姿态测量子系统外文翻译.docx
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姿态测量子系统外文翻译.docx
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姿态测量子系统外文翻译
毕业设计(论文)外文资料翻译
系别:
电子信息系
专业:
通信工程
班级:
B090310
姓名:
贠鸽
学号:
B********
外文出处:
知网
附件:
1.原文;2.译文
2013年03月
AttitudeDeterminationSubsystem
Spacecraftattitudedeterminationistheprocessofestimatingtheorientationofaspacecraftbymakingremoteobservationsofothercelestialbodiesorreferencepoints.Combinationsofthesesensorobservationsareusedtogenerateamoreaccurateestimateofspacecraftrotationalattitude.
Attitudeestimatesmustbecalculatedquicklyandcontinuouslyduringtheentireoperationallifeofthemission.Duringnormaloperations,theproblemisrecursive—theattitudefilterbasingnewpredictionsonpresentandpriorsensorinformation.Theattitudefiltermustalsoestimatefromactivationwhenthespacecraftisfirstinitiatedandnopriordataisavailable.Sincespacecraftorientationwillvarywithtimeandtask,orientationchangesmustbetracked,andratesandaccelerativetorquesestimated.
Thesystemsdesignedtocarryout3-axisattitudedeterminationareinevitablycomplex,butmuststillbedesignedwiththeutmostcaretoperformthetaskasreliablyaspossible.Any,eventemporarymalfunctionispotentiallyserious,damagingfragileinstruments,breakingcommunicationslinks,upsettingmeasurementsanddisruptingpowergeneration.
1.ConventionalSystems
Modernattitudesystemscarryavarietyofdifferentsensorinstrumentstocoverthedifferentmodesofoperation.Dataprocessingisbyaradiation-hardenedcomputeroflimitedpowerandmemory.Systemsadoptamode-orientedapproachusingdifferentsensorsandalgorithmstoderiveattitudeforeachspacecraftactivity.Changesbetweencertainmodesmayoccurautomatically(particularlythoseassociatedwithacquisition),butmostchangesaremoreusuallyinitiatedbycommandfromground.Eachmodehasadifferentsetofcontrollawsandusesonlyasubsetofthespacecraft’ssensors.
Theoverallsafetyofthespacecraftisprotectedbyanemergencymode,whichautomaticallyactivatesifothercontrolmodesfail.Oftenimplementedasanentirelyhardwiredsystem,oratleastsecurelypartitionedfromthemainattitudecontroltaskintheattitudeprocessor,theemergencymodepreventsuntimelylossofthespacecraftintheeventoffailureorglitch(singleeventup-set).Itemploysseparatesensorsensuringthatintheeventofasinglesensorfailure,thespacecraftmayberecovered.Attitudedeterminationandcontrolisusuallyminimal,thesystemsimplylockingsolararraysontothesun,minimisingangularratetopreventdamagetodeployedboomsandsensors,notifyinggroundcontrolofemergencymodeactivationandawaitingfurtherinstructions.
Anemergencymodeisanimportantelementintheconventionalapproachtoattitudesystemfaulttolerance.Reliabilityestimatesarecalculatedundertheassumptionthattheattitudesystemmayundergoandrecoverfromasinglefaulteventinoneofthesystemcomponentsduringtheoperationallifetimeofthespacecraft.Toachieveasingleeventfaulttoleranceitiscommontoduplicatetheentireattitudesubsystemonboardthespacecraft.TheattitudesubsystemintheISOmissionhaseveryessentialelementdoubledorquadrupled,dependingonitsreliability,andistypical.IntheeventofafaultinattitudesubsystemA,theemergencymoderecoversthespacecraft,andthegroundcontrolteamactivatesanidenticalsubsystemB.Thereisalsooftensomecross-strappingofthesystemstoallowcombinationsofcomponentsfrombothAandBsystemstobeusedintheeventoffurtherfailure.Theapproachofduplicatingentiresystemsisknownasnumericalredundancy.Thedisadvantageswiththestrategyarediscussedinalatersection.
Whileattitudedeterminationsubsystemsaredesignedwithawidevarietyofrequirements,missionsandspacecraft,thesensorssuitestheyutiliseareoftenverysimilar.Thefollowingsectionsdescribethreecategoriesofsensordevices:
ratesensors,coarsepointingsensorsandfinepointingsensors.
2.RateSensors
Figure1Aringlasergyropack(Bae)
Forinitialacquisitionmodes,wherethespinrateofthespacecraftmustbecontrolledtoattainafirstinertiallock,setsofratesensorsareemployedforeachspacecraftaxis.Gyroscopedevicesarewidelyused.Highprecisionmechanicalgyrosarecommonbutsufferfromhighdriftratesandarenotalwaysreliable.SolidstatedevicessuchasringlasergyrosFigure2areincreasinglyconsideredfornewmissions,asarequartzratesensors.
Thesedevicesallmeasurerotationalmotionaroundaprincipalaxis.Theyareusuallymountedinthesameconfigurationasreactionwheelsinpacksoffour:
inatetrahedralarrangementorwiththreemountedorthogonallyandafourthataskewangletoallowsomeredundancytofailure.Sincetheirsignalmustbeintegratedtogiveangularposition,signalerrormakesthemunsuitableformeasuringabsoluteanglesasthemeasurementwilldriftwithtime.Ratemeasurementislimitedtoapredeterminedrangeconfiguredtomeetthecontrolsystemspecifications.
3.CoarsePointingSensors
Coarsecontroliscarriedoutusingsunsensors,simpledevicesconsistofsolarcellsandbaffles.AcrudeestimateofsunincidenceangleisderivedfromthedifferentialvoltagesgeneratedbyopposingcellsFigure2.Theyareoftenusedaspartofthehardwiredemergencysunre-acquisitionsystem,dedicatedtore-aligningthespacecraftwiththesunwhenthemaincontrolmodefails.
Figure2ACoarseSunSensor(EngineeringdiagramTPDLtd.)
Spacecraftalsoutiliseotherbrightobjectssuch,astheearthorbrightstars,forcrudeattitudecalculation.Earthsensorsscantheskydetectingtheplanetedgesbythermalemission.Orientationinformationisgeneratedtoapproximately0.05,butlargererrorscanbecausedbyanomalousatmosphericconditions.Crudestarsensorsrecordthepositionofaparticularpre-determinedbrightcelestialbody,usingitslocationtoalignspacecraftaxes.Theseinstrumentsareusedforcoarseattitudestabilisation,afteracquisition.TheGlobalPositionSystem(GPS)isalsofindingincreasingapplicationinspacecraft.Withtwoantennae,satellitesinearthorbitmaynotonlylocateposition,butalsocalculateorientation.Thesystemislimitedtosystemsrequiringonlyacrudeattitudelock(Ý0.5o),butisinexpensiveanduseswellproventechnology.
4.FinePointingSensors
Finepointingcontrolisalmostinvariablybystarcamera.Recently,designshaveconvergedtowardssimplecamerasystemsrecording2DimagesusingaChargedCoupledDevice(CCD)chip(similartothoseusedinmodernvideocameras).AnopticalarrangementconsistingoftwoorthreelensesimagesstarlightenteringthecameraontothesurfaceoftheCCDFigure3.TheCCDcapturestheviewasagridofpixelsofvaryingintensitiesreadyforprocessing.Itissometimescooledtominimisethermalnoiseandtherebytoimproveresolution.Theopticsoftendeliberatelydefocustheimagetoallowstarcentroiddetermination—theaccuratelocationofeachstartosub-pixelprecision.
Imageframesareprocessedbyadedicatedcomputerderivingstarmovementinformationorstarpositionandbrightnessinformation,dependingonthemodeofoperation.Algorithmsaresimple,incapableoftrackingmorethanafewstarsormovementsabovefractionsofapixel.Smallonboardcataloguesareoccasionallyusedforcomparisonwithimages,butabsolutelocationisalmostalwayscarriedoutontheground.Accuraciesforstar-cameratypeinstrumentsarequotedbetween1and100(3ó)arcsecforprincipalaxesdependentonthenumberofCCDpixels,thesizeofthefield-of-viewandtheamountofstar-centroidlocationdetermination.
Figure3AMediumAccuracyStarTracker30ox40oFieldofView(AleniaSpazio)
AtypicalexampleofoneofthehigheraccuracydevicesistheOfficineGalileoStartrackerusedonanumberofspacecraft.ItconsistsofaPeltiercooledCCDarray(384×288pixels)lookingata4.2—3.16segmentofthesky.Largebafflespreventstrayreflectionsenteringtheopticsandallowobservationofstarpositionsuptoaquotedminimum55anglefromthesun.Anopticalarrangement(focallength0.12m)producesadefocusedimageofasmallportionofthecelestialsphere,blurringstarimagestoa3×3pixelsegmentofthearray.Analogueelectronicssumtheoutputsofeachpixeloveranintegrationperiod(typically0.5seconds),andanelectronicthresholdingcircuitreducestheamountofpixeldatapassedtothedevice’sprocessor.Theprocessorimplementsastar-centroidlocatingalgorithmandcrudetrackingalgorithms,magnifyingtherawinstrumentpixelstarpositionbyafactorof166togivepositionsona63744—47808grid(quantisedto0.24arcsec).Aworstcasenoiseanalysisquotesastandarddeviationof2.2arcsecforattitudedeterminationtrackingfiveknownstars.Thedeviceisreportedtobeabletomapandtrackstarsinthefieldofviewtoalimitingmagnitudeof+8.
5.DataProcessingandSensorFusion
Tocalculatespacecraftattitude,informationfromdifferentsensorsmustbefusedtogethertoformonecoherentestimate.Theproblemisagainoftenbrokenintomodes,differentsensordataandfusingtechniquesbeingusedbydifferentcontrolmodes.
Duringslewcontrol,spacecraftrateisobservedusuallysolelybygyroscopeorratesensor,eitheruseddirectlyorlightlyfilteredtoremovesomesensornoisewithoutintroducinglargelagsininstrumentresponse.Somesimplepost-processingcalculatesorthogonalratesofrotationinthespacecraftframefromnon-orthogonallymountedsensors.Systemresponseisoccasionallyimprovedbytheintegrationofaccelerometerdatawithratedatadirectlysensingtheimpulsesappliedtothespacecraft.Sincethrusterimpulses
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