精确的GPS定位：前景与挑战

精确的GPS定位：前景与挑战精确的GPS定位：前景与挑战摘要现在，基于GPS载波相位定位是一个不可缺少的工具，广泛应用于精确的导航、测量、大地测量。为了解决这样的各种应用，精确的GPS定位技术的许多技术已经被开发。几乎所有的技术都涉及“相对”定位--GPS接收器/天线的坐标确定，借助于在一个固定的碱或参考接收机的测量。从本质上讲，所有这些技术可以根据一个小数量的属性分类。技术实施是在后处理，还是实时模式？该方案是否涉及静态或动态的定位？接收器间的距离比较短（说<10公里的），还是很长的（例如>千公里）？是一个单一的基站参与，还是接收器的参考网络？等。这些属性也决定了数据处理策略，应保证精确和可靠的定位结果。超过过去二十年的精确的GPS定位起到了作用，类似F1赛车。也就是说，基于载波相位GPS定位正研究新的硬件上的挑战，新的数据处理算法和新的运作程序，然后纳入主流的测量和导航的“产品”。在本文中，对高精度GPS定位的挑战，进展和前景进行讨论，特别强调确定的限制和为解决这些问题的近期和中期的前景发表评论。1简介全球定位系统（GPS），是由美国国防部研发的全天候的，全球性的，基于卫星的、精密时钟的定位系统，在20世纪80年代初，服务于普通测量和导航。高精度的差分定位的标准模式需要一个位于“基站”参考GPS接收机，基站坐标是已知的，而所述第二用户的GPS接收器同时跟踪同一个卫星信号。对两个接收机的载波相位数据的组合和处理，相对于参考接收器用户接收机的坐标确定。然而，由于使用的载波相位数据在整个系统的复杂性，而测量是模糊的，在软件处理数据时，需要纳入“整周模糊度”（AR）算法。GPS用户接收机硬件的发展已经走了由明显的方式向提高性能的AR（HanRizos，1997b）。从用户接收到距离最近的参考接收机可从几公里到几百公里不等。由于接收器分离的增加，距离依赖偏见的问题，因此，整周模糊度可靠性的解决将成为一个更大的挑战。另一方面，在过去的15年，“GPS大地测量学”的发展如此成功，即使没有AR，“十亿分之几”的相对精度现在是可以实现的。但是，对于所谓的“高生产率”，基于GPS技术载波相位，当使用少量数据时，AR是至关重要的（不像“GPS大地测量”技术）。因此，基于载波相位定位是进步的研发创新的结果。除了在AR技术的进展（HanRizos，1997b），在过去的十年左右的几个重要的发展导致了这种高精度性能也可在实时–说，在外地，后立即测量的制作，并在从参考接收机的数据传输完毕（二）来处理场接收器。精确的实时定位甚至可能在GPS接收器在运动。这些系统通常被称作RTK系统（“实时运动”），并制定切实可行的许多时间关键型应用程序，如机器控制GPS-RTK使用GPS导航，土方工程和基坑，自动拖运卡车业务，和其他的自主机器人导航中的应用。创新是关键：实时操作通过参考和用户接收机之间的通信链路的规定（兰利，1993；Talbot，1996），和移动计算功能内置到用户接收设备进行必要的计算。有效的整周模糊度算法能利用改进的GPS接收机硬件（允许双频率，高品质的测量）（HanRizos，1997b）。AR是即使用户接收机是运动的执行（所谓的“对飞”，AR，或otf-ar），和后AR定位能力的静态和动态定位同样适用（Landau&Euler,1992;Han&Rizos,1997b）。不幸的是，这样的进步是“脆弱的”，因为仍有不可靠和高效的许多应用要求。如果GPS信号没有进行跟踪和锁定损失，在调查开始整周模糊度的解决可以保持整个GPS动态定位的跨度。然而，GPS卫星信号偶尔阴影（例如，由于建筑在城市峡谷环境），或暂时阻断（例如，当接收器通过一座桥或隧道），在大多数情况下，整周模糊度值是“丢失”，必须重新确定。与现有的商用GPS系统的短距离应用不同，这个过程可以从几秒到几分钟。在这个“重新初始化”时期的GPS载波范围的数据无法获得，因此有“无效”的时间，直到已收集到足够的数据解决歧义。如果中断GPS信号出现反复，模糊“重新初始化”，至少是一个刺激，和糟糕的商业RTK定位系统一样的一个显著的弱点。所有的GPS制造商的目标是发展理想的实时精确的GPS定位系统，可以很容易的方式提供需要定位结果，在目前使用基于差分GPS伪距（DGPS）技术的情况下，通常将是1-5米级定位精度。另一个发展，也来自于“GPS”，GPS用户需要购买和操作一个载波相位跟踪GPS接收机的观念创新，但依赖于一个参考接收器由第三方经营的网络。（这可以归因于的全球网络操作主持下的国际GPS服务成功的连续运行，以及越来越多各种目的的地方或区域的永久GPS网络的建立–Rizosetal.1999.）。这样的网络可以让“服务提供者”提供用户–通过参考接收器所需的数据传输的实时服务，也可以通过Web处理服务。如顶级的GPS接收机的成本问题，时间AR，从参考接收器的距离（S），可见卫星数，最小化的多路径干扰，参考接收器等操作，可以考虑为约束的高精度GPS定位（Han&Rizos,1996c）。在过去的几年中，通过处理一些主要的约束，已经有了几个重要的发展，并显著提高了商业GPS产品和服务：（a）一定的条件下，分米级定位精度是可能实现的，即使当基线长度已达数百公里的长度。例如，通过网络基于GPS载波相位定位技术的实现。（b）可靠的otf-ar在短时间内，即使只是一次测量的时段，是可能实现的。给出了很短的时间AR的周跳的概念，或有“重新初始化”的歧义，毫无意义，因为所谓的“瞬间”的光学传递函数（IOTF）是为所有时代的动态定位的正常模式（Rizos&Han,1998）。（c）第三代双频GPS接收机能够在两个波段频率载波相位和伪距测量是非常快速的，是otf-ar或iotf-ar.的必要前提。（d）在GPS接收器/天线本身改进多路径限制。（e）连续运行参考接收器网络或基于Web支持实时的精确的GPS导航和测量。这些都是在许多国家/城市建立和发展的创新服务，为广泛的用户提供了机会。此外，全球跟踪网络的IGS“骨干”的功能为精确的静态和动态GPS定位。（f）使用集成GPS，GLONASS接收器-他们没有任何作用及GPS只接收？必须强调的是，大学研究所为创新的GPS动态定位技术的发展做出了重要贡献。在几乎所有的情况下，大学的研究人员已经开发出了必要的算法，并证明了新技术的可行性。商业产品和业务的实现也将随之而来。本文的重点是基于载波相位动态GPS定位的现状、前景和挑战。尽管在超精密GPS静态定位技术（GPS大地测量技术，主要解决地球动力学，大地测量和地球科学的应用）的发展已经取得了巨大的进步，克服移动用户接收机厘米级定位精度的挑战将最终有利于一个更广泛的用户群体。此外，目前许多大学的R＆D的研究人员在仪器制造商支持下正在开发该项目。事实上，从精确的导航和定位的角度，静态模式测量可以被认为是运动模式定位的一种特殊情况。2基于载波相位GPS动态定位在上世纪90年代，现在的GPS精密定位技术是大学进行的研究的结果，随后已经由GPS制造商演变为以载波相位为基础的“GPS”产品。特别是在过去的十年左右，一些发展已经实现，提供高精度的实时定位--这就是，在外地实时测量的操作，并把参考接收器的数据发送到用户接收机的计算机进行处理。实时定位甚至可能在GPS接收器在运动（AR用OTF算法进行）的情况下实现。这些系统通常被称作RTK系统（“实时运动”），并制定切实可行的关键实时应用，如机器控制使用的GPS、GPS引导的发掘、集装箱港口作业等。参考文献Chen,X.,S.Han,C.Rizos&P.C.Goh,2000,Improvingreal-timepositioningefficiencyusingtheSingaporeIntegratedMultipleReferenceStationNetwork(SIMRSN),13thInt.Tech.MeetingoftheSatelliteDivisionoftheU.S.Inst.ofNavigation,SaltLakeCity,Utah,19-22September,9-18.Colombo,O.L.&C.Rizos,1996,TestinghighaccuracylongrangecarrierphaseDGPSinAustralasia,IAGSymp.115,"GPSTrendsinPreciseTerrestrial,Airborne,andSpaceborneApplications",pub.Springer,226-230.Dai,L.,J.Wang,C.Rizos&S.Han,2001,Real-timecarrierphaseambiguityresolutionforGPS/GLONASSreferencestationnetworks,tobepres.Int.Symp.onKinematicSystemsinGeodesy,Geomatics&Navigation(KIS2001),Banff,Canada,5-8June.Han,S.,1995,AmbiguityrecoveryforGPSlongrangekinematicpositioning,8thInt.Tech.MeetingoftheSat.Div.oftheU.S.Inst.ofNavigation,PalmSprings,California,12-15Sept.,349-360.Han,S.,1997a,Carrierphase-basedlong-rangeGPSkinematicpositioning,PhDDissertation,UNISURVrept.no.S-49,SchoolofGeomaticEngineering,TheUniversityofNewSouthWales,185pp.Han,S.,1997b,Qualitycontrolissuesrelatingtoambiguityresolutionforreal-timeGPSkinematicpositioning,J.ofGeodesy,71(6),351-361.Han,S.&E.Mok,1997,Validationcriteriaandaccuracyestimationoftheambiguityfunctionmethod,GeomaticsResearchAustralasia.67,67-82.Han,S.&C.Rizos,1995a,Asuggestedprocedureforon-the-flyambiguityresolutionforlongrangekinematicpositioning,4thInt.Conf.onDifferentialSatelliteNavigationSystems,Bergen,Norway,24-28April,PaperNo.67,8pp.Han,S.&C.Rizos,1995b,Anewmethodofconstructingmulti-satelliteambiguitycombinationsforimprovedambiguityresolution,8thInt.Tech.MeetingoftheSat.Div.oftheU.S.Inst.ofNavigation,PalmSprings,California,12-15Sept.,1145-1153.Han,S.&C.Rizos,1996a,Validationandrejectioncriteriaforintegerleastsquaresestimation,SurveyReview,33(260),375-382.Han,S.&C.Rizos,1996b,IntegratedmethodforinstantaneousambiguityresolutionusingnewgenerationGPSreceivers,IEEEPositionLocation&Navig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dinatesaredeterminedwiththeaidofmeasurementsalsomadeatastationarybaseorreferencereceiver.Inessenceallofthesetechniquesmaybecategorisedaccordingtoasmallnumberofattributes.Isthetechniqueimplementedinthepost-processedorreal-timemode?Doesthescenarioinvolvestaticorkinematicpositioning?Istheinter-receiverdistancecomparativelyshort(say<10km)orverylong(e.g.>1000km)?Isasinglebasestationinvolvedorareferencenetworkofreceivers?andsoon.Eachoftheseattributesalsodeterminesthedataprocessingstrategiesthatshouldbeemployedtoensureaccurateandreliablepositioningresults.Overthelasttwodecades'preciseGPSpositioning'hasplayedarolesimilartoF1motorracing.Thatis,challengestocarrierphase-basedGPSpositioningspurresearchonnewhardware,newdataprocessingalgorithmsandnewoperationalprocedures,whicharethenincorporatedintomainstreamsurveyingandnavigation'products'.Inthispaper,thechallenges,progressandoutlookforhighprecisionGPSpositioningwillbediscussed,withparticularemphasisonidentifyingtheconstraintsandcommentingontheprospectsforaddressingtheminthenearandmediumterm.1.INTRODUCTIONTheGlobalPositioningSystem(GPS)isanall-weather,global,satellite-based,round-theclockpositioningsystemdevelopedbytheU.S.DepartmentofDefense,thatbecameavailabletotheciviliansurveyingandnavigationcommunityintheearly1980s.ThestandardmodeofhighaccuracydifferentialpositioningrequiresonereferenceGPSreceivertobelocatedata"basestation"whosecoordinatesareknown,whiletheseconduserGPSreceiversimultaneouslytracksthesamesatellitesignals.Whenthecarrierphasedatafromthetworeceiversiscombinedandprocessed,theuserreceiver'scoordinatesaredeterminedrelativetothereferencereceiver.However,theuseofcarrierphasedatacomesatacostintermsofoverallsystemcomplexitybecausethemeasurementsareambiguous,requiringtheincorporationofan"ambiguityresolution"(AR)algorithmwithinthedataprocessingsoftware.DevelopmentsinGPSuserreceiverhardwarehavegoneasignificantwaytowardsimprovingtheperformanceofAR(Han&Rizos,1997a).Thedistancefromtheuserreceivertothenearestreferencereceivermayrangefromafewkilometrestohundredsofkilometres.Asthereceiverseparationincreases,theproblemsofaccountingfordistance-dependentbiasesgrowsand,asaconsequence,reliableambiguityresolutionbecomesanevengreaterchallenge.Ontheotherhand,developmentsin"GPSGeodesy"havebeensosuccessfulinthelast15years,thatrelativeaccuraciesof"afewpartsperbillion"arenowpossibleevenwithoutAR.However,forso-called"highproductivity"carrierphase-basedGPStechniques,ARiscrucialwhensmallamountsofdataareused(unlikethecasefor"GPSGeodesy"techniques).Hencecarrierphase-basedpositioningistheresultofprogressiveR&Dinnovations.InadditiontoadvancesinARtechniques(Han&Rizos,1997b),overthelastdecadeorsoseveralsignificantdevelopmentshaveresultedinthishighaccuracyperformancealsobeingavailablein'real-time'–thatis,inthefield,immediatelyfollowingthemakingofmeasurements,andafterthedatafromthereferencereceiverhasbeentransmittedtothe(second)fieldreceiverforprocessing.Precisereal-timepositioningisevenpossiblewhentheGPSreceiverisinmotion.ThesesystemsarecommonlyreferredtoasRTKsystems("realtime-kinematic"),andmakefeasibletheuseofGPS-RTKformanytime-criticalapplicationssuchasmachinecontrol,GPS-guidedearthworks/excavations,automatedhaultruckoperations,andotherautonomousroboticnavigationapplications.Thecrucialinnovationsthereforeare:Real-timeoperationthroughtheprovisionofcommunicationlinksbetweenreferenceanduserreceivers(Langley,1993;Talbot,1996),andmobilecomputingcapabilitiesbuiltintotheuserreceiverequipmenttocarryoutthenecessarycalculations.EfficientambiguityresolutionalgorithmsabletotakeadvantageofimprovementsinGPSreceiverhardware(thatallowdual-frequency,highqualitymeasurementstobemade)(Han&Rizos,1997a).ARbeingimplementedevenastheuserreceiverisinmotion(so-called"on-the-fly"AR,orOTF-AR),andthepost-ARpositioningcapabilitybeingequallyapplicabletostaticandkinematicpositioning(Landau&Euler,1992;Han&Rizos,1997b).Unfortunately,suchadvancesare'fragile'becausetherearestillnotyetasreliableandefficientasdemandedbymanyapplications.IftheGPSsignalsweretrackedandloss-of-lockneveroccurred,theintegerambiguitiesresolvedatthebeginningofasurveycouldbekeptforthewholeGPSkinematicpositioningspan.However,theGPSsatellitesignalsareoccasionallyshaded(forexample,duetobuildingsin"urbancanyon"environments),ormomentarilyblocked(forexample,whenthereceiverpassesunderabridgeorthroughatunnel),andinmostcasestheintegerambiguityvaluesare'lost'andmustberedetermined.ThisprocesscantakefromafewsecondsuptoseveralminuteswithpresentcommercialGPSsystemsforshort-rangeapplications.Duringthis"re-initialisation"periodtheGPScarrier-rangedatacannotbeobtained,andhencethereis'dead'timeuntilsufficientdatahasbeencollectedtoresolvetheambiguities.IfinterruptionstotheGPSsignalsoccurrepeatedly,ambiguity"reinitialisation"is,attheveryleast,anirritation,andatworseasignificantweaknessofcommercialGPS-RTKpositioningsystems.ThegoalofallGPSmanufacturersistodeveloptheidealreal-timepreciseGPSpositioningsystem,abletodeliverpositioningresults,ondemand,inaseasyamannerasispresentlythecaseusingpseudo-range-baseddifferentialGPS(DGPS)techniques,whichtypicallydeliverpositioningaccuraciesoftheorderof1-5metres.Anotherdevelopmentthatalsoresultsfrominnovationsin"GPSGeodesy"istheconceptofGPSusersneedingtoonlypurchaseandoperateonecarrierphase-trackingGPSreceiver,butthenrelyonanetworkofreferencereceiversoperatedbyathirdparty.(Thiscanbeattributedtothespectacularsuccessofthecontinuously-operatingglobalGPSnetworkoperatedundertheauspicesoftheInternationalGPSService,aswellastheincreasingnumberoflocalorregionalpermanentGPSnetworksestablishedforavarietyofpurposes–Rizosetal.,1999.)Suchnetworkscouldallow"serviceproviders"toofferreal-timeservicestousers–throughthenecessarytransmissionofreferencereceiverdata,orpost-processingservicesviatheWeb.Issuessuchasthecostoftop-of-the-lineGPSreceivers,time-to-AR,distancefromreferencereceiver(s),numberofvisiblesatellites,minimisiationofmultipathdisturbance,operationofreferencereceivers,etc.,canbeconsideredconstraintstohighpreciseGPSpositioning(Han&Rizos,1996c).Overthelastfewyearsseveralimportantdevelopmentshaveoccurredthataddresssomeofthemainconstraints,andofferhopeforsignificantlyimprovedcommercialGPSproductsandservices:(a)Undercertainconditionsdecimetre-levelpositioningaccuracyhasbeenpossibleevenwhenthebaselinelengthshavebeenuptohundredsofkilometresinlengthvia,e.g.,theimplementationofnetworkGPScarrierphase-basedpositioningtechniques.(b)ReliableOTF-ARintheshortestperiodoftimepossible,evenwithjustonemeasurementepoch,ispossible.Givenveryshort'time-to-AR'thenotionofcycleslips,orhavingto"re-initialise"theambiguities,hasnomeaningbecauseso-called'instantaneous'OTF(IOTF)isthenthenormalmodeofkinematicpositioningforallepochs(Rizos&Han,1998).(c)Thirdgenerationdual-frequencyGPSreceiverscapableofmakingcarrierphaseandpseudo-rangemeasurementsonthetwoL-bandfrequenciesisanecessaryprerequisiteforveryfastOTF-ARorIOTF-AR.(d)ImprovedmultipathmitigationwithintheGPSreceivers/antennasthemselves.(e)Continuously-operatingreferencereceivernetworksthatsupportreal-timeorWeb-basedpreciseGPSnavigationandsurveying.Thesearebeingestablishedinmanycountries/cities,andoffertheopportunitytodevelopinnovativeservicestoawiderangeofusers.Inaddition,theglobaltrackingnetworkoftheIGSfunctionsasthe'backbone'forprecisestaticandkinematicGPSpositioning.(f)The