Satellite.docx

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