无人机专业英语 课件 Chap4 Satellite Navigation

EnglishforUnmannedaerialvehicle无人机专业英语第一部分第二部分导航的发展与原理卫星导航的分类Unit4Satellitenavigation——Classification

、HistoryandtheorySatnav=satellite['sætəlaɪt]navigation[nævɪ'geɪʃ(ə)n]卫星导航geo-spatial[dʒi:əʊˈspeɪʃəl]地理空间Longitude['lɒn(d)ʒɪtju:d]经度latitude['lætɪtju:d]纬度Transmit[trænz'mɪt]传输Satellite['sætəlaɪt]tracking['trækɪŋ]卫星追踪Synchronisation[sɪnŋkrənaɪ'zeʃən]同步GNSS=globalnavigationsatellitesystem全球导航卫星系统MEO=medium['miːdɪəm]Earthorbit

['ɔːbɪt]中地球轨道NewWords&PhrasesOrbital['ɔ:bɪt(ə)l]inclination[ɪnklɪ'neɪʃ(ə)n]轨道倾角SBAS=SatelliteBasedAugmentationSystems星基增强系统GBAS=GroundBasedAugmentationSystems地基增强系统WAAS=WideAreaAugmentationSystem广域增强系统EGNOS=EuropeanGeostationary[dʒiːə(ʊ)'steɪʃ(ə)n(ə)rɪ]NavigationOverlay[əʊvə'leɪ]Service欧洲同步卫星导航覆盖服务MSAS=Multi-FunctionalSatelliteAugmentationSystem多功能卫星增强系统LAAS=

LocalAreaAugmentationSystem

局域增强系统NewWords&PhrasesSpectrum['spektrəm]光谱DGPS=Differential[,dɪfə'renʃ(ə)l]GPS差分GPSRTK=RealTimeKinematic[,kɪnɪ'mætɪk]实时动态terrestrial

[tə'restrɪəl]陆地的Deviate['di:vɪeɪt]偏离Ephemeris[ɪ'fem(ə)rɪs]星历表(记录卫星运行参数)Atomic[ə'tɒmɪk]clock原子钟Trilateration[,trailætə'reiʃən]三边测量/三球交汇Ionosphere[aɪ'ɒnəsfɪə]电离层NewWords&PhrasesTangent[ˈtændʒənt]切线Radionavigation[,reɪdɪəʊ,nævɪ'ɡeɪʃən]无线电导航

compatible[kəm'pætɪb(ə)l]兼容的CBOC=Composite['kɒmpəzɪt]Binary['baɪnərɪ]Offset['ɒfset]Carrier

['kærɪə]

复合二进制偏移载波GEO=Geostationary[dʒiːə(ʊ)'steɪʃ(ə)n(ə)rɪ]orbit['ɔ:bɪt]对地静止轨道GSO=Geosynchronous[,dʒiːə(ʊ)'sɪŋkrənəs]orbit地球同步轨道Military['mɪlɪt(ə)rɪ]军事的Sensitivity[sensɪ'tɪvɪtɪ]敏感性NewWords&Phrasespseudo-random['sjuːdəʊ,'rændəm]code伪随机码UHF=Ultra['ʌltrə]HighFrequency超高频Plastic['plæstɪk]塑料的Troposphere['trɒpə(ʊ)sfɪə]对流层NewWords&PhrasesA

satellitenavigation

or

satnav

systemisasystemthatuses

satellites

toprovideautonomousgeo-spatialpositioning.Itallowssmall

electronic

receiverstodeterminetheirlocation(longitude,

latitude,and

altitude/elevation)tohighprecision(withinafewmetres)using

timesignals

transmittedalonga

lineofsight

by

radio

fromsatellites.卫星导航或卫星导航系统是利用卫星来提供自主地理空间定位的系统。它是通过卫星发射无线电报时信号，并沿直线传输，再根据小型电子接收器确定其位置信息(包括维度、经度、高度/海拔高度)，达到定位的高精度(几米内)。Satellite

NavigationThesystemcanbeusedforprovidingposition,navigationorfortrackingthepositionofsomethingfittedwithareceiver(satellitetracking).Thesignalsalsoallowtheelectronicreceivertocalculatethecurrentlocaltimetohighprecision,whichallowstimesynchronisation.Satnavsystemsoperateindependentlyofanytelephonicorinternetreception,thoughthesetechnologiescanenhancetheusefulnessofthepositioninginformationgenerated.该系统可用于提供位置、导航或跟踪装有接收机的物体的位置（卫星跟踪）。电子接收器可以对当地时间进行校准来提高信号的精度，使得时间同步。卫星导航系统独立于任何电话或互联网接收操作，尽管这些技术可以增强定位信息的有用性。Satellite

NavigationAsatellitenavigationsystemwithglobalcoveragemaybeknownasa

globalnavigationsatellitesystem

(GNSS).AsofDecember2016,onlythe

UnitedStates'

GlobalPositioningSystem

(GPS),Russia's

GLONASSandthe

EuropeanUnion‘s

Galileo

areglobaloperationalGNSSs.具有全球覆盖范围的卫星导航系统可称为全球导航卫星系统(GNSS)。截至2016年12月，只有美国的全球定位系统(GPS)，俄罗斯的GLONASS（格洛纳斯）和欧盟的Galileo（伽利略）系统是全球运行的GNSS。Satellite

NavigationTheEuropeanUnion’sGalileoGNSSisscheduledtobefullyoperationalby2020.

China

isintheprocessofexpandingitsregional

BeiDouNavigationSatelliteSystem

intotheglobal

BeiDou-2GNSS

by2020.India,

France

and

Japan

areintheprocessofdevelopingregionalnavigationandaugmentationsystemsaswell.欧盟伽利略全球导航卫星系统计划到2020年全面运行。到2020年，中国的区域性北斗导航卫星系统也将扩展到全球性的北斗2号全球导航卫星系统。印度、法国和日本也在开发区域导航和增强系统。Satellite

NavigationGlobalcoverageforeachsystemisgenerallyachievedbya

satelliteconstellation

of18–30

mediumEarthorbit

(MEO)satellitesspreadbetweenseveral

orbitalplanes.Theactualsystemsvary,butuse

orbitalinclinations

of>50°and

orbitalperiods

ofroughlytwelvehours(atanaltitudeofabout20,000kilometresor12,000miles).每个系统的全球覆盖通常是由18至30颗中地球轨道(MEO)卫星组成的卫星星座实现的，这些卫星分布在几个轨道平面之间。实际系统各不相同，但使用的轨道倾角大于50°，轨道周期约为12小时(高度约20000公里或12000英里)。Satellite

NavigationSatellitenavigationsystemsthatprovideenhancedaccuracyandintegritymonitoringusableforcivilnavigationareclassifiedasfollows:GNSS-1isthefirstgenerationsystemandisthecombinationofexistingsatellitenavigationsystems(GPSandGLONASS),with

SatelliteBasedAugmentationSystems

(SBAS)or

GroundBasedAugmentationSystems

(GBAS).Part1ClassificationIntheUnitedStates,thesatellitebasedcomponentisthe

WideAreaAugmentationSystem

(WAAS),inEuropeitisthe

EuropeanGeostationaryNavigationOverlayService

(EGNOS),andinJapanitisthe

Multi-FunctionalSatelliteAugmentationSystem

(MSAS).Groundbasedaugmentationisprovidedbysystemslikethe

LocalAreaAugmentationSystem

(LAAS).GNSS-2

isthesecondgenerationofsystemsthatindependentlyprovidesafullciviliansatellitenavigationsystem,exemplifiedbytheEuropeanGalileopositioningsystem.Part1ClassificationThesesystemswillprovidetheaccuracyandintegritymonitoringnecessaryforcivilnavigation;includingaircraft.ThissystemconsistsofL1andL2frequencies(inthe

Lband

oftheradiospectrum)forciviluseandL5forsystemintegrity.DevelopmentisalsoinprogresstoprovideGPSwithciviluseL2andL5frequencies,makingitasystem.这些系统将提供民用导航所必需的准确性和完整性监测；包括飞行器。该系统由L1和L2频率段组成（在无线电频谱的L波段），用于民用，L5频率段用于系统完整性。民用L2和L5频率在GPS的应用尚在开发，使之成为GNSS-2系统。Part1ClassificationCoreSatellitenavigationsystems,currentlyGPS(UnitedStates),GLONASS(RussianFederation),Galileo(EuropeanUnion)andBeiDouNavigationSatelliteSystem(BDS,China).GlobalSatelliteBasedAugmentationSystems(SBAS)suchasOmnistarandStarFire.RegionalSBASincludingWAAS(US),EGNOS(EU),MSAS(Japan)andGAGAN(India).RegionalscaleGBASsuchasCORSnetworks.Part1ClassificationRegionalSatelliteNavigationSystemssuchasChina'sBeidou,India'sNAVIC,andJapan'sproposedQZSS.ContinentalscaleGroundBasedAugmentationSystems(GBAS)forexampletheAustralianGRASandthejointUSCoastGuard,CanadianCoastGuard,USArmyCorpsofEngineersandUSDepartmentofTransportationNationalDifferentialGPS(DGPS)service.LocalGBAStypifiedbyasingleGPSreferencestationoperatingRealTimeKinematic(RTK)corrections.Part1ClassificationGroundbased

radionavigation

haslongbeenpracticed.The

DECCA,LORAN,GEE

and

Omega

systemsusedterrestrial

longwave

radio

transmitters

whichbroadcastaradiopulsefromaknown"master"location,followedbyapulserepeatedfromanumberof"slave"stations.台卡、罗兰、奇(GEE)和欧米伽系统使用地面长波无线电发射机，从已知的“主”位置发射无线电脉冲，并跟随若干重复脉冲。Part2—HistoryandtheoryThedelaybetweenthereceptionofthemastersignalandtheslavesignalsallowedthereceivertodeducethedistancetoeachoftheslaves,providinga

fix.主信号和从信号的接收之间的延迟使得接收机能够推断到每个从信号之间的距离，从而提供一个确定值。Thefirstsatellitenavigationsystemwas

Transit,asystemdeployedbytheUSmilitaryinthe1960s.Transit'soperationwasbasedonthe

Dopplereffect:thesatellitestravelledonwell-knownpathsandbroadcasttheirsignalsonawell-known

radiofrequency.Part2—HistoryandtheoryThereceivedfrequencywilldifferslightlyfromthebroadcastfrequencybecauseofthemovementofthesatellitewithrespecttothereceiver.Bymonitoringthisfrequencyshiftoverashorttimeinterval,thereceivercandetermineitslocationtoonesideortheotherofthesatellite,andseveralsuchmeasurementscombinedwithapreciseknowledgeofthesatellite'sorbitcandetermineaparticularposition.由于卫星相对于接收机的移动，接收频率将与发射频率略有不同。通过在短时间内监测这一频率变化接收器可以确定它在卫星的一边或另一边的位置，而几个这样的测量，再加上卫星轨道的精确知识，就可以确定详细的位置。Part2—HistoryandtheoryPartofanorbitingsatellite'sbroadcastincludeditspreciseorbitaldata.Inordertoensureaccuracy,the

USNavalObservatory(USNO)

continuouslyobservedthepreciseorbitsofthesesatellites.Asasatellite'sorbitdeviated,theUSNOwouldsendtheupdatedinformationtothesatellite.Subsequentbroadcastsfromanupdatedsatellitewouldcontainitsmostrecent

ephemeris.轨道卫星广播的一部分包括其精确的轨道数据。为了保证精确性，美国海军天文台(USNO)不断观测这些卫星的精确轨道。当卫星的轨道偏离时，USNO将向卫星发送更新的信息。随后从更新的卫星广播将包含其最近的星历表（记录卫星运行参数）。Part2—HistoryandtheoryModernsystemsaremoredirect.Thesatellitebroadcastsasignalthatcontainsorbitaldata(fromwhichthepositionofthesatellitecanbecalculated)andtheprecisetimethesignalwastransmitted.现代系统更直接。卫星广播一个包含轨道数据的信号(卫星的位置可以从该数据中计算)，以及信号发送的精确时间。Theorbital

ephemeris

istransmittedinadatamessagethatissuperimposedonacodethatservesasatimingreference.轨道星历表作为时间基准，传输的是迭代的数据编码。Part2—HistoryandtheoryThesatelliteusesan

atomicclock

tomaintainsynchronizationofallthesatellitesintheconstellation.Thereceivercomparesthetimeofbroadcastencodedinthetransmissionofthree(atsealevel)orfourdifferentsatellites,therebymeasuringthetime-of-flighttoeachsatellite.Severalsuchmeasurementscanbemadeatthesametimetodifferentsatellites,allowingacontinualfixtobegeneratedinrealtimeusinganadaptedversionof

trilateration.卫星使用原子钟来保持星座中所有卫星的同步。接收机通过比较三颗或是四颗不同的卫星(相对于海平面)传输编码的广播时间。从而测量每颗卫星的渡越时间。这样的测量可以同时对不同的卫星进行，可通过适当的三边测量来进行连续实时地修正。Part2—HistoryandtheoryEachdistancemeasurement,regardlessofthesystembeingused,placesthereceiveronasphericalshellatthemeasureddistancefromthebroadcaster.Bytakingseveralsuchmeasurementsandthenlookingforapointwheretheymeet,afixisgenerated.However,inthecaseoffast-movingreceivers,thepositionofthesignalmovesassignalsarereceivedfromseveralsatellites.每一种距离测量，无论使用什么系统，都将接收器放置在播送设备所测量距离的球面上。通过几次这样的测量，然后寻找交汇点，就会产生一个确定值。然而，在快速移动接收器的情况下，信号移动的位置是从多个卫星接收到的信号。Part2—HistoryandtheoryInaddition,theradiosignalsslowslightlyastheypassthroughtheionosphere,andthisslowingvarieswiththereceiver'sangletothesatellite,becausethatchangesthedistancethroughtheionosphere.Thebasiccomputationthusattemptstofindtheshortestdirectedlinetangenttofouroblatesphericalshellscentredonfoursatellites.此外，无线电信号在通过电离层时会稍微减慢，且这种减速会随接收器与卫星的角度而变化，因为这会改变通过电离层的距离。因此，基本计算试图找出四颗卫星上四个扁球壳的最短有向线切线。Part2—HistoryandtheorySatellitenavigationreceiversreduceerrorsbyusingcombinationsofsignalsfrommultiplesatellitesandmultiplecorrelators,andthenusingtechniquessuchas

Kalmanfiltering

tocombinethenoisy,partial,andconstantlychangingdataintoasingleestimateforposition,time,andvelocity.卫星导航接收机利用多颗卫星和多个相关器的信号组合，然后利用卡尔曼滤波等技术对局部噪声进行组合，从而减小误差。并不断地将数据转换为位置、时间和速度的单一估计。Part2—HistoryandtheoryBESTREGARDSTOALLOFYOU!THANKS!EnglishforUnmannedaerialvehicle无人机专业英语第三部分第四部分区域卫星导航系统全球卫星导航系统Unit4Satellitenavigation——GNSS、Regionalnavigationsatellitesystems、HowdoestheGPSwork?第五部分GPS工作原理Satnav=satellite['sætəlaɪt]navigation[nævɪ'geɪʃ(ə)n]卫星导航geo-spatial[dʒi:əʊˈspeɪʃəl]地理空间Longitude['lɒn(d)ʒɪtju:d]经度latitude['lætɪtju:d]纬度Transmit[trænz'mɪt]传输Satellite['sætəlaɪt]tracking['trækɪŋ]卫星追踪Synchronisation[sɪnŋkrənaɪ'zeʃən]同步GNSS=globalnavigationsatellitesystem全球导航卫星系统MEO=medium['miːdɪəm]Earthorbit

['ɔːbɪt]中地球轨道NewWords&PhrasesOrbital['ɔ:bɪt(ə)l]inclination[ɪnklɪ'neɪʃ(ə)n]轨道倾角SBAS=SatelliteBasedAugmentationSystems星基增强系统GBAS=GroundBasedAugmentationSystems地基增强系统WAAS=WideAreaAugmentationSystem广域增强系统EGNOS=EuropeanGeostationary[dʒiːə(ʊ)'steɪʃ(ə)n(ə)rɪ]NavigationOverlay[əʊvə'leɪ]Service欧洲同步卫星导航覆盖服务MSAS=Multi-FunctionalSatelliteAugmentationSystem多功能卫星增强系统LAAS=

LocalAreaAugmentationSystem

局域增强系统NewWords&PhrasesSpectrum['spektrəm]光谱DGPS=Differential[,dɪfə'renʃ(ə)l]GPS差分GPSRTK=RealTimeKinematic[,kɪnɪ'mætɪk]实时动态terrestrial

[tə'restrɪəl]陆地的Deviate['di:vɪeɪt]偏离Ephemeris[ɪ'fem(ə)rɪs]星历表(记录卫星运行参数)Atomic[ə'tɒmɪk]clock原子钟Trilateration[,trailætə'reiʃən]三边测量/三球交汇Ionosphere[aɪ'ɒnəsfɪə]电离层NewWords&PhrasesTangent[ˈtændʒənt]切线Radionavigation[,reɪdɪəʊ,nævɪ'ɡeɪʃən]无线电导航

compatible[kəm'pætɪb(ə)l]兼容的CBOC=Composite['kɒmpəzɪt]Binary['baɪnərɪ]Offset['ɒfset]Carrier

['kærɪə]

复合二进制偏移载波GEO=Geostationary[dʒiːə(ʊ)'steɪʃ(ə)n(ə)rɪ]orbit['ɔ:bɪt]对地静止轨道GSO=Geosynchronous[,dʒiːə(ʊ)'sɪŋkrənəs]orbit地球同步轨道Military['mɪlɪt(ə)rɪ]军事的Sensitivity[sensɪ'tɪvɪtɪ]敏感性NewWords&Phrasespseudo-random['sjuːdəʊ,'rændəm]code伪随机码UHF=Ultra['ʌltrə]HighFrequency超高频Plastic['plæstɪk]塑料的Troposphere['trɒpə(ʊ)sfɪə]对流层NewWords&PhrasesGPSTheUnitedStates'GlobalPositioningSystem(GPS)consistsofupto32

mediumEarthorbit

satellitesinsixdifferent

orbitalplanes,withtheexactnumberofsatellitesvaryingasoldersatellitesareretiredandreplaced.Operationalsince1978andgloballyavailablesince1994,GPSiscurrentlytheworld'smostusedsatellitenavigationsystem.Part3GlobalnavigationsatellitesystemsGLONASSTheformerly

Soviet,andnow

Russian,

GlobalNavigationSatelliteSystem,orGLONASS,isaspace-basedsatellitenavigationsystemthatprovidesacivilianradionavigation-satelliteserviceandisalsousedbytheRussianAerospaceDefenceForces.Thefullorbitalconstellationof24GLONASSsatellitesenablesfullglobalcoverage.Part3GlobalnavigationsatellitesystemsGalileoThe

EuropeanUnion

and

EuropeanSpaceAgency

agreedinMarch2002tointroducetheirownalternativetoGPS,calledthe

Galileopositioningsystem.Galileobecameoperationalon15December2016(globalEarlyOperationalCapability(EOC))

AtanestimatedcostofEUR3.0billion,thesystemof30

MEO

satelliteswasoriginallyscheduledtobeoperationalin2010.Theoriginalyeartobecomeoperationalwas2014.

Thefirstexperimentalsatellitewaslaunchedon28December2005.

Part3GlobalnavigationsatellitesystemsGalileoGalileoisexpectedtobecompatiblewiththe

modernizedGPS

system.ThereceiverswillbeabletocombinethesignalsfrombothGalileoandGPSsatellitestogreatlyincreasetheaccuracyofposition.Galileoisexpectedtobeinfullservicein2020andatasubstantiallyhighercost.ThemainmodulationusedinGalileoOpenServicesignalisthe

CompositeBinaryOffsetCarrier

(CBOC)modulation.伽利略开放业务信号的主要调制方式是复合二进制偏移载波(CBOC)调制。Part3GlobalnavigationsatellitesystemsBeiDou-2China

hasindicatedtheyplantocompletetheentiresecondgenerationBeiDouNavigationSatelliteSystem(BDSorBeiDou-2,formerlyknownasCOMPASS),byexpandingcurrentregional(Asia-Pacific)serviceintoglobalcoverageby2020.TheBeiDou-2systemisproposedtoconsistof30

MEO

satellitesandfivegeostationarysatellites.A16-satelliteregionalversion(coveringAsiaandPacificarea)wascompletedbyDecember2012.Part3GlobalnavigationsatellitesystemsBeiDou-1Chineseregional(Asia-Pacific,16satellites)networktobeexpandedintothewholeBeiDou-2globalsystemwhichconsistsofall35satellitesby2020.中国区域(亚太，16颗卫星)网络将扩展到整个北斗-2全球系统，到2020年由35颗卫星组成。Part4RegionalnavigationsatellitesystemsNAVICThe

NAVIC

or

NavigationwithIndianConstellation

isanautonomousregionalsatellitenavigationsystemdevelopedby

IndianSpaceResearchOrganisation

(ISRO)whichwouldbeunderthetotalcontrolof

Indiangovernment.ThegovernmentapprovedtheprojectinMay2006,withtheintentionofthesystemcompletedandimplementedon28April2016.Itwillconsistofaconstellationof7navigationalsatellites.

3ofthesatelliteswillbeplacedinthe

Geostationaryorbit(GEO)

andtheremaining4inthe

Geosynchronousorbit(GSO)

tohavealargersignalfootprintandlowernumberofsatellitestomaptheregion.Part4RegionalnavigationsatellitesystemsNAVICItisintendedtoprovideanall-weatherabsolutepositionaccuracyofbetterthan7.6metersthroughout

India

andwithinaregionextendingapproximately1,500

kmaroundit.

它的目的是在整个印度及其周围约1500公里的区域，全天候绝对定位精度超过7.6米。印度完整的目标已说明控制，空间段、地面段和用户接收器都是在印度建造的。AgoalofcompleteIndiancontrolhasbeenstated,withthe

spacesegment,

groundsegment

anduserreceiversallbeingbuiltinIndia.Part4RegionalnavigationsatellitesystemsQZSSTheQuasi-ZenithSatelliteSystem(QZSS),isaproposedthree-satelliteregional

timetransfer

systemandenhancementfor

GPS

covering

Japan.ThefirstdemonstrationsatellitewaslaunchedinSeptember2010.Part4RegionalnavigationsatellitesystemsTheSatelliteNetworkTheGPSsatellitestransmitsignalstoaGPSreceiver.Thesereceiverspassivelyreceivesatellitesignals;theydonottransmit.GPSreceiversrequireanunobstructedviewofthesky,sotheycanonlybeusedeffectivelyoutdoors.EarlyreceiversdidnotperformwellwithinforestedareasorneartallbuildingsbutlaterreceiverdesignssuchasSiRFStarIII,MTKetchaveovercomethisandimprovedperformanceandsensitivitymarkedly.GPSoperationsdependonaveryaccuratetimereference,whichisprovidedbyatomicclocksonboardthesatellites.Part5GPS—Howitworks？TheSatelliteNetworkEachGPSsatellitetransmitsdatathatindicatesitslocationandthecurrenttime.AllGPSsatellitessynchronizeoperationssothattheserepeatingsignalsaretransmittedatthesameinstant.Thesignals,movingatthespeedoflight,arriveataGPSreceiveratslightlydifferenttimesbecausesomesatellitesarefurtherawaythanothers.Part5GPS—Howitworks？TheSatelliteNetworkAstheFig3-1shows,thedistancetotheGPSsatellitescanbedeterminedbyestimatingtheamountoftimeittakesfortheirsignalstoreachthereceiver.WhenthereceiverestimatesthedistancetoatleastfourGPSsatellites,asitcancalculateitspositioninthreedimensions.Part5GPS—Howitworks？Fig.3-1SignalsfrommultiplesatellitesTheSatelliteNetworkOnceithasinformationonhowfarawayatleastthreesatellitesare,yourGPSreceivercanpinpointyourlocationusingaprocesscalledtrilateration(seetheFig.3-2).Part5GPS—Howitworks？Fig.3-2trilaterationTheSatelliteNetworkImagineyouarestandingsomewhereonEarthwiththreesatellitesintheskyaboveyou.IfyouknowhowfarawayyouarefromsatelliteA,thenyouknowyoumustbelocatedsomewhereontheredcircle.IfyoudothesameforsatellitesBandC,youcancalculatedyourlocationbyseeingwherethethreecirclesintersect.ThisisjustwhatyourGPSreceiverdoes,althoughitusesoverlappingspheresratherthancircles.Part5GPS—Howitworks？TheSatelliteNetworkThemoresatellitesthereareabovethehorizonthemoreaccuratelyyourGPSunitcandeterminewhereyouare.地平线上的卫星越多，你的全球定位系统就越能准确地确定你在哪里。Part5GPS—Howitworks？TheSatelliteNetworkAsshowninFig3-3,thereareatleast24operationalGPSsatellitesatalltimesplusanumberofspares.

Thesatellites,operatedbytheUSDOD,orbitwithaperiodof12hours(twoorbitsperday)ataheightofabout11,500milestravelingat9000mph(3.9km/sor14,000kph).Groundstationsareusedtopreciselytrackeachsatellite'sorbit.Part5GPS—Howitworks？Fig.3-3TheNavstarGPSConstellation如图3-3所示，在任何时候至少有24颗可运行的GPS卫星及其备件。这些卫星由美国国防部运行，运行周期为12小时(每天两次)。在大约11，500英里的高度以每小时9000英里(3.9公里/秒或14，000公里/小时)的速度行驶。地面站用于精确跟踪每颗卫星的轨道。Part5GPS—Howitworks？HowPositionisDeterminedAGPSreceiver"knows"thelocationofthesatellitesbecausethatinformationisincludedinthetransmitted

Ephemerisdata.Byestimatinghowfarawayasatellite

is,thereceiveralso"knows"itislocatedsomewhereonthesurfaceofanimaginaryspherecentredatthesatellite.GPS接收机“知道”卫星的位置，因为该信息包含在发送星历数据中。通过估计一颗卫星的距离，接收器也“知道”它是位于以卫星为中心的假想球体表面的某个地方。Itthendeterminesthesizesofseveralspheres,oneforeachsatelliteandthereforeknowsthereceiverislocatedwherethesespheresintersect.Part5GPS—Howitworks？GPSAccuracyTheaccuracyofapositiondeterminedwithGPSdependsonthetypeofreceiver.MostconsumerGPSunitshaveanaccuracyofaboutplusorminus10m.OthertypesofreceiversuseamethodcalledDifferentialGPS(DGPS)toobtainmuchhigheraccuracy.DGPSrequiresanadditionalreceiverfixedataknownlocationnearby.Observationsmadebythestationaryreceiverareusedtocorrectpositionsrecordedbytherovingunits,producinganaccuracygreaterthan1meter.固定接收器所作的观测用于校正流动单元所记录的位置，从而产生大于1米的精度。Part5GPS—Howitworks？HowIsTheSignalTimed?AllGPSsatelliteshaveseveralatomicclocks.Thesignalthatissentoutisarandomsequence,eachpartofwhichisdifferentfromeveryother,calledpseudo-randomcode.Thisrandomsequenceisrepeatedcontinuously.所有的GPS卫星都有几个原子钟。发出的信号是随机序列，每个部分都不同，称为伪随机码。这个随机序列是持续更新的。Part5GPS—Howitworks？HowIsTheSignalTimed?AllGPSreceiversknowthissequenceandrepeatitinternally.Therefore,satellitesandthereceiversmustbeinsynch.Thereceiverpicksupthesatellite'stransmissionandcomparestheincomingsignaltoitsowninternalsignal.Bycomparinghowmuchthesatellitesignalislagging,thetraveltimebecomesknown.所有GPS接收机都知道这个序列，并在内部重复。因此，卫星和接收器必须同步。接收机接收卫星的传输，并将接收到的信号与自己的内部信号进行比较。通过比较卫星信号的滞后程度，就知道了传输时间。Part5GPS—Howitworks？Whatdoesthesignalconsistof?GPSsatellitestransmittworadiosignals.ThesearedesignatedasL1andL2.ACivilianGPSusestheL1signalfrequency(1575.42MHz)intheUHFband.Thesignalstravelbylineofsight,meaningtheywillpassthroughclouds,glass,plasticetcbutwillnottravelthroughsolidobjectssuchasbuildingsandmountains.全球定位系统卫星发射两个无线电信号。它们被命名为L1和L2。民用全球定位系统在超高频波段使用L1信号频率(1575.42兆赫)。这些信号是通过直线传播的，能够穿过云，玻璃，塑料等，但不会穿越实体物体，如建筑物和山脉。Part5GPS—Howitworks？Whatdoesthesignalconsistof?TheGPSsignalcontainsthreedifferentbitsofinformation—apseudorandomcode,almanacdataandephemerisdata.The

pseudorandomcode

issimplyanI.D.codethatidentifieswhichsatelliteistransmittinginformation.YoucanoftenviewthisnumberonyourGPSunit'ssatelliteinformationpage,thenumberattachedtoeachsignalbaridentifieswhichsatellitesit'sreceivingasignalfrom.Part5GPS—Howitworks？Whatdoesthesignalconsistof?Almanacdata

isdatathatdescribestheorbitalcoursesofthesatellites.Everysatellitewillbroadcastalmanacdataforeverysatellite.YourGPSreceiverusesthisdatatodeterminewhichsatellitesitexpectstoseeinthelocalsky.Itcanthendeterminewhichsatellitesitshouldtrack.WithAlmanacdatathereceivercanconcentrateonthosesatellitesitcanseeandforgetaboutthosethatwouldbeoverthehorizonandoutofview.Almanacdataisnotpreciseandcanbevalidformanymonths.Part5GPS—Howitworks？Whatdoesthesignalconsistof?Ephemerisdata

isdatathattellstheGPSreceiverwhereeachG