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外文翻译--全球卫星定位系统.doc

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外文翻译--全球卫星定位系统.doc

1本科毕业论文外文文献及译文文献、资料题目GlobalPositioningSystem文献、资料来源http//en.wikipedia.org/wiki/Global_Positioning_System文献、资料发表(出版)日期2009.01.03院(部)土木工程学院专业土木工程班级姓名学号指导教师翻译日期外文文献2GlobalPositioningSystemTheGlobalPositioningSystemGPSisaglobalnavigationsatellitesystemGNSSdevelopedbytheUnitedStatesDepartmentofDefenseandmanagedbytheUnitedStatesAirForce50thSpaceWing.ItistheonlyfullyfunctionalGNSSintheworld,canbeusedfreely,andisoftenusedbyciviliansfornavigationpurposes.TheGlobalPositioningSystemGPSisasatellitebasednavigationsystemmadeupofanetworkof24satellitesplacedintoorbitbytheU.S.DepartmentofDefense.GPSwasoriginallyintendedformilitaryapplications,butinthe1980s,thegovernmentmadethesystemavailableforcivilianuse.GPSworksinanyweatherconditions,anywhereintheworld,24hoursaday.TherearenosubscriptionfeesorsetupchargestouseGPS.Itusesaconstellationofbetween24and32mediumEarthorbitsatellitesthattransmitpreciseradiowavesignals,whichallowGPSreceiverstodeterminetheircurrentlocation,thetime,andtheirvelocity.ItsofficialnameisNAVSTARGPS.AlthoughNAVSTARisnotanacronym,1afewbackronymshavebeencreatedforit.Sinceitbecamefullyoperationalin1993,GPShasbecomeawidelyusedaidtonavigationworldwide,andausefultoolformapmaking,landsurveying,commerce,scientificuses,andhobbiessuchasgeocaching.Also,theprecisetimereferenceisusedinmanyapplicationsincludingthescientificstudyofearthquakesandasarequiredtimesynchronizationmethodforcellularnetworkprotocolssuchastheIS95standardforCDMA.GPSsatellitescircletheearthtwiceadayinaverypreciseorbitandtransmitsignalinformationtoearth.GPSreceiverstakethisinformationandusetriangulationtocalculatetheusersexactlocation.Essentially,theGPSreceivercomparesthetimeasignalwastransmittedbyasatellitewiththetimeitwasreceived.ThetimedifferencetellstheGPSreceiverhowfarawaythesatelliteis.Now,withdistancemeasurementsfromafewmoresatellites,thereceivercandeterminetheuserspositionanddisplayitontheunitselectronicmap.AGPSreceivermustbelockedontothesignalofatleastthreesatellitestocalculatea2Dpositionlatitudeandlongitudeandtrackmovement.Withfourormoresatellitesinview,thereceivercandeterminetheusers3Dpositionlatitude,longitudeandaltitude.Oncetheuserspositionhasbeendetermined,theGPSunitcancalculateotherinformation,suchasspeed,bearing,track,tripdistance,distancetodestination,sunriseandsunsettimeandmore.3HistoryThefirstsatellitenavigationsystem,Transit,usedbytheUnitedStatesNavy,wasfirstsuccessfullytestedin1960.Usingaconstellationoffivesatellites,itcouldprovideanavigationalfixapproximatelyonceperhour.In1967,theU.S.NavydevelopedtheTimationsatellitewhichprovedtheabilitytoplaceaccurateclocksinspace,atechnologythatGPSreliesupon.Inthe1970s,thegroundbasedOmegaNavigationSystem,basedonsignalphasecomparison,becamethefirstworldwideradionavigationsystem.ThedesignofGPSisbasedpartlyonsimilargroundbasedradionavigationsystems,suchasLORANandtheDeccaNavigatordevelopedintheearly1940s,andusedduringWorldWarII.AdditionalinspirationfortheGPScamewhentheSovietUnionlaunchedthefirstmanmadesatellite,Sputnikin1957.AteamofU.S.scientistsledbyDr.RichardB.KershnerweremonitoringSputniksradiotransmissions.Theydiscoveredthat,becauseoftheDopplereffect,thefrequencyofthesignalbeingtransmittedbySputnikwashigherasthesatelliteapproached,andlowerasitcontinuedawayfromthem.Theyrealizedthatsincetheyknewtheirexactlocationontheglobe,theycouldpinpointwherethesatellitewasalongitsorbitbymeasuringtheDopplerdistortion.AfterKoreanAirLinesFlight007wasshotdownin1983afterstrayingintotheUSSRsprohibitedairspace,3PresidentRonaldReaganissuedadirectivemakingGPSfreelyavailableforcivilianuseasacommongood.4Thesatelliteswerelaunchedbetween1989and1993.Initiallythehighestqualitysignalwasreservedformilitaryuse,whilethesignalavailableforcivilianusewasintentionallydegradedSelectiveAvailability,SA.SelectiveAvailabilitywas4endedin2000,improvingtheprecisionofcivilianGPSfromabout100mtoabout20m.OfcrucialimportanceforthefunctionofGPSistheplacementofatomicclocksinthesatellites,firstproposedbyFriedwardtWinterbergin1955.5Onlythencantherequiredpositionaccuracybereached.TimelineIn1972,theUSAirForceCentralInertialGuidanceTestFacilityHollomanAFBconducteddevelopmentalflighttestsoftwoprototypeGPSreceiversoverWhiteSandsMissileRange,usinggroundbasedpseudosatellites.SatellitenumbersBlockLaunchPeriodSatelliteslaunchedCurrentlyinserviceI1978–198510110II1985–199090IIA1990–19971913IIR1997–2004121112IIRM2005–20097126IIF2009–201101020

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