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测绘工程专业英语全文翻译（Unit20~30）Unit20UnderstandingtheGPS（认识GPS）WhatIsGPS?（什么是GPS）TheglobalPositioningSystem(GPS)operatedbytheU.S.DepartmentOFDefense(DOD)isasatellite-basedsystemthatcanbeusedtolocatepositionsanywhereontheearth（.全球定位系统（GPS），由美国国防部管理，是一个基于人造卫星的系统，可以用来在全球任何地方定位）GPSprovidescontinuous(24hours/day),real-time,3-dimensionalpositioning,navigationandtimingworldwide.（GPS提供全世界范围内的全天候，实时，三维坐标，导航和授时的功能。）AnypersonwithaGPSreceivercanaccessthesystem,anditcanbeusedforanyapplicationthatrequireslocationcoordinates.（任何拥有一台GPS接收机的人都可以使用这套系统，可以应用于任何需要位置坐标的工作中）TheGPSsystemconsistsofthreesegments:⑴thespacesegment:theGPSsatellitesthemselves,⑵thecontrolsegment,operatedbytheU.S.military,and⑶theusersegment,whichincludesbothmilitaryandcivilianusersandtheirGPSequipments（.GPS由三个部分组成：⑴太空部分：GPS卫星自己，⑵地面控制部分，由美国军方控制，⑶用户部分，包括军方和民用用户和他们的装备。）SpaceSegment:TheGPSConstellation（太空部分：GPS星座）ThefirstGPSsatellitewaslaunchedbytheU.S.AirForceinearly1978.（第一颗GPS卫星由美国空军在1978年的早些时候发射的）Therearenow24satellitesorbitingtheearthatanaltitudeofabout10,900miles.（现在有24颗卫星在绕地球大约10900英里的高度运行）Thehighaltitudeinsuresthatthesatelliteorbitsarestable,preciseandpredictable,andthatthesatellites’motionthroughspaceisnotaffectedbyatmosphericdrag.（这么高的高度确保了卫星轨道是稳定、精确和可知的，并且确保了卫星穿过太空的运行不受到大气阻力的影响）Therearefoursatellitesineachof6orbitalplanes.（6个轨道平面中每个有4颗卫星。）Eachplaneisinclined55degreesrelativetotheequator,whichmeansthatsatellitescrosstieequatortiltedata55degreesangle.（每个平面相对于赤道面倾斜55度，这就意味着这些卫星以55度角穿过赤道）Thesystemisdesignedtomaintainfulloperationalcapabilityeveniftwoofthe24satellitesfail.（这套系统被设计成即使24颗卫星的2颗故障也能维持全工作能力）These24satellitesmakeupafullGPSconstellation.（这24颗卫星组成了一个完整的GPS星座）TheGPSsatellitesarepoweredprimarilybysun-seekingsolarpanels,withnicadbatteriesprovidingsecondarypower.（GPS卫星主要由太阳定向太阳能电池板提供动力，镍镉蓄电池提供备用动力）EachGPSsatellitehasfouratomicclocksonboard,onlyoneofwhichisinuseatatime.ThesehighlyaccurateatomicclocksenableGPStoprovidethemostaccuratetimingsystemthatexists.（每一个GPS卫星有四个原子钟，某一时刻只有一台在使用。这些高精度原子钟能够使GPS提供最准确的时间系统）ControlSegment:U.S.DODMonitoring（控制部分：美国国防部监控）TheU.S.DepartmentOfDefensemaintainsamastercontrolstationatFalconAirForceBaseinColoradoSprings.（美国国防部维持了一个主控站，在科罗拉多斯普林斯的佛肯空军基地）TherearefourothermonitorstationslocatedinHawaii,AscensionIsland,DiegoGarciaandKwajalein.（还有4个监控站座落在夏威夷、阿松森岛、迭哥加西亚岛、卡瓦加兰岛）TheDODstationsmeasurethesatelliteorbitsprecisely.（DOD站精确测量卫星轨道）Anydiscrepanciesbetweenpredictedorbitsandactualorbitsaretransmittedbacktothesatellites.（任何预报轨道和实际轨道之间的偏差被传回卫星。）Thesatellitescanthenbroadcastthesecorrections,alongwiththeotherpositionandtimingdata,sothataGPSreceiverontheearthcanpreciselyestablishthelocationofeachsatelliteitistracking.（然后卫星可以广播这些改正值——连同位置和时间数据一起，以便地球上的GPS接收机可以精确确定它所跟踪的每个卫星的位置）UserSegment:MilitaryandCivilianGPSUsers（用户部分：军方和民用GPS用户）TheU.S.militaryusesGPSfornavigation,reconnaissance,andmissileguidancesystems.（美国军方把GPS用在导航、侦察和导弹制导系统上CivilianuseofGPSdevelopedatthesametimeasmilitaryuseswerebeingestablished,andhasexpandedfarbeyondoriginalexpectations.（GPS的民用在军用建立起之时就发展起来了，并且已经发展的远远超过起初的期望）TherearecivilianapplicationsforGPSinalmosteveryfield,fromsurveyingtotransportationtonaturalresourcemanagementtoagriculture.（GPS的民用用途几乎用在每个领域，从测量到交通到自然资源管理到农业）MostcivilianusesofGPS,however,fallintooneofthreecategories:navigation,positioningandtiming.（然而，GPS的大部分民用用途，分为三种类型：导航、定位和授时。）HowDoesGPSWork?（GPS如何工作）AGPSreceivercalculatesitspositionbyatechniquecalledsatelliteranging,whichinvolvesmeasuringthedistancebetweentheGPSreceiverandtheGPSsatellitesitistracking.（GPS接收机利用一项被称为卫星测距的技术，计算它自己的位置，涉及到GPS接收机到它所跟踪的GPS卫星的距离测量）Therange(therangeareceivercalculatesisactuallyapseudorange,oranestimateorangeratherthanatruerange)ordistanceismeasuredaselapsedtransittime.（距离（接收机测的距离实际上是伪距，或者说是距离估值，而不是真实距离）是用传播时间来测的）Thepositionofeachsatelliteisknown,andthesatellitestransmittheirpositionsaspartofthe“messages”theysendviaradiowaves.（每个卫星的位置已知，并且卫星将它们的位置作为它们用无线电波发出的“讯文”的一部分传送出去）TheGPSreceiveronthegroundistheunknownpoint,andmustcomputeitspositionbasedontheinformationitreceivesfromthesatellites.（地面GPS接收机是未知点，必需基于它从卫星收到的这些信息计算它的位置。）ThefirststepinmeasuringthedistancebetweentheGPSreceiverandasatelliterequiresmeasuringthetimeittakesforthesignaltotravelfromthesatellitetothereceiver.（测量GPS接收机和卫星的第一步，要求测出从卫星到接收机的信号传播时间。）Oncethereceiverknowshowmuchtimehaselapsed,thetraveltimeofthesignalmultipliesthespeedoflight(becausethesatellitesignalstravelatthespeedoflight,approximately186,000milespersecond)tocomputethedistance.（一旦接收机知道用了多少时间，用信号的传播时间乘上光速（因为卫星信号以光速传播，大约186,000英里每秒）来计算距离。）Distancemeasurementstofoursatellitesarerequiredtocomputea3-dimensional(latitude,longitudeandaltitude)position.（计算一个三维（纬度、精度和高程）位置需要4颗卫星的距离值）Inordertomeasurethetraveltimeofthesatellitesignal,thereceiverhastoknowwhenthesignalleftthesatelliteandwhenthesignalreachedthereceiver.（为了测出卫星信号的传播时间，接收机需要知道信号是什么时候离开卫星的和什么时候到达接收机的）Knowingwhenthesignalreachesthereceiveriseasy,theGPSreceiverjust“checks”itsinternalclockwhenthesignalarrivestoseewhattimeitis（.确定信号到达接收机的时间容易，GPS接收机只需在信号到达时“检查”自己内置的钟，看看是什么时间）Buthowdoesit“know”whenthesignalleftthesat