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外文翻译--什么是GPS以及GPS如何工作.doc外文翻译--什么是GPS以及GPS如何工作.doc -- 5 元

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WhyGPSandHowItWorksWhatsGPSNavigationandpositioningarecrucialtosomanyactivitiesandyettheprocesshasalwaysbeenquitecumbersome.Overtheyearsallkindsoftechnologieshavetriedtosimplifythetask,buteveryonehashadsomedisadvantage.Finally,theU.S.DepartmentofDefenseDoDdecidedthatthemilitaryhadtohaveasuperpreciseformofworldwidepositioning.Andfortunatelytheyhadthekindofmoney.Ittooktobuildsomethingreallygood.TheresultistheGlobalPositioningSystem,asystemthatschangednavigationforever.TheGlobalPositioningSystemGPSisaworldwideradionavigationsystemformedfromaconstellationof24satellitesandtheirgroundstation.GPSusesthesemanmadestarsasreferencepointstocalculatepositionsaccuratetoamatterofmeters.Infact,withadvancedformsofGPSyoucanmakemeasurementstobetterthanacentimeter.Inasenseitslikegivingeverysquaremeterontheplanetauniqueaddress.GPSreceivershavebeenminiaturizedtojustafewintegratedcircuitsandsoarebecomingveryeconomical.Andthatmakesthetechnologyaccessibletovirtuallyeveryone.ThesedaysGPSisfindinguswayintocars,boats,planes,constructionequipment,moviemakinggear,farmmachinery,evenlaptopcomputers.SoonGPSwillbecomealmostasbasicasthetelephone.Indeeditjustmaybecomeauniversalutility.WorkinginFiveLogicalStepsInprinciple,GPSoperatesinfivelogicalstepsStep1TriangulatingfromsatellitesImprobableasitmayseem,thewholeideabehindGPSistousesatellitesinspaceasreferencepointsforlocationshereonearth.Thatsabigideageometrically,asbyvery,veryaccuratelymeasuringourdistancefromthreesatelliteswecantriangulateourpositionanywhereonearth.Supposewemeasureourdistancefromasatelliteandfindittobe11,000miles.Knowingthatwere11,000milesfromaparticularsatellitenarrowsdownallthepossiblelocationswecouldbeinthewholeuniversetothesurfaceofaspherethatiscenteredonthissatelliteandhasaradiusof11,000miles.Next,saywemeasureourdistancetoasecondsatelliteandfindoutthatits12,000milesaway.Thattellsusthatwerenotonlyonthefirstspherebutwerealsoonaspherethats12,000milesfromthesecondsatellite.Orinotherwords,weresomewhereonthecirclewherethesetwospheresintersect.Ifwethenmakeameasurementfromathirdsatelliteandfindthatwere13,000milesfromthatone,thatnarrowsourpositiondownevenfarther,tothetwopointswherethe13,000milespherecutsthroughthecirclethatstheintersectionofthefirsttwospheres.Sobyrangingfromthreesatelliteswecannarrowourpositiontojusttwopointsinspace.Todecidewhichoneisourtruelocationwecouldmakeafourthmeasurement.ButusuallyoneofthetwopointsisaridiculousanswereithertoofarfromEarthoranimpossiblevelocityandcanberejectedwithoutameasurement.Step2MeasuringdistancefromasatelliteWesawinthelastsectionthatapositioniscalculatedfromdistancemeasurementstoatleastthreesatellites.ButhowcanyoumeasurethedistancetosomethingthatsfloatingaroundinspaceWedoitbytiminghowlongittakesforasignalsentfromthesatellitetoarriveatourreceiver.Inasense,thewholethingboilsdowntothosevelocitytimestraveltimemathproblemswedidinhighschool.InthecaseofGPSweremeasuringaradiosignalsothevelocityisgoingtobethespeedoflightorroughly186,000milespersecond.Theproblemismeasuringthetraveltime.Butassumingwehavepreciseclocks,howdowemeasuretraveltimeToexplainitletsuseagoofyanalogySupposetherewasawaytogetboththesatelliteandthereceivertostartplayingTheStarSpangledBanneratprecisely12Noon.Ifsoundcouldreachusfromspacewhich,ofcourse,isridiculous,thenstandingatthereceiverwedheartwoversionsofTheStarSpangledBanner,onefromourreceiverandonefromthesatellite.Thesetwoversionswouldbeoutofsync.Theversioncomingfromthesatellitewouldbealittledelayedbecauseithadtotravelover11,000miles.Ifwewantedtoseejusthowdelayedthesatellitesversionwas,wecouldstartdelayingthereceiversversionuntiltheyfellintoperfectsync.Theamountwehavetoshiftbackthereceiversversionisequaltothetraveltimeofthesatellitesversion.SowejustmultiplythattimetimesthespeedoflightandBLNGO.Wevegotourdistancetothesatellite.ThatsbasicallyhowGPSworks.OnlyinsteadofTheStarSpangledBannerthesatellitesandreceiversusesomethingcalledaPseudoRandomCodewhichisprobablyeasiertosingthanTheStarSpangledBanner.ThePseudoRandomCodePRCisafundamentalpartofGPS.Physicallyitsjustaverycomplicateddigitalcode,orinotherwords,acomplicatedsequenceofonandoffpulses.Thesignalissocomplicatedthatitalmostlookslikerandomelectricalnoise.HencethenamePseudoRandom.Step3GettingperfecttimingIfmeasuringthetraveltimeofaradiosignalisthekeytoGPSthenourstopwatcheshadbetterbeverygood,becauseiftheirtimingisoffbyjustathousandthofasecond,atthespeedoflight,thattranslatesintoalmost200milestoerrorOnthesatelliteside,timingisalmostperfectbecausetheyhaveincrediblypreciseatomicclocksonboard.ButwhataboutourreceivershereonthegroundRememberthatboththesatelliteandthereceiverneedtobeabletopreciselysynchronizetheirpseudorandomcodestomakethesystemwork.Ifourreceiversneededatomicclockswhichcostupwardsof50Kto100KGPSwouldbealameducktechnology.Nobodycouldaffordit.LuckilythedesignersofGPScameupwithabrilliantlittletrickthatletsusgetbywithmuchlessaccurateclocksinourreceivers.ThistrickisoneofthekeyelementsofGPSandasanaddedsidebenefititmeansthateveryGPSreceiverisessentiallyanatomicaccuracyclock.Thesecrettoperfecttimingistomakeanextrasatellitemeasurement.Ifthreeperfectmeasurementscanlocateapointin3dimensionalspace,thenfourimperfect
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