精確的GPS定位：前景與挑戰(zhàn)

精確的GPS定位：前景與挑戰(zhàn)精確的GPS定位：前景與挑戰(zhàn)摘要現(xiàn)在，基于GPS載波相位定位是一個不可缺少的工具，廣泛應(yīng)用于精確的導(dǎo)航、測量、大地測量。為了解決這樣的各種應(yīng)用，精確的GPS定位技術(shù)的許多技術(shù)已經(jīng)被開發(fā)。幾乎所有的技術(shù)都涉及“相對”定位--GPS接收器/天線的坐標(biāo)確定，借助于在一個固定的堿或參考接收機的測量。從本質(zhì)上講，所有這些技術(shù)可以根據(jù)一個小數(shù)量的屬性分類。技術(shù)實施是在后處理，還是實時模式？該方案是否涉及靜態(tài)或動態(tài)的定位？接收器間的距離比較短（說<10公里的），還是很長的（例如>千公里）？是一個單一的基站參與，還是接收器的參考網(wǎng)絡(luò)？等。這些屬性也決定了數(shù)據(jù)處理策略，應(yīng)保證精確和可靠的定位結(jié)果。超過過去二十年的精確的GPS定位起到了作用，類似F1賽車。也就是說，基于載波相位GPS定位正研究新的硬件上的挑戰(zhàn)，新的數(shù)據(jù)處理算法和新的運作程序，然后納入主流的測量和導(dǎo)航的“產(chǎn)品”。在本文中，對高精度GPS定位的挑戰(zhàn)，進展和前景進行討論，特別強調(diào)確定的限制和為解決這些問題的近期和中期的前景發(fā)表評論。1簡介全球定位系統(tǒng)（GPS），是由美國國防部研發(fā)的全天候的，全球性的，基于衛(wèi)星的、精密時鐘的定位系統(tǒng)，在20世紀(jì)80年代初，服務(wù)于普通測量和導(dǎo)航。高精度的差分定位的標(biāo)準(zhǔn)模式需要一個位于“基站”參考GPS接收機，基站坐標(biāo)是已知的，而所述第二用戶的GPS接收器同時跟蹤同一個衛(wèi)星信號。對兩個接收機的載波相位數(shù)據(jù)的組合和處理，相對于參考接收器用戶接收機的坐標(biāo)確定。然而，由于使用的載波相位數(shù)據(jù)在整個系統(tǒng)的復(fù)雜性，而測量是模糊的，在軟件處理數(shù)據(jù)時，需要納入“整周模糊度”（AR）算法。GPS用戶接收機硬件的發(fā)展已經(jīng)走了由明顯的方式向提高性能的AR（HanRizos，1997b）。從用戶接收到距離最近的參考接收機可從幾公里到幾百公里不等。由于接收器分離的增加，距離依賴偏見的問題，因此，整周模糊度可靠性的解決將成為一個更大的挑戰(zhàn)。另一方面，在過去的15年，“GPS大地測量學(xué)”的發(fā)展如此成功，即使沒有AR，“十億分之幾”的相對精度現(xiàn)在是可以實現(xiàn)的。但是，對于所謂的“高生產(chǎn)率”，基于GPS技術(shù)載波相位，當(dāng)使用少量數(shù)據(jù)時，AR是至關(guān)重要的（不像“GPS大地測量”技術(shù)）。因此，基于載波相位定位是進步的研發(fā)創(chuàng)新的結(jié)果。除了在AR技術(shù)的進展（HanRizos，1997b），在過去的十年左右的幾個重要的發(fā)展導(dǎo)致了這種高精度性能也可在實時–說，在外地，后立即測量的制作，并在從參考接收機的數(shù)據(jù)傳輸完畢（二）來處理場接收器。精確的實時定位甚至可能在GPS接收器在運動。這些系統(tǒng)通常被稱作RTK系統(tǒng)（“實時運動”），并制定切實可行的許多時間關(guān)鍵型應(yīng)用程序，如機器控制GPS-RTK使用GPS導(dǎo)航，土方工程和基坑，自動拖運卡車業(yè)務(wù)，和其他的自主機器人導(dǎo)航中的應(yīng)用。創(chuàng)新是關(guān)鍵：實時操作通過參考和用戶接收機之間的通信鏈路的規(guī)定（蘭利，1993；Talbot，1996），和移動計算功能內(nèi)置到用戶接收設(shè)備進行必要的計算。有效的整周模糊度算法能利用改進的GPS接收機硬件（允許雙頻率，高品質(zhì)的測量）（HanRizos，1997b）。AR是即使用戶接收機是運動的執(zhí)行（所謂的“對飛”，AR，或otf-ar），和后AR定位能力的靜態(tài)和動態(tài)定位同樣適用（Landau&Euler,1992;Han&Rizos,1997b）。不幸的是，這樣的進步是“脆弱的”，因為仍有不可靠和高效的許多應(yīng)用要求。如果GPS信號沒有進行跟蹤和鎖定損失，在調(diào)查開始整周模糊度的解決可以保持整個GPS動態(tài)定位的跨度。然而，GPS衛(wèi)星信號偶爾陰影（例如，由于建筑在城市峽谷環(huán)境），或暫時阻斷（例如，當(dāng)接收器通過一座橋或隧道），在大多數(shù)情況下，整周模糊度值是“丟失”，必須重新確定。與現(xiàn)有的商用GPS系統(tǒng)的短距離應(yīng)用不同，這個過程可以從幾秒到幾分鐘。在這個“重新初始化”時期的GPS載波范圍的數(shù)據(jù)無法獲得，因此有“無效”的時間，直到已收集到足夠的數(shù)據(jù)解決歧義。如果中斷GPS信號出現(xiàn)反復(fù)，模糊“重新初始化”，至少是一個刺激，和糟糕的商業(yè)RTK定位系統(tǒng)一樣的一個顯著的弱點。所有的GPS制造商的目標(biāo)是發(fā)展理想的實時精確的GPS定位系統(tǒng)，可以很容易的方式提供需要定位結(jié)果，在目前使用基于差分GPS偽距（DGPS）技術(shù)的情況下，通常將是1-5米級定位精度。另一個發(fā)展，也來自于“GPS”，GPS用戶需要購買和操作一個載波相位跟蹤GPS接收機的觀念創(chuàng)新，但依賴于一個參考接收器由第三方經(jīng)營的網(wǎng)絡(luò)。（這可以歸因于的全球網(wǎng)絡(luò)操作主持下的國際GPS服務(wù)成功的連續(xù)運行，以及越來越多各種目的的地方或區(qū)域的永久GPS網(wǎng)絡(luò)的建立–Rizosetal.1999.）。這樣的網(wǎng)絡(luò)可以讓“服務(wù)提供者”提供用戶–通過參考接收器所需的數(shù)據(jù)傳輸?shù)膶崟r服務(wù)，也可以通過Web處理服務(wù)。如頂級的GPS接收機的成本問題，時間AR，從參考接收器的距離（S），可見衛(wèi)星數(shù)，最小化的多路徑干擾，參考接收器等操作，可以考慮為約束的高精度GPS定位（Han&Rizos,1996c）。在過去的幾年中，通過處理一些主要的約束，已經(jīng)有了幾個重要的發(fā)展，并顯著提高了商業(yè)GPS產(chǎn)品和服務(wù)：（a）一定的條件下，分米級定位精度是可能實現(xiàn)的，即使當(dāng)基線長度已達數(shù)百公里的長度。例如，通過網(wǎng)絡(luò)基于GPS載波相位定位技術(shù)的實現(xiàn)。（b）可靠的otf-ar在短時間內(nèi)，即使只是一次測量的時段，是可能實現(xiàn)的。給出了很短的時間AR的周跳的概念，或有“重新初始化”的歧義，毫無意義，因為所謂的“瞬間”的光學(xué)傳遞函數(shù)（IOTF）是為所有時代的動態(tài)定位的正常模式（Rizos&Han,1998）。（c）第三代雙頻GPS接收機能夠在兩個波段頻率載波相位和偽距測量是非?？焖俚?，是otf-ar或iotf-ar.的必要前提。（d）在GPS接收器/天線本身改進多路徑限制。（e）連續(xù)運行參考接收器網(wǎng)絡(luò)或基于Web支持實時的精確的GPS導(dǎo)航和測量。這些都是在許多國家/城市建立和發(fā)展的創(chuàng)新服務(wù)，為廣泛的用戶提供了機會。此外，全球跟蹤網(wǎng)絡(luò)的IGS“骨干”的功能為精確的靜態(tài)和動態(tài)GPS定位。（f）使用集成GPS，GLONASS接收器-他們沒有任何作用及GPS只接收？必須強調(diào)的是，大學(xué)研究所為創(chuàng)新的GPS動態(tài)定位技術(shù)的發(fā)展做出了重要貢獻。在幾乎所有的情況下，大學(xué)的研究人員已經(jīng)開發(fā)出了必要的算法，并證明了新技術(shù)的可行性。商業(yè)產(chǎn)品和業(yè)務(wù)的實現(xiàn)也將隨之而來。本文的重點是基于載波相位動態(tài)GPS定位的現(xiàn)狀、前景和挑戰(zhàn)。盡管在超精密GPS靜態(tài)定位技術(shù)（GPS大地測量技術(shù)，主要解決地球動力學(xué)，大地測量和地球科學(xué)的應(yīng)用）的發(fā)展已經(jīng)取得了巨大的進步，克服移動用戶接收機厘米級定位精度的挑戰(zhàn)將最終有利于一個更廣泛的用戶群體。此外，目前許多大學(xué)的R＆D的研究人員在儀器制造商支持下正在開發(fā)該項目。事實上，從精確的導(dǎo)航和定位的角度，靜態(tài)模式測量可以被認為是運動模式定位的一種特殊情況。2基于載波相位GPS動態(tài)定位在上世紀(jì)90年代，現(xiàn)在的GPS精密定位技術(shù)是大學(xué)進行的研究的結(jié)果，隨后已經(jīng)由GPS制造商演變?yōu)橐暂d波相位為基礎(chǔ)的“GPS”產(chǎn)品。特別是在過去的十年左右，一些發(fā)展已經(jīng)實現(xiàn)，提供高精度的實時定位--這就是，在外地實時測量的操作，并把參考接收器的數(shù)據(jù)發(fā)送到用戶接收機的計算機進行處理。實時定位甚至可能在GPS接收器在運動（AR用OTF算法進行）的情況下實現(xiàn)。這些系統(tǒng)通常被稱作RTK系統(tǒng)（“實時運動”），并制定切實可行的關(guān)鍵實時應(yīng)用，如機器控制使用的GPS、GPS引導(dǎo)的發(fā)掘、集裝箱港口作業(yè)等。參考文獻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&NavigationSymp.,Atlanta,Georgia,22-26April,254-261.Han,S.&C.Rizos,1996c,Progressandconstraintsofreal-timecarrierphase-basedmarineGPSpositioning,IAGSymp.117,"Gravity,Geoid&MarineGeodesy",Tokyo,Japan,30September-5October,712-719.Han,S.&C.Rizos,1996d,GPSnetworkdesignanderrormitigationforreal-timecontinuousarraymonitoringsystems,9thInt.Tech.MeetingoftheSat.Div.oftheU.S.Inst.ofNavigation,KansasCity,Missouri,17-20Sept.,1827-1836.Han,S.&C.Rizos,1997a,AninstantaneousambiguityresolutiontechniqueformediumrangeGPSkinematicpositioning,10thInt.Tech.MeetingoftheSat.Div.oftheU.S.Inst.ofNavigation,KansasCity,Missouri,16-19Sept.,1789-1800.Han,S.,&C.Rizos,1997b,ComparingGPSambiguityresolutiontechniques,GPSWorld,8(10),54-61.Han,S.&C.Rizos,1999,TheimpactoftwoadditionalcivilianGPSfrequenciesonambiguityresolutionstrategies,55thNationalMeetingU.S.InstituteofNavigation,"NavigationalTechnologyforthe21stCentury",Cambridge,Massachusetts,28-30June,315-321.Han,S.,C.Rizos&R.Abbot,1999,Seasurfacedeterminationusinglong-rangekinematicGPSpositioningandLaserAirborneDepthSoundertechniques,MarineGeodesy,22(3),195-203.Hatch,R.R.,J.Jung,P.Enge&B.Pervan,2000,CivilianGPS:Thebenefitsofthreefrequencies,GPSSolutions,3(4),1-9.Higgins,M.B.&N.C.Talbot,2001,Centimetresforeveryone:InitialresultsfromanAustralianVirtualReferenceStationnetworkpilotproject,tobepres.5thInt.Symp.onSatelliteNavigationTechnology&Applications,Canberra,Australia,24-27July.Hudnut,K.W.&J.A.Behr,1998,ContinuousGPSmonitoringofstructuraldeformationatPacoimaDam,California,SeismologicalRes.Letters,69(4),299-308.Kleusberg,A.,1990,ComparingGPSandGLONASS,GPSWorld,1(6),52-54.Landau,H.&H.J.Euler,1992,On-the-flyambiguityresolutionforprecisiondifferentialpositioning,5thInt.Tech.MeetingoftheSat.Div.oftheU.S.Inst.ofNavigation,Albuquerque,NewMexico,22-24Sept.,607-613.Landau,H.&U.Vollath,1996,CarrierphaseambiguityresolutionusingGPSandGLONASSsignals,9thInt.Tech.MeetingoftheSat.Div.oftheU.S.Inst.ofNavigation,KansasCity,Missouri,17-20Sept.,917-923.Langley,R.B.,1993,CommunicationlinksforDGPS,GPSWorld,4(5),47-51.Langley,R.B.,1994,RTCMSC-104DGPSstandards,GPSWorld,5(5),48-53.Leick,A.,J.Li,J.Beser&G.Mader,1995,ProcessingGLONASScarrierphaseobservations-theoryandfirstexperience,8thInt.Tech.MeetingoftheSat.Div.oftheU.S.Inst.ofNavigation,PalmSprings,California,12-15Sept.,1041-1047.Mok,E.,1999,ReliablesingleepochGPSprocessingalgorithmforstaticdeformationmonitoring,GeomaticsResearchAustralasia,70,95-117Raquet,J.&G.Lachapelle,2001,RTKpositioningwithMultipleReferenceStations,GPSWorld,12(4),48-53.Rizos,C.&S.Han,1998,PrecisekinematicapplicationsofGPS:Prospectsandchallenges,BoletimCi.Geodesicas,Curitibo,Brazil,3,3-33.Rizos,C.,S.Han&C.Roberts,1997,Permanentautomaticlow-costGPSdeformationmonitoringsystems:errormitigationstrategiesandsystemarchitecture,10thInt.Tech.MeetingoftheSat.Div.oftheU.S.Inst.ofNavigation,KansasCity,Missouri,16-19Sept.,909-917.Rizos,C.,S.Han,H.Y.Chen&P.C.Goh,1999,ContinuouslyoperatingGPSreferencestationnetworks:newalgorithmsandapplicationsofcarrierphase-based,medium-range,staticandkinematicpositioning,in"Quovadisgeodesia…?",specialpublicationtocelebrateProf.ErikW.Grafarend's60thbirthday,Dept.ofGeodesy&Geoinformatics,UniversityofStuttgart,ISSN0933-2839,367-378.Talbot,N.C.,1996,Compactdatatransmissionstandardforhigh-precisionGPS,9thInt.Tech.MeetingoftheSat.Div.oftheU.S.Inst.ofNavigation,KansasCity,Missouri,17-20Sept.,861-871.Talbot,N.C.,K.Zhang,M.Hale&J.Millner,2001,GPSNet:AVictorianpermanentGPStrackingnetwork,tobepres.5thInt.Symp.onSatelliteNavigationTechnology&Applications,Canberra,Australia,24-27July.Teunissen,P.J.G.,1994,Anewmethodforfastcarrierphaseambiguityestimation,IEEEPositionLocation&NavigationSymp.,LasVegas,Nevada,11-15April,562-573.Wang,J.,C.Rizos,M.P.Stewart&A.Leick,2001,GPSandGLONASSintegration:Modellingandambiguityresolutionissues,GPSSolutions,5(1),inprint.Wanninger,L.,1995,Improvedambiguityresolutionbyregionaldifferentialmodellingoftheionosphere,8thInt.Tech.MeetingoftheSat.Div.oftheU.S.Inst.ofNavigation,PalmSprings,California,12-15Sept.,55-62.Wübbena,G.,A.Bagge,G.Seeber,V.B?der&P.Hankemeier,1996,Reducingdistancedependenterrorsforreal-timepreciseDGPSapplicationsbyestablishingreferencestationnetworks,9thInt.Tech.MeetingoftheSat.Div.oftheU.S.Inst.ofNavigation,KansasCity,Missouri,17-20Sept.,1845-1852.PreciseGPSPositioning:ProspectsandChallengesChrisRizosSchoolofGeomaticEngineeringTheUniversityofNewSouthWalesSydneyNSW2052,AUSTRALIABIOGRAPHYChrisRizos,B.Surv.(UNSW)Ph.D.(UNSW)isprofessorandleaderoftheSatelliteNavigationandPositioning(SNAP)GroupatUNSW.HeisSecretaryofSection1,"Positioning",oftheInternationalAssociationofGeodesy(IAG),aFellowoftheIAG,andaFellowoftheAustralianInstituteofNavigation.ABSTRACTCarrierphase-basedGPSpositioningisnowanindispensabletoolforawiderangeofpreciseapplicationsinnavigation,surveyingandgeodesy.Toaddresssuchavarietyofapplications,manyimplementationsofpreciseGPStechniqueshavebeendeveloped.Almostalltechniquesinvolve'relative'positioning,inwhichoneGPSreceiver/antenna'scoordinatesaredeterminedwiththeaidofmeasurementsalsomadeatastationarybaseorreferencereceiver.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