GPS 通信系统 外文文献 英文文献

附录A译文全球移动通信系统移动系统跨越世界性成功标志是越来越朝着个人化、方便化方向发展。在商业活动中，人们必须使用移动电话，以便无论何时何地都能实现电话的功能。在快速的个人生活中，移动电话已成为一种必须，而不仅仅是为了方便。不像固定通信系统那样，很大程度上依靠技术和通信标准，移动通信系统随着个人通信系统的革命而发生变化。对移动通信系统而言，要获得调整后的武夫，有三个关键因素，即价格、电话的大小和重量以及网络的花费和质量。如果上述因素实现有困难，特别是前两个，那么市场的发展将严格受限。固定电话的服务是全球的，相互联系的范围从同轴电缆到光纤，以及人造卫星。世界通信标准是不同的，但随着普通接口以及对接口转化，相互之间的联系能发生改变。随着漫游的创建，需要一个复杂的网络工作系统，这对于移动通信而言是一个非常复杂的问题。因此，移动通信的通信标准问题比固定通信系统标准问题更关键，此外，在移动通信领域无线电频谱分配问题也非常使人烦恼。移动通信系统是最初工作在频带为450MHz模拟方式（现在仍然有），后来随着数字式GSM发展,工作在频带为900MHz,之后随着个人通信系统的发展,工作的频带为1800MHz。移动通信系统的历史可分为几代。第一代为美国的先进移动电话系统（AMPS）,欧洲大部分的全通路通信系统（TACS）,以及北欧的移动电话系统（NMTS）,这些都是模拟系统。第二代由第一个非常标准的计划支配这个计划由欧洲特殊移动通信系统委员会（GSM）制定，这个设计作为全球移动通信系统。GSM系统基于蜂窝通信原理，其最早作为一个概念由美国贝尔实验室的工程师们提出，这一思想出自于增加网络容量的需要以及解决网络堵塞的问题。在人口稠密地区运行的广播式移动网络系统会由于很少的几个用户同时呼叫而引起堵塞。蜂窝系统的功能在于允许频率复用。蜂窝的概念由两个特征定义，即频率复用和小区分裂。频率复用的区域相隔非常远，不会产生同一通道的干扰问题。这允许传递同时呼叫，超过了理论上的频谱容量。当需要一个小区的通信量到达最大，小区分裂是必须的，那么这个小区被分为一个更小的小区。蜂窝系统的小区通常被描绘成六角形，一组七个、九个或十二个。附录B外文文献GSM(GlobalSystemforMobileCommunication)Thesuccessofmobilesystemsacrosstheworldisasignthatcommunicationismovingtowardsamorepersonalized,convenientsystem.Peoplewhohavetouseamobilephoneonbusinesssoonbegintorealizethattheabilitytophoneanytime,anyplaceinone'spersonalliferapidlybecomesanecessity,notaconvenience.Thespeedandrapiditywithwhichthepersonalcommunicationsrevolutiontakesplaceis,unlikefixedtransmissionsystems,highlydependentontechnologyandcommunicationstandards.Formobilethethreekeyelementstoachiexdngsendeetake-uparethecost,thesizeandtheweightofthephone,andthecostandqualityofthelink.Ifanyofthesearewrong,especiallythefirsttwo,thenmarketgrowthisliabletobeseverelyrestricted.Thefixedtelephoneserviceisglobalandtheinterconnectionvariesfromcoaxialcabletoopticalfiberandsatellite.Thenationalstandardsaredifferent,butwithcommoninterfacesandinterfaceconversion,interconnectioncantakeplace.Formobiletheproblemisfarmorecomplex,withtheneedtoroamcreatinganeedforcomplex,networksandsystems.Thusinmobilethequestionofstandardsisfarmorecrucialtosuccessthanfixedsystems.Inaddition,thereisalsothevexedquestionofspectrumallocationinthemobilearea.Mobilesystemsoriginallyoperatedinanaloguemode(andstilldo)inthe450MHzbandmovinglaterto900MHzwithdigitalGSMandthento1800MHzwithpersonalcommunicationssystems.Thehistorytomobilitycansplitintogenerations.Thefirstgenerationsystemsweretheadvancedmobilephonesystems(AMPS)intheUS,totalaccesscommunicationsystems(TACS)inmostofEuropeandNordicmobiletelephonesystem(NMTS);whichwereallanaloguesystems.ThesecondgenerationisverymuchdominatedbythestandardfirstsetoutinEuropebythegroupspecialmobile(GSM)committee,whichwasdesignedasaglobalmobilecommunicationsystem.TheGSMsystemisbasedonacellularcommunicationprinciplewhichwasfirstproposedasaconceptinthe1940'sbyBellSystemengineersintheUS.Theideacameoutoftheneedtoincreasenetworkcapacityandgotroundthefactthatbroadcastmobilenetworks,operatingindenselypopulatedareas,couldbejammedbyaverysmallnumberofsimultaneouscalls.Thepowerofthecellularsystemwasthatitallowedfrequencyreuse.Thecellularconceptisdefinedbytwofeatures,frequencyreuseandcellsplitting.Frequencyreusecomesintoplaybyusingradiochannelsonthesamefrequencyincoverageareasthatarefarenoughapartnottocauseco-channelinterference.Thisallowshandlingofsimultaneouscallsthatexceedthetheoreticalspectralcapacity.Cellsplittingisnecessatywhenthetrafficdemandonacellhasreachedthemaximumandthecellisthendividedintoamicro-cellularsystem.Theshapeofcellinacellularsystemisalwaysdepictedasahexagonandtheclustersizecanbeseven,nineortwelve.TheGSMsystemrequiresanumberoffunctionstobecreatedforafullyoperationalmobilesystem.Thecellcoverageareaiscontrolledbyabasestationwhichisitselfmadeupoftwoelements.Thefirstelementisthetransmissionsystemwhichcommunicatesouttothemobileandalsoreceivesinformationfromittosetupandmaintaincallswhenactuallyinoperation.ThebaseStationtransceiver(BST)iscontrolledbythebasestationcontroller(BSC),whichcommunicateswiththemobileswitchingcenter(MSC)-theessentiallinktothelocalpublicswitchedtelephonenetwork(PSTN),andtothesubscriberdatawhichisstoredinregisterswithinthesystem.ThesubscriberregistersallowtheGSMsystemtocheckasubscriberwhorequeststheuseofthenetwork,allowaccessandthensetupthechargingfunction,etc.TheGSMsystemwasallocatedpartofthe900MHzbandatthe1978WorldAdministrationConference(WAC),theactualbandsbeing890to915MHzfortheuplinktransmissionand935to960MHzforthedownlink.Theaccessmethodistimedivisionmultipleaccess(TDMA).TheGSMsystemoperatesinabursttransmissionmodewith124radiochannelsinthe900MHzband,andtheseburstscancarrydifferenttypesofinformation.Thefirsttypeofinformationisspeech,whichiscodedat6.5kb/sor13kb/s.Thesecondtypeisdata,whichcanbesentat3.6kb/s,6kb/sor12.6kb/s.Theretwoformsoftransmissionaretheusefulpartsofthetransmission,buthavetobesupportedbyoverheadinformationwhichissentincontrolchannels(CCH).TheuseofdigitalradiotransmissionandtheadvancedhandoveralgorithmsbetweenradiocellsinGSMnetworkallowsforsignificantlybetterfrequencyusagethaninanaloguecellularsystems,thusincreasingthenumberofsubscribersthatcanbeserved.SinceGSMprovidesacommonstandard,cellularsubscriberswillalsobeabletousetheirtelephonesovertheentireGSMservicearea.RoamingisfullyautomaticbetweenandwithinallcountriescoveredbyGSMsystem.Inadditiontointernationalroaming,GSMprovidesnewservices,suchashigh-speeddatacommunication,facsimileandshortmessageservice.TheGSMtechnicalspecificationsaredesignedtoworkinconcertwithotherstandards,e.g.ISDN.Inter-workingbetweenthestandardsisinthiswayassured.Inthelongtermperspectivecellularsystems,usingadigitaltechnologywillbecometheuniversalmethodoftelecommunication.ThethirdgenerationmobilecommunicationsystemcurrentlybeingdevelopedinEuropeisintendedtointegrateallthedifferentsendeesofthesecondgenerationsystemsandcoveramuchwiderrangeofbroadbandsendees(voice,data,video,multimedia)consistentandcompatiblewithtechnologydevelopmentstakingplacewithinthefixedtelecommunicationnetworks.RadioReceiverAmodernradioreceiver,whoseinputsignalstothisradioareamplitude-modulatedradiowaves.Thebasicelectroniccircuitsinclude:antenna,tuner,mixer,localoscillator,IFamplifier,audiodetector,AFamplifier,loudspeaker,andpowersupply.Anyantennasystemcapableofradiatingelectricalenergyisalsoabletoabstractenergyfromapassingradiowave.Sinceeverywavepassingthereceivingantennaincludesitsownvoltageintheantennaconductor,itisnecessarythatthereceivingequipmentbecapableofseparatingthedesiredsignalfromtheunwantedsignalsthatarealsoinducingvoltagesintheantenna.Thisseparationismadeonthebasisofthedifferenceinfrequencybetweentransmittingstationsandiscarriedoutbytheuseofresonantcircuits,whichcanbemadetodiscriminateverystronglyinfavorofaparticularfrequency.Ithasalreadybeenpointedthat,bymakingantennacircuitresonanttoaparticularfrequency,theenergyabstractedfromradiowavesofthatfrequencywillbemuchgreaterthantheenergyfromwavesofotherfrequencies;thisalonegivesacertainamountofseparationbetweensignals.Stillgreaterselectiveactioncanbeobtainedbytheuseofadditionalsuitablyadjustedresonantcircuitslocatedsomewhereinthereceiverinsuchawayastorejectallbutthedesiredsignal.Theabilitytodiscriminatebetweenradiowavesofdifferentfrequenciesiscalledselectivityandtheprocessofadjustingcircuitstoresonancewiththefrequencyofadesiredsignalisspokenofastuning.Althoughintelligibleradiosignalshavebeenreceivedfromthestationsthousandsofmilesdistant,usingonlytheenergyabstractedfromtheradiowavebythereceivingantennamuchmoresatisfactorytotheradio-frequencycurrentsbeforedetection,inwhichcaseitiscalledradio-frequencyamplificationoritmaybeappliedtotherectifiedcurrentsafterdetection,inwhichcaseitiscalledaudio-frequencyamplification.Theuseofamplificationmakespossiblethesatisfactoryreceptionofsignalsfromwavesthatwouldotherwisebetooweaktogiveanaudibleresponse.Theprocessbywhichthesignalbeingtransmittedisreproducedfromtheradio-frequencycurrentspresentatthereceiveriscalleddetection,orsometimesdemodulation.Wheretheintelligenceistransmittedbyvaryingtheamplitudeoftheradiatedwave,detectionisaccomplishedbyrectifyingtheradiofrequencycurrent.Therectifiedcurrentthusproducedvariesinaccordancewiththesignaloriginallymodulatedonthewaveradiatedatthetransmitterandsoreproducesthedesiredsignal.Thus,whenthemodulatedwaveisrectified,theresultingcurrentisseentohaveanaveragevaluethatvariesinaccordancewiththeamplitudeoftheoriginalsignal.Receivercircuitaremadeupaofanumberofstages.Astageisasingletransistorconnectedtocomponentswhichprovideoperatingvoltagesandcurrentsandalsosignalvoltagesandcurrents.Eachstagehasitsinputcircuitfromwhichthesignalcomesinanditoutputcircuitfromwhichthesignal,usuallyamplified,goesout.Whenonestagefollowsanother,theoutputcircuitofthefirstfeedsthesignaltothecircuitofthesecond.Andsothesignalsamplified,stagebystage,untilitstrongenoughtooperatetheloudspeaker.MobileCommunicationCordlessTelephoneSystemsCordlesstelephonesystemsarefullduplexcommunicationsystemsthatuseradiotoconnectaportablehandsettoadedicatedbasestation,whichisthenconnectedtoadedicatedtelephonelinewithaspecifictelephonenumberonthepublicswitchedtelephonenetwork(PSTN).Infirstgenerationcordlesstelephonesystems(manufacturedinthe1980's),theportableunitcommunicatesonlytothededicatedbaseunitandonlyoverdistancesofafewtensofmeters.EarlycordlesstelephonesoperatesolelyasextensiontelephonestoatransceiverconnectedtoasubscriberlineonthePSTNandareprimarilyforin-homeuse.SecondgenerationcordlesstelephoneshaverecentlybeenintroducedwhichallowsubscriberstousetheirhandsetsatmanyoutdoorlocationswithinurbancenterssuchasLondonorHongKong.Moderncordlesstelephonesaresometimescombinedwithpagingreceiverssothatasubscribermayfirstbepagedandthenrespondtothepageusingthecordlesstelephone.Cordlesstelephonesystemsprovidetheuserwithlimitedrangeandmobility,asitisusuallynotpossibletomaintainacalliftheusertravelsoutsidetherangeofthebasestation.Typicalsecondgenerationbasestationsprovidecoveragerangesuptoafewhundredmeters.CellularTelephoneSystemsAcellulartelephonesystemprovidesawirelessconnectiontothePSTNforanyuserlocationwithintheradiorangeofthesystem.Cellularsystemsaccommodatealargenumberofusersoveralargegeographicarea,withinalimitedfrequencyspectrum.Cellularradiosystemsprovidehighqualityservicethatisoftencomparabletothatofthelandlinetelephonesystems.Highcapacityisachievedbylimitingthecoverageofeachbasestationtransmittertoasmallgeographicareacalledacellsothatthesameradiochannelsmaybereusedbyanotherbasestationlocatedsomedistanceaway.Asophisticatedswitchingtechniquecalledahandoffenablesacalltoproceeduninterruptedwhentheusermovesfromonecelltoanother.Abasiccellularsystemconsistsofmobilestation,basestationsandamobileswitchingcenter(MSC).TheMobileSwitchingCenterissometimescalledamobiletelephoneswitchingoffice(MTSO),sinceitisresponsibleforconnectingallmobilestothePSTNinacellularsystem.Eachmobilecommunicationviaradiowithoneofthebasestationsandmaybehanded-offtoanynumberofbasestationsthroughoutthedurationofacall.Themobilestationcontainsatransceiver,anantenna,andcontrolcircuitry,andmaybemountedinavehicleorusedasaportablehand-heldunit.Thebasestationsconsistofseveraltransmittersandreceiverswhichsimultaneouslyhandlefullduplexcommunicationsandgenerallyhavetowerswhichsupportseveraltransmittingandreceivingantennas.ThebasestationservesasabridgebetweenallmobileusersinthecellandconnectsthesimultaneousmobilecallsviatelephonelinesormicrowavelinkstotheMSC.TheMSCcoordinatestheactivitiesofallofthebasestationsandconnectstheentirecellularsystemtothePSTN.AtypicalMSChandless100ooocellularsubscribersand5000simultaneousconversationsatatime,andaccommodatesallbillingandsystemmaintenancefunctions,aswell.Inlargecities,severalMSCsareusedbyasinglecarrier.WhyGPSandHowItWorks?WhafsGPS?Navigationandpositioningarecrucialtosomanyactivitiesandyettheprocesshasalwaysbeenquitecumbersome.Overtheyearsallkindsoftechnologieshavetriedtosimplifythetask,buteveryonehashadsomedisadvantage.Finally,theU.S.DepartmentofDefense(DoD)decidedthatthemilitaryhadtohaveasuperpreciseformofworldwidepositioning.Andfortunatelytheyhadthekindofmoney.Ittooktobuildsomethingreallygood.TheresultistheGlobalPositioningSystem,asystemthat'schangednavigationforever.TheGlobalPositioningSystem(GPS)isaworldwideradio-navigationsystemformedfromaconstellationof24satellitesandtheirgroundstation.GPSusesthese“man-madestars”asreferencepointstocalculatepositionsaccuratetoamatterofmeters.Infact,withadvancedformsofGPSyoucanmakemeasurementstobetterthanacentimeter.Inasenseit'slikegivingeverysquaremeterontheplanetauniqueaddress.GPSreceivershavebeenminiaturizedtojustafewintegratedcircuitsandsoarebecomingveryeconomical.Andthatmakesthetechnologyaccessibletovirtuallyeveryone.ThesedaysGPSisfindinguswayintocars,boats,planes,constructionequipment,moviemakinggear,farmmachinery,evenlaptopcomputers.SoonGPSwillbecomealmostasbasicasthetelephone.Indeeditjustmaybecomeauniversalutility.WorkinginFiveLogicalStepsInprinciple,GPSoperatesinfivelogicalsteps:Step1：TriangulatingfromsatellitesImprobableasitmayseem,thewholeideabehindGPSistousesatellitesinspaceasreferencepointsforlocationshereonearth.That'sabigideageometrically,asbyvery,veryaccuratelymeasuringourdistancefromthreesatelliteswecan“triangulate”ourpositionanywhereonearth.Supposewemeasureourdistancefromasatelliteandfindittobe11,000miles.Knowingthatwe're11,000milesfromaparticularsatellitenarrowsdownallthepossiblelocationswecouldbeinthewholeuniversetothesurfaceofaspherethatiscenteredonthissatelliteandhasaradiusof11,000miles.Next,saywemeasureourdistancetoasecondsatelliteandfindoutthatit's12,000milesaway.Thattellsusthatwerenotonlyonthefirstspherebutwe'realsoonaspherethat's12,000milesfromthesecondsatellite.Orinotherwords,we'resomewhereonthecirclewherethesetwospheresintersect.Ifwethenmakeameasurementfromathirdsatelliteandfindthatwe're13,000milesfromthatone,thatnarrowsourpositiondownevenfarther,tothetwopointswherethe13,000milespherecutsthroughthecirclethat'stheintersectionofthefirsttwospheres.Sobyrangingfromthreesatelliteswecannarrowourpositiontojusttwopointsinspace.Todecidewhichoneisourtruelocationwecouldmakeafourthmeasurement.Butusuallyoneofthetwopointsisaridiculousanswer(eithertoofarfromEarthoranimpossiblevelocity)andcanberejectedwithoutameasurement.Step2:MeasuringdistancefromasatelliteWesawinthelastsectionthatapositioniscalculatedfromdistancemeasurementstoatleastthreesatellites.Buthowcanyoumeasurethedistancetosomethingthat'sfloatingaroundinspace?Wedoitbytiminghowlongittakesforasignalsentfromthesatellitetoarriveatourreceiver.Inasense,thewholethingboilsdowntothose''velocitytimestraveltime”mathproblemswedidinhighschool.InthecaseofGPSwe'remeasuringaradiosignalsothevelocityisgoingtobethespeedoflightorroughly186,000milespersecond.Theproblemismeasuringthetraveltime.Butassumingwehavepreciseclocks,howdowemeasuretraveltime?Toexplainitlefsuseagoofyanalogy:Supposetherewasawaytogetboththesatelliteandthereceivertostartplaying“TheStar-SpangledBanner”atprecisely12Noon.Ifsoundcouldreachusfromspace(which,ofcourse,isridiculous),GSM系统需要很多功能，以产生全操作移动通信系统。小区的覆盖区域受基站控制，基站本身由两部分组成。第一部分为射频系统。当在实际工作时，它向移动台发送信息，也从移动台接受信息以建立和保持童话。基站的无线电收发信机（BST）由基站控制流（BSC）控制，无线电收发信机与移动交换中心（MSC）通信一一这是到当地公用市话网（PSTN）的必由之路，并且用户数据都存储在此系统的寄存器中，用户寄存器使GSM系统检查需要使用网络的用户，允许接入并建立收费功能等。GSM系统在1978年被世界无线电管理委员会划为频带为900MHz的一部分，实际上行传播连路的频带为89。〜915MHz,下.行传播连路的频带为935〜960MHz。使用时分多址方式（TDMA）,GSM在频带为900MHz下用124个无线信道传递突发串。这些突发串能携带不同类型的信息。第一类是语音信息，其每秒编码6.5K或13K位。第二类是数字信息，每秒传送3.6K或12.6K位。这两种是非常有用的传送方式，但必须受控制信道（CCH）发布的管理信息所支持。GSM网使用数字无线传输和先进的无线越区切算方法，可以得到比模拟蜂窝系统好得多的频率利用率，因而增加了服务的用户数。由于GSM支持通用标准所以蜂窝用户在整个GSM服务区也能够使用他们的手机，能够在GSM覆盖区域内自动漫游。此外，也能进行国际漫游。GSM还提供一些新服务：如告诉数据通信，传真和短消息服务。GSM技术的规范性能设计成与其他标准一致的工作方式，例如：ISDN。在这种方式确保下，两种标准间能互相工作。从长远目光看蜂窝系统，使用数字技术将成为世界性的通信方法。0前在欧洲正在开发第三代移动通信系统，其目的是要综合第二代系统的所有业务并覆盖更广泛的业务（话音、数据、视频、多媒体）范围，而且还要与固定电信网络的技术发展保持一致和兼容。无线电接收机无线电接收机，输入信号为调幅无线电波。它的基本组成包括天线、调谐贿赂、混频器、本振电路、中频放大器、检波器、音频放大器、喇叭、电源等。任何天线系统既能辐射无线电波又能接收无线电波。任何经过天线的无线电波均能在天线中感应电压，因此，接收机必须能够从天线所收到的所有信号中分离出有用信号。这个分离过程是根据发射端发射的信号频率不同，利用调谐回路完成的。调谐贿赂能够有效地从众多频率中选择出某一个特定频率。通过天线调谐回路对某一特定频率的谐振，可以使天线从这一特定频率中吸收的能量比从其他频率中吸收的能量大得多，这样，就从某种thenstandingatthereceiverwe'dheartwoversionsof“TheStar-SpangledBanner,Monefromourreceiverandonefromthesatellite.Thesetwoversionswouldbeoutofsync.Theversioncomingfromthesatellitewouldbealittledelayedbecauseithadtotravelover11,000miles.Ifwewantedtoseejusthowdelayedthesatellite'sversionwas,wecouldstartdelayingthereceiver'sversionuntiltheyfellintoperfectsync.Theamountwehavetoshiftbackthereceiver'sversionisequaltothetraveltimeofthesatellite'sversion.SowejustmultiplythattimetimesthespeedoflightandBLNGO.Wevegotourdistancetothesatellite.ThafsbasicallyhowGPSworks.Onlyinsteadof“TheStar-SpangledBannerwthesatellitesandreceiversusesomethingcalleda“PseudoRandomCode^-whichisprobablyeasiertosingthan“TheStar-SpangledBanner”.ThePseudoRandomCode(PRC)isafundamentalpartofGPS.Physicallyifsjustaverycomplicateddigitalcode,orinotherwords,acomplicatedsequenceof“on”and"off'pulses.Thesignalissocomplicatedthatitalmostlookslikerandomelectricalnoise.Hencethename“Pseudo-Random.”Step3：GettingperfecttimingIfmeasuringthetraveltimeofaradiosignalisthekeytoGPSthenourstopwatcheshadbetterbeverygood,becauseiftheirtimingisoffbyjustathousandthofasecond,atthespeedoflight,thattranslatesintoalmost200milestoerror!Onthesatelliteside,timingisalmostperfectbecausetheyhaveincrediblypreciseatomicclocksonboard.Butwhataboutourreceivershereontheground?Rememberthatboththesatelliteandthereceiverneedtobeabletopreciselysynchronizetheirpseudo-randomcodestomakethesystemwork.Ifourreceiversneededatomicclocks(whichcostupwardsof$5OKto$iooK)GPSwouldbealameducktechnology.Nobodycouldaffordit.LuckilythedesignersofGPScameupwithabrilliantlittletrickthatletsusgetbywithmuchlessaccurateclocksinourreceivers.ThistrickisoneofthekeyelementsofGPSandasanaddedsidebenefititmeansthateveryGPSreceiverisessentiallyanatomic-accuracyclock.Thesecrettoperfecttimingistomakeanextrasatellitemeasurement.Ifthreeperfectmeasurementscanlocateapointin3-dimensionalspace,thenfourimperfectmeasurementscandothesamething.Ifeverythingwereperfectthanallofoursatelliterangeswouldintersectatasinglepoint.Butimperfectclocks,afourthmeasurement,doneasacross-check,willNOTintersectwiththefirstthree.Sothereceiver'scomputersays"Uh-oh!”Thereisadiscrepancyinmymeasurements.Imustnotbeperfectlysyncedwithuniversaltime.Sinceanyoffsetfromuniversaltimewillaffectallofourmeasurements,thereceiverlooksforasinglecorrectionfactorthatitcansubtractfromallitstimingmeasurementsthatwouldcausethemalltointersectatasinglepoint.Thatcorrectionbringsthereceiver'sclockbackintosyncwithuniversaltime,andbingo!一you'vegotatomicaccuracytimerightinthepalmofyourhand.Onceithasthatcorrectionitappliestoalltherestofitsmeasurementsandnowwe'vegotprecisepositioning.OneconsequenceofthisprincipleisthatanydecentGPSreceiverwillneedtohaveatleastfourchannelssothatitcanmakethefourmeasurementssimultaneously.Step4:KnowingwhereasatelliteisinspaceTheAirForcehasinjectedeachGPSsatelliteintoaverypreciseorbit,accordingtotheGPSmasterplan.OnthegroundallGPSreceivershaveanalmanacprogrammedintotheircomputersthattellsthemwhereintheskyeachsatelliteis,momentbymoment.ThebasicorbitsarequiteexactbutjusttomakethingsperfecttheGPSsatelliteareconstantlymonitoredbytheDepartmentofDefense.Theyuseverypreciseradartocheckeachsatellite'sexactaltitude,positionandspeed.Theerrorsthey'recheckingforarecalledephemeriserrorsbecausetheyaffectthesatellite'sorbitorephemeris.Theseerrorsarecausedbygravitationalpullsfromthemoonandsunandbythepressureofsolarradiationonthesatellites.Theerrorsareusuallyveryslightbutifyouwantgreataccuracytheymustbetakenintoaccount.OncetheDoDhasmeasuredasatellite'sexactposition,theyrelaythatinformationbackuptothesatelliteitself.Thesatellitethenincludesthisnewcorrectedpositioninformationinthetimingsignalsit'sbroadcasting.SoaGPSsignalismorethanjustpseudo-randomcodefortimingpurposes.Italsocontainsanavigationmessagewithephemerisinformationaswell.Step5:CorrectingerrorsUptonowwe'vebeentreatingthecalculationsthatgointoGPSveryabstractly,asifthewholethingwerehappeninginavacuum.ButintherealworldtherearelotsofthingsthatcanhappentoaGPSsignalthatwillmakeitslifelessthanmathematicallyperfect.Togetthemostoutofthesystem,agoodGPSreceiverneedstotakeawidevarietyofpossibleerrorsintoaccount.Forexample,oneofthebasicassumptionswe'vebeenusingsofarisnotexactlytrue.We'vebeensayingthatyoucalculatedistancetoasatellitebymultiplyingasignafstraveltimebythespeedoflight.Butthespeedoflightisonlyconstantinavacuum.AsaGPSsignalpassesthroughthechargedparticlesoftheionosphereandthenthroughthewatervaporinthetroposphereitgetssloweddownabit,andthiscreatesthesamekindoferrorasbadclocks.Typicaldelaymightbeonatypicalday.Thisiscalledmodelingandithelpsbut,ofcourse,atmosphericconditionsarerarelyexactlytypical.Anotherwaytogetahandleontheseatmosphereinducederrorsistocomparetherelativespeedsoftwodifferentsignals.Thisdualfrequencymeasurementisverysophisticatedandisonlypossiblewithadvancedreceivers.Fortunatelyalloftheseinaccuraciesstilldon'tadduptomuchofanerror.AndaformofGPScalledDifferentialGPScansignificantlyreducetheseproblems.DifferentialGPSinvolvestheuseoftworeceivers.OnemonitorsvariationsintheGPSsignalandcommunicatesthosevariationstotheotherreceiver.Thesecondreceivercanthencorrectitscalculationsforbetteraccuracy.TherequestforgreaterandgreateraccuracyhasspawnedanassortmentofvariationsonbasicGPStechnology.PuttingGPStoPracticalUseGPStechnologyhasmaturedintoaresourcethatgoesfarbeyonditsoriginaldesigngoals.Thesedaysscientists,sportsmen,farmers,soldiers,pilots,surveyors,hikers,deliverydrivers,sailors,dispatchers,lumberjacks,fire-fighters,andpeoplefrommanyotherwalksoflifeareusingGPSinwaysthatmaketheirworkmoreproductive,safer,andsometimeseveneasier.Location:DeterminingabasicpositionThefirstandmostobviousapplicationofGPSisthesimpledeterminationofapositionorlocation.GPSisthefirstpositioningsystemtoofferhighlypreciselocationdataforanypointontheplanet,inanyweather.Thatalonewouldbeenoughtoqualifyitasamajorutility,buttheaccuracyofGPSandthecreativityofitsusersispushingitintosomesurprisingrealms.Knowingthepreciselocationofsomething,orsomeone,isespeciallycriticalwhentheconsequencesofinaccuratedataaremeasuredinhumanterms.Forexample,whenScottO'Grady'saircraftwasshotdowninBosnia,GPShelpedrescuersfindhim.Sometimesanexactreferencelocatorisneededforextremelyprecisescientificwork.Justgettingtotheworld'stallestmountainwastricky,butGPSmademeasuringthegrowthofMt.Everesteasy.Thedatacollectedstrengthenedpastwork,butalsorevealedthatastheKhumbuglaciermovestowardEverest'sBaseCamp,themountainitselfisgettingtaller.Navigation:GettingfromonelocationtoanotherGPShelpsyoudetermineexactlywhereyouare,butsometimesimportanttoknowhowtogetsomewhereelse.GPSwasoriginallydesignedtoprovidenavigationinformationforshipsandplanes.Soit'snosurprisethatwhilethistechnologyisappropriatefornavigatingonwater,it'salsoveryusefulintheairandontheland.It'sinterestingthatthesea,oneofouroldestchannelsoftransportation,hasbeenrevolutionizedbyGPS,thenewestnavigationtechnology.Theworld'sfirstGPSreceiverwasintroducedformarinenavigationin1985.Andasyouwouldexpect,naxdgatingtheworld'soceansandwatenvaysismoreprecisethanever.Byprovidi