Satellite.docx
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Satellite
英文资料与中文翻译
Satellites
1.OVERVIEW
Asatelliteisaradiofrequencyrepeater.New-generationsatellitesareregenerative;thatis,theyhaveonboardprocessingcapabilitymakingthemmoreofanintelligentunitthanamererepeater(moreissaidofonboardprocessinginSec.2.9).Thiscapabilityenablesthesatellitetocondition,amplify,orreformatreceiveduplinkdataandroutethedatatospecifiedlocations,oractuallyregeneratedataonboardthespacecraftasopposedtosimplyactingasarelaystationbetweentwoormoregroundstations.
AtypicalsatellitewithonboardprocessorsistheNASAAdvancedCommunicationsTechnologySatellite(ACTS)showninFig.1.1.ItwaspartofthepayloadontheSpaceShuttleDiscoverylaunchedonSeptember12,1993.AccordingtoNASA,itssatelliteweighs3250lb(1477.3kg)andmeasures47.1ft(14.36m)fromtiptotipofthesolararraysand29.9ft(9.11m)acrossthemainreceivingandtransmittingantennareflectors,withaheightof15.2ft(4.63m)fromthespacecraftseparationplanetothetipofthehighesthighestantenna.Thesolararraysprovideapproximately1.4kilowatts.Themaincommunicationantennasarea7.2-ft(2.19-m)receivingantennaanda10.8-ft(3.29-m)transmittingantenna.Wedescribemoreaboutsatellitecomponents’designlaterinthischapter,astatelitecomprisesseveralindividualequipmentcalledatransponderatermderivedfromtransmitterandresponder.
TheblockdiagramshowninFig.1.2mayrepresentatransponderunit.Asseeninthefigure,atranspondermaybedescribedasasystemcomposedbasicallyofabandpassfilterrequiredtoselecttheparticularchannel’sbandfrequencies,afrequencytranslatorthatchangesfrequenciesfromoneleveltoanother,andanoutputamplifier.Onceamplified,thechannelsarerecombinedinanoutputmultiplexerforthereturntransmission.Allthesedevicesmustbestableovertheiroperatingtemperaturerangetomaintainthedesiredrejectioncharacteristics.Thefunctionalityofthesedevices(eachcomponentblockinFig.1.2),isaddressedlaterinthischapter.Atranspondermaychannelthesatellitecapacitybothinfrequencyandinpowerandmaybeaccessedbyoneorseveralcarriers.
Inmostsystemapplications,onesatelliteservesmanyearthstations.Withtheassistanceofearthstations,fixedortransportable,satellitesareopeninganeweraforglobalsatellitemultiaccesschannels’datatransmissionandbroadcastofmajornewsevents,live,fromanywhereintheworld.Commercialandoperationalneedsdictatethedesignandcomplexityofsatellites.Themostcommonexpectedsatelliteattributesincludethefollowing:
1.Improvedcoverageareasandqualityservices,andfrequencyreuseability
2.Compatibilityofsatellitesystemwithothersystemsandexpand-abilityofcurrentsystemthatenhancesfutureoperations
3.High-gain,multiplehoppingbeamantennasystemsthatpermitsmaller-apertureearthstations
4.IncreasedcapacityrequirementsthatallowseveralG/seccommu-nicationbetweenusers
5.Competitivepricing
Futuretrendsinsatelliteantennas(concerningdesignandcomplexity)arelikelytobedictatedfromthestatusofthesatellitetechnology,trafficgrowth,emergingtechnology,andcommercialactivities.
Thenexttwosectionsexaminethetypeofsatellitesandthemajorcharacteristicsthatdeterminethesatellitepathrelativetotheearth.Thesecharacteristicsareasfollows:
1.Orbitaleccentricityoftheselectedorbit
2.Periodoftheorbit
3.Elevationangle;theinclinationoftheorbitalplanerelativetothereferenceaxis
FIGURE1.1Geometryofasatellite.(CourtesyofNASA.)
FIGURE1.2Basictransponderarrangement.
2、TypesofSatellites
Thereare,ingeneral,fourtypesofsatellite:
Geostationarysatellite(GEO)
Highellipticalorbitingsatellite(HEO)
Middle-earthorbitingsatellite(MEO)
Low-earth-orbitingsatellite(LEO)
AnHEOsatelliteisaspecializedorbitinwhichasatellitecontinuouslyswingsveryclosetotheearth,loopsoutintospace,andthenrepeatsitsswingbytheearth.Itisanellipticalorbitapproximately18,000to35,000kmabovetheearth’ssurface,notnecessarilyabovetheequator.HEOsaredesignedtogivebettercoveragetocountrieswithhighernorthernorsouthernlatitudes.Systemscanbedesignedsothattheapogeeisarrangedtoprovidecontinuouscoverageinaparticulararea.Bydefinition,anapogeeisthehighestaltitude-pointoftheorbit,thatis,thepointintheorbitwherethesatelliteisfarthestfromtheearth.Toclarifysomeoftheterminology,weprovide,Fig.1.3,whichshowsthegeometricpropertiesofanellipticalorbit.Bygeometry,
(2.1)
(2.2)
Wheretheeccentricity,ortheamountbywhichtheellipsedepartsfroma
circle,is:
(2.3)
FIGURE2.3Geometricpropertiesofanellipticalorbit
=semifocallength;
=semiparameter;Sm=semiminoraxis;r=radiusdistance,focustoorbitpath;
=positionangle.
Thegeneralequationofanellipsecanthusbewrittenas
Itisapparentfrom(2.4)thatife=0,theresultinglocusisacircle.
AnMEOisacircularorbit,orbitingapproximately8,000to18,000kmabovetheearth’ssurface,againnotnecessarilyabovetheequator.AnMEOsatelliteisacompromisebetweenthelowerorbitsandthegeosynchronousorbits.MEOsystemdesigninvolvesmoredelaysandhigherpowerlevelsthansatellitesinthelowerorbits.However,itrequiresfewersatellitestoachievethesamecoverage.
LEOsatellitesorbittheearthingridsthatstretchapproximately160to1,600kmabovetheearth’ssurface.Thesesatellitesaresmall,areeasytolaunch,andlendthemselvestomassproductiontechniques.AnetworkofLEOsatellitestypicallyhasthecapacitytocarryvastamountsoffacsimile,electronicmail,batchfile,andbroadcastdataatgreatspeedandcommunicatetoendusersthroughterrestriallinksonground-basedstations.Withadvancesintechnology,itwillnotbelonguntilutilitycompaniesareaccessingresidentialmeterreadingsthroughanLEOsystemortransportagenciesandpoliceareaccessingvehicleplates,monitoringtrafficflow,andmeasuringtruckweightsthroughanLEOsystem.
IntheUnitedStates,thethreesatellitetypes(HEO,MEO,andLEO)arecollectivelycalledLEOs;thatis,low-earthorbitingsatellitesystems.Byfrequencydesignation,theLEOsaregroupedasbigandlittleLEOs,asdescribedinTable2.1.
LEOsaresubjecttoaerodynamicdragcausedbyresistanceoftheearth’satmospheretothesatellitepassage.Theexactvalueoftheforcecausedbythedragdependsonatmosphericdensity,theshapeofthesatellite,andthesatellite’svelocity.Thisforcemaybeexpressedintheform
Fd=-0.5
CdAeqv2kg-m/sec2(2.5)
where
=atmosphericdensity.Thisdensityisaltitude-dependent,anditsariationis
exponential.
Cd=coefficientofaerodynamicdrag.
Aeq=equivalentsurfaceareaofthesatellitethatisperpendiculartothe
velocity,v.
v=velocityofthesatellitewithrespecttotheatmosphere.Themagnitudeofthis
velocityisdiscussedinSec.2.2.
Ifthemassmsofthesatelliteisknown,theaccelerationadduetoaerodynamicdragcanbeexpressedas
Theeffectofthedragisadecreaseoftheorbit’ssemimajoraxisduetothedecreaseinitsenergy.Acircularorbitremainsassuch,butitsaltitudedecreaseswhereasitsvelocityincreases.Duetodrag,theapogeeintheellipticalorbitbecomeslowerand,asaconsequence,theorbitgraduallybecomescircular.Thelongertheinfluenceontheorbit,theslowerthesatellitebecomes,anditeventuallyfallsfromorbit.Aerodynamicdragismoresignificantatlowaltitudes(200to400km)andnegligibleonlyabout3000kmbecause,inspiteofthelowvalueofatmosphericdensityencounteredatthealtitudesofthesatellites,theirhighorbitalvelocityimpliesthatperturbationsduetodragareverysignificant.
TABLE2.1FrequencyClassificationofLEOs[1]
Ageostationaryorbitisanonretrogradecircularorbitintheequatorialplanewithzeroeccentricityandzeroinclination.Thesatelliteremainsfixed(stationary)inanapparentpositionrelativetotheearth;about35,784kmawayfromtheearthifitselevationangleisorthogonal(90)totheequator.Itsperiodofrevolutionissynchronizedwiththatoftheearthininertialspace.Thegeometricconsiderationsforageostationarysatellitecommunicationsystemarediscussedlaterinthetext.
CommercialGEOsprovidefixedsatelliteservice(FSS)intheCandKubandsoftheradiospectrum.SomeGEOsusetheKubandtoprovidecertainmobileservices.TheInternationalTelecommunicationUnion(ITU)hasallocatedsatellitebandsinvariouspartsoftheradiospectrumfromVHFto275GHz.Table2.2showssatellitecommunicationsfrequencybandsandtheservicestheyperform,whileTable2.3showstypicallinksfrequencybands.
FrequencybandsintheUHFaresuitableforcommunicatingwithsmallormobileterminals,fortelevisionbroadcasting,andformilitaryfleetcommunication.Thebandoffrequenciessuitableforanearth-space-earthradiolinkisbetween450MHzand20GHz.Frequenciesbetween20and50GHzcanbeusedbutwouldbesubjecttoprecipitationattenuation.However,ifanavailabilitygreaterthan99.5%isrequired,aspecialprovisionsuchasdiversityreceptionandadaptivepowercontrolwouldneedtobeemployed.Higherfrequenciesaremoresuitableforintersatellitelinks(ISL)andmaybecomeuseableastheorbitalcongestionarisesatthelowerfrequencies.Anotherbenefitofhigher-frequencycommunicationsystemsisthatsystemcomponentsgenerallybecomesmaller.Forsatellites,thistranslatestolighterweight,lowerpower,andreducedcost,andmoreimportantly,itmean