin+-0.050.south-northstationkeepingmaneuvers:Asatellitewhichisnominallygeostationaryalsodriftinlatitude,inclinationtochangeatarateofabout0.85deg/year,resultinacyclicchangeintheinclinationgoingfrom0to14.670in26.6years.Thenorth-southstationkeepingtolerancessatelliteinthe6/4GHzmustbekeptwithin+-0.10,andinthe14/12GHzwithin+-0.050.Chapter7TheSpaceSegment7.4StationkeepingSatellitealtitudealsowillshowvariationsofabout+-0.1percentofthenominalGSOheight(72km,longitudeandlatitude147km)2020/11/2511Chapter7TheSpaceSegment7.4Stationkeeping2020/11/2512Chapter7TheSpaceSegment7.5Thermalcontrolsatellitesaresubjecttolargethermalgradients,receivingsun’sradiationononesidewhiletheothersidefacesintospace.Thesatellite’sequipmentshouldoperateasnearlyaspossibleinastabletemperatureenvrioment.2020/11/2513Chapter7TheSpaceSegment7.6TT&CsubsystemThetelemetry,tracking,andcommandsubsystemperformsseveralroutinefunctionsaboardthespacecraft.1.Telemetry(telemetering):measurementatadistanceOperationofgeneratinganelectricalsignalproportionaltothequantitybeingmeasuredandencodingandtransmittingthistoadistantstation.Telemetrydatainclude:attitudeinformationfromsunandearthsensors;environmentalinformationsuchasmagneticfieldintensityanddirection,thefrequencyofmeteorite(陨石)impact(撞击),andsoon.Spacecraftinformationsuchastemperaturespowersupplyvoltages,andstored-fuelpressure,equipmentaboarddata.2.Commandsubsystem:receivethesignalsfromtheearthstationofteninresponsetotelemeteredinformation.Demodulation,decodeandroutethesetotheexecutingequipment.3.Tracking:beaconsignalsranging(needisotropicantenna)2020/11/2514Chapter7TheSpaceSegment7.6TT&Csubsystem2020/11/2515Chapter7TheSpaceSegment7.7TransponderTransponderisaseriesofinterconnectedunitswhichformsasinglechannelbetweenthereceiveandtransmitantennainacommunicationssatellite.Someoftheunitsutilizedbyatransponderinagivenchannelmaybecom