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GPS SATELLITE 1 2 3 SURVEYING 4 5 Third Edition 6 7 8 9 10 11 12 13 14 [-3 ALFRED LEICK 15 16 17 Lin 18 — 19 34 * 20 —— 21 No 22 * PgE 23 24 25 [-3 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 JOHNWILEY&SONS,INC. 1 2 3 4 5 6 7 Thisbookisprintedonacid-freepaper. 8 9 Copyright©2004byJohnWiley&Sons,Inc.Allrightsreserved. 10 PublishedbyJohnWiley&Sons,Inc.,Hoboken,NewJersey 11 PublishedsimultaneouslyinCanada 12 13 Nopartofthispublicationmaybereproduced,storedinaretrievalsystem,ortransmittedinanyform 14 orbyanymeans,electronic,mechanical,photocopying,recording,scanning,orotherwise,exceptas [-4] permittedunderSection107or108ofthe1976UnitedStatesCopyrightAct,withouteithertheprior 15 writtenpermissionofthePublisher,orauthorizationthroughpaymentoftheappropriateper-copyfee 16 totheCopyrightClearanceCenter,Inc.,222RosewoodDrive,Danvers,MA01923,(978)750-8400, 17 fax(978)750-4470,oronthewebatwww.copyright.com.RequeststothePublisherforpermission Lin 18 shouldbeaddressedtothePermissionsDepartment,JohnWiley&Sons,Inc.,111RiverStreet, — 19 Hoboken,NJ07030,(201)748-6011,fax(201)748-6008,e-mail:[email protected]. 62 * 20 —— LimitofLiability/DisclaimerofWarranty:Whilethepublisherandauthorhaveusedtheirbestefforts 21 inpreparingthisbook,theymakenorepresentationsorwarrantieswithrespecttotheaccuracyor Nor 22 completenessofthecontentsofthisbookandspecificallydisclaimanyimpliedwarrantiesof * PgE 23 merchantabilityorfitnessforaparticularpurpose.Nowarrantymaybecreatedorextendedbysales 24 representativesorwrittensalesmaterials.Theadviceandstrategiescontainedhereinmaynotbesuitable foryoursituation.Youshouldconsultwithaprofessionalwhereappropriate.Neitherthepublishernor 25 [-4] authorshallbeliableforanylossofprofitoranyothercommercialdamages,includingbutnotlimited 26 tospecial,incidental,consequential,orotherdamages. 27 28 Forgeneralinformationonourotherproductsandservicesorfortechnicalsupport,pleasecontactour 29 CustomerCareDepartmentwithintheUnitedStatesat(800)762-2974,outsidetheUnitedStatesat (317)572-3993orfax(317)572-4002. 30 31 Wileyalsopublishesitsbooksinavarietyofelectronicformats.Somecontentthatappearsinprintmay 32 notbeavailableinelectronicbooks.FormoreinformationaboutWileyproducts,visitourwebsiteat 33 www.wiley.com. 34 LibraryofCongressCataloging-in-PublicationData: 35 36 Leick,Alfred. 37 GPSsatellitesurveying/AlfredLeick.—3rded. p. cm. 38 Includesbibliographicalreferencesandindex. 39 ISBN0-471-05930-7(cloth) 40 1.Artificialsatellitesinsurveying. 2.GlobalPositioningSystem. I.Title. 41 TA595.5.L45 2004 42 526.9'82—dc21 2003049651 43 PrintedintheUnitedStatesofAmerica 44 45 10 9 8 7 6 5 4 3 2 1 1 CHAPTER 1 2 3 4 5 6 7 8 9 10 11 12 [Fi 13 14 [1] 15 INTRODUCTION 16 17 Lin 18 — 19 0.0 20 —— 21 Anewandexcitingeraofpositioningonland,onthesea,andinspacebeganwith Sho 22 thelaunchofthefirstglobalpositioningsystem(GPS)satelliteonFebruary22,1978. PgE 23 Theprimarypurposeofthesatellitesystemwastomeettheneedsofthemilitaryand 24 nationalsecurity,inregardstopositioningandtiming,ona24-hourperdaybasisall 25 aroundtheworldandunderallweatherconditions.Verysoon,however,thepotential [1] 26 benefitsofGPSforcivilianapplicationsbecameapparent,withthatnumberrapidly 27 increasingandnoendinsighttwentyplusyearslater. 28 The satellites transmit at frequencies L1 (1575.42 MHz) and L2 (1227.6 MHz) 29 modulated with two types of codes and the navigation message. The codes are the 30 civilian C/A-code and the encrypted military P(Y)-codes. At present the L1 carrier 31 ismodulatedwithbothtypesofcodes,whereasL2ismodulatedwithaP-codeonly. 32 ModernizedGPSwilltransmitasecondcivilcodeonL2andathirdcivilcodeona 33 newcarrierL5(1176.45MHz). 34 Therearetwotypesofobservablesofinteresttousers.Oneofthemisthepseu- 35 dorange,whichequalsthedistancebetweenthesatelliteandthereceiverplussmall 36 correctivetermsduetoreceiverandsatelliteclockerrors,theimpactoftheionosphere 37 andtroposphereonsignalpropagation,andmultipath.Giventhegeometricpositions 38 ofthesatellitesasafunctionoftime,i.e.,satelliteephemeris,fourpseudorangesare 39 inprinciplesufficienttocomputethelocationofthereceiveranditsclockcorrection. 40 Pseudoranges are a measure of the travel time of the codes, C/A or P(Y). The sec- 41 ond observable, the carrier phase, is the difference between the received phase and 42 thephaseofthereceiveroscillatorattheepochofmeasurement.Receiversarepro- 43 grammedtomakephaseobservationsatthesameequallyspacedepochs.Inaddition, 44 receiverskeeptrackofthenumberofcompletecyclesreceivedsincethebeginning 45 1 2 INTRODUCTION 1 of a measurement. Thus, the actual output is the accumulated phase observable at 2 pre-setepochs. 3 Governmentpolicies(SPS,2001)currentlydefineastandardpositioningservice 4 (SPS) based on the C/A-code observations and a precise positioning service (PPS) 5 based on P(Y)-code observations. SPS and PPS address “classical satellite” navi- 6 gationmethodswhereonereceiverobservesseveralsatellitesinordertodetermine 7 itsgeocentricposition,usingthebroadcastephemeris.Typically,apositioniscom- 8 puted for every epoch of observation. The advantages of relative positioning have 9 longbeenrecognizedasawaytosatisfythehighaccuracyrequirementsofgeodesy, 10 surveying,andothergeosciences.Inrelativepositioning,alsocalleddifferentialpo- 11 sitioning,therelativelocationbetweenco-observingreceiversisdetermined.Inthis 12 casemanycommonerrorscancel,ortheirimpactissignificantlyreduced.Duringthe 13 pioneeringyearsofGPS,thereappearedtobeacleardistinctionbetweenapplications 14 in navigation and surveying. This distinction, if ever real, has rapidly disappeared. [2], 15 Whereas navigation solutions used to incorporate primarily pseudorange observa- 16 tions,surveyingsolutionshavealwaysbeenbasedonthemillimeter-accuratecarrier 17 phaseobservations.Modernapproachescombinebothtypesofobservablesinanop- Lin 18 timalmanner.ThisleadstoaunifiedGPSpositioningtheoryforbothsurveyingand — 19 navigation.Theavailabilityofprecise,postprocessedephemerides—evenpredicted 0.0 20 preciseephemerides—allowsforsingle-pointpositioningthatisbetterthanspecified —— 21 for SPS or even PPS. Powerful processing algorithms reduce the time required for Lon 22 datacollection,soastorendereventhedistinctionbetweenstatic(bothreceiversare PgE 23 static)andkinematic(atleastonereceivermoves)techniquesunnecessary. 24 Theachievableaccuracyverymuchdependsonmanyfactorsthatwillbedetailed 25 throughout this book. In order to emphasize the characteristic difference between [2], 26 geocentricandrelativepositionaccuracy,letussimplystatethatgeocentricposition 27 accuracyrangesfrommeterstodecimeters,whereastherelativepositionaccuracyis 28 at the centimeters to millimeters level. The secrets that make GPS such a powerful 29 positioning device can be readily explained. At the center is the ability to measure 30 carrierphasestoabout1/100ofacycle,whichequalsabout2mminlineardistance. 31 Thehighfrequencies(L1andL2)penetratetheionosphererelativelywell.Because 32 thetimedelaycausedbytheionosphereisinverselyproportionaltothesquareofthe 33 frequency,carrierphaseobservationsatbothfrequenciescanbeusedtomodeland, 34 thus,eliminatemostionosphericeffects.Dual-frequencyobservationsareparticularly 35 usefulwhenthestationseparationislargeandwhenshorteningtheobservationtimeis 36 important.Therehasbeensignificantprogressinthedesignofstableclocksandtheir 37 miniaturization,providingprecisetimingatthesatellite.TheGPSsatelliteorbitsare 38 stablebecauseatsuchhighsatellitealtitudesonlythemajorgravitationalforcesaffect 39 theirmotion.Therearenoatmosphericdrageffectsactingonsatellites.Theimpactof 40 thesunandthemoonontheorbitsissignificantbutcanbecomputedaccurately.The 41 remaining worrisome physical aspects are solar radiation pressure on the satellites, 42 as well as the tropospheric delay and multipath effects on signal propagation. On 43 thealgorithmicside,muchisgainedbyusinglinearcombinationsofthebasicphase 44 observables. For example, unwanted parameters are eliminated and certain effects 45 neednotbemodeled.Letthereceiverskandmobservesatellitepatthesametime. The difference between these two phase observations is called a single-difference HISTORICALPERSPECTIVE 3 1 observable.Itcanbereadilyshownthatsingledifferencesarelargelyindependentof 2 satellitefrequencyoffsetandlineardrift.Next,assumethattwosingledifferencesare 3 available,onereferringtosatellitep andonetosatelliteq.Thedifferencebetween 4 thesetwosingledifferences,calledthedouble-differenceobservable,islargelyinde- 5 pendentofreceiverclockerrors.Finally,takingthedifferenceoftwodoublediffer- 6 encesthatrefertodifferentepochsyieldsthetriple-differenceobservable.Thislast 7 typeofobservationisusefulforinitialprocessingandscreeningofthedata. 8 Single-, double-, or triple-difference processing yields the relative location be- 9 tweentheco-observingreceiversandisusuallyreferredtoasthevectorbetweenthe 10 stations. Because the satellites are at a finite distance from the earth, there is also 11 a“geocentricpositioningcomponent”totheseobservableswhichis,asamatterof 12 fact,afunctionofthebaselinelength.Inpractice,theabsolutelocationofthebaseline 13 mustbesufficientlyknowninordernottodegradetherelativepositioningcapability. 14 Thistopicwillbediscussedlater.Byitself,oneaccuratevectorbetweenstationsis [3] 15 generallynotofmuchuse,atleastinsurveying.Ofcourse,onecanaddthevectorto 16 thegeocentricpositionofthe“known”stationandformallycomputethegeocentric 17 positionofthenewstation.Theproblemwiththisprocedureisthattheuncertaintyof Lin 18 the“known”stationistransferredinfulltothenewstation.Also,despiteallofmodern — 19 technology,thevectorsthemselvescanstillbeinerror.Possibilitiesofmisidentify- 0.0 20 ing ground marks, centering errors, misreading antenna heights, etc., can never be —— 21 completelyavoided.Likeotherobservations,theGPSvectorobservationsaremost Lon 22 effectively controlled by a least-squares network adjustment consisting of a set of PgE 23 redundantvectors.Suchnetworksolutionsmakeitpossibletoassessthequalityof 24 the observations, validate the correctness of statistical data, and detect (and possi- 25 bly remove) existing blunders. Therefore, the primary result of a GPS survey is a [3] 26 polyhedron of stations whose accurate relative locations have been controlled by a 27 least-squaresadjustment. 28 29 30 1.1 HISTORICALPERSPECTIVE 31 32 A summary of GPS development and performance to date is detailed in Table 1.1. 33 Because the scope of GPS research and application development is so broad and 34 conductedbyresearchersallovertheglobe,itisimpossibletogiveacomprehensive 35 listing.Table1.1,therefore,merelydemonstratestheextraordinarilyrapiddevelop- 36 mentoftheGPSpositioningsystem. 37 GPSmadeitsdebutinsurveyingandgeodesywithabigbang.Duringthesummer 38 of1982,thetestingoftheMacrometerreceiver,developedbyC.C.Counselmanat 39 M.I.T.,verifiedaGPSsurveyingaccuracyof1–2partspermillion(ppm)ofthestation 40 separation.Baselinesweremeasuredrepeatedlyusingseveralhoursofobservations 41 tostudythisnewsurveyingtechniqueandtogaininitialexperiencewithGPS.Dur- 42 ing 1983 a thirty (plus)-station first-order network densification in the Eifel region 43 ofGermanywasobserved(Bocketal.,1985).Thisprojectwasajointeffortbythe 44 StateSurveyingOfficeofNorthRhein-Westfalia,aprivateU.S.firm,andscientists 45 fromM.I.T.Inearly1984,thegeodeticnetworkdensificationofMontgomeryCounty (Pennsylvania)wascompleted.Thesoleguidanceofthisprojectrestedwithaprivate 4 HISTORICALPERSPECTIVE 1 TABLE1.1 GPSDevelopmentandPerformanceataGlance 2 1978 LaunchoffirstGPSsatellite 3 4 1982 PrototypeMacrometertestingatM.I.T. 5 Geodeticnetworkdensification(Eifel,Germany) 1983 6 PresidentReaganoffersGPStotheworld“freeofcharge” 7 Geodeticnetworkdensification(MontgomeryCounty,Pennsylvania) 8 1984 EngineeringsurveyatStanford 9 Remondi’sdissertation 10 PrecisegeoidundulationdifferencesforEifelnetwork 11 Codelessdualbandobservations 12 1985 KinematicGPSsurveying 13 Antennaswapforambiguityinitialization 14 [4], FirstinternationalsymposiumonprecisepositioningwithGPS 15 16 Challengeraccident(January28) 1986 17 10cmaircraftpositioning Lin 18 1987 JPLbaselinerepeatabilityteststo0.2–0.04ppm — 19 5.3 LaunchoffirstBlockIIsatellite 20 OTFsolution —— 21 1989 WideareadifferentialGPS(WADGPS)concepts Nor 22 U.S.CoastGuardGPSInformationCenter(GPSIC) PgE 23 1990 GEOID90forNAD83datum 24 25 NGSephemerisservice [4], 1991 26 GIG91experiment(January22–February13) 27 IGScampaign(June21–September23) 28 Initialsolutionstodealwithantispoofing(AS) 1992 29 NarrowcorrelatorspacingC/A-codereceiver 30 Attitudedeterminationsystem 31 Real-timekinematicGPS 32 ACSMadhoccommitteeonaccuracystandards 33 OrangeCountyGIS/cadastraldensification 34 1993 Initialoperationalcapability(IOC)onDecember8 35 1–2ppbbaselinerepeatability 36 LAMBDA 37 IGSservicebeginningJanuary1 38 Antispoofingimplementation(January31) 39 RTCMrecommendationsondifferentialGPS(Version2.1) 40 1994 NationalSpatialReferenceSystemCommittee(NGS) 41 Multiple(single-frequency)receiverexperimentsforOTF 42 Proposaltomonitortheearth’satmospherewithGPS(occultations) 43 Fulloperationalcapability(FOC)onJuly17 44 1995 Precisepointpositioning(PPP)atJPL 45 HISTORICALPERSPECTIVE 5 1 TABLE1.1 (Continued) 2 1996 PresidentialDecisionDirective,firstU.S.GPSpolicy 3 4 VicepresidentannouncessecondGPScivilsignalat1227.60MHz 1998 JPL’sautomatedGPSdataanalysisserviceviaInternet 5 6 VicepresidentannouncesGPSmodernizationinitiativeandthirdcivilGPSsignal 7 1999 at1176.45MHz 8 IGDG(Internet-basedglobaldifferentialGPS)atJPL 9 Selectiveavailabilitysettozero 10 2000 GPSJPObeginsmodificationstoIIR-MandIIFsatellites 11 12 13 14 GPSsurveyingfirm(CollinsandLeick,1985).Alsoin1984,GPSwasusedatStan- [5] 15 fordUniversityforahigh-precisionGPSengineeringsurveytosupportconstruction 16 forextendingtheStanfordLinearAccelerator(SLAC).Terrestrialobservations(an- 17 glesanddistances)werecombinedwithGPSvectors.TheStanfordprojectyielded Lin 18 a truly millimeter-accurate GPS network, thus demonstrating, among other things, — 19 thehighqualityoftheMacrometerantenna.Thisaccuracycouldbeverifiedthrough 0.5 20 comparisonwiththealignmentlaserattheaccelerator,whichreproducesastraight —— 21 line within one-tenth of a millimeter (Ruland and Leick, 1985). Therefore, by the No 22 middleof1984,1–2ppmGPSsurveyinghadbeendemonstratedbeyondanydoubt. PgE 23 No visibility was required between the stations. Data processing could be done on 24 a microcomputer. Hands-on experience was sufficient to acquire most of the skills 25 neededtoprocessthedata—i.e.,first-ordergeodeticnetworkdensificationsuddenly [5] 26 becamewithinthecapabilityofindividualsurveyors. 27 President Reagan offered GPS free of charge for civilian aircraft navigation in 28 1983oncethesystembecamefullyoperational.Thisannouncementwasmadeafter 29 the Soviet downing of the Korean Air flight 007 over the Korea Eastern Sea. This 30 announcementcanbeviewedasthebeginningofsharingarrangementsofGPSfor 31 militaryandcivilianusers. 32 Engelisetal.(1985)computedaccurategeoidundulationdifferencesfortheEifel 33 network,demonstratinghowGPSresultscanbecombinedwithorthometricheights, 34 as well as what it takes to carry out such combinations accurately. New receivers 35 became available—e.g., the dual-frequency P-code receiver TI-4100 from Texas 36 Instruments—whichwasdevelopedwiththesupportofseveralfederalagencies.Ladd 37 etal.(1985)reportedonasurveyusingcodelessdual-frequencyreceiversandclaimed 38 1ppminallthreecomponentsofavectorinaslittleas15minutesofobservationtime. 39 Thus, the move toward rapid static surveying had begun. Around 1985, kinematic 40 GPS became available (Remondi, 1985). Kinematic GPS refers to ambiguity-fixed 41 solutions that yield centimeter (and better) relative accuracy for a moving antenna. 42 Theonlyconstraintonthepathofthemovingantennaisvisibilityofthesamefour 43 (atleast)satellitesatbothreceivers.Remondiintroducedtheantennaswappingtech- 44 niquefortherapidinitializationofambiguities.Antennaswappingmadekinematic 45 positioninginsurveyingmoreefficient. 6 HISTORICALPERSPECTIVE 1 ThedeploymentofGPSsatellitescametoasuddenhaltduetothetragicJanuary 2 28,1986Challenger accident.SeveralyearspasseduntiltheDeltaIIlaunchvehicle 3 wasmodifiedtocarryGPSsatellites.However,thetheoreticaldevelopmentscontin- 4 ued at full speed. They were certainly facilitated by the publication of Remondi’s 5 (1984) dissertation, the very successful First International Symposium on Precise 6 PositioningwiththeGlobalPositioningSystem(Goad,1985),andaspecialtycon- 7 ferenceonGPSheldbytheAmericanSocietyofCivilEngineersinNashvillein1988. 8 Kinematic GPS was used for decimeter positioning of airplanes relative to re- 9 ceiversontheground(Mader,1986;KrabillandMartin,1987).Thegoalofthesetests 10 was to reduce the need for traditional and expensive ground control in photogram- 11 metry.Theseearlysuccessesnotonlymadeitclearthatpreciseairplanepositioning 12 wouldplayamajorroleinphotogrammetry,buttheyalsohighlightedtheinterestin 13 positioningotherremotesensingdevicesinairplanesandspacecraft. 14 LichtenandBorder(1987)reportrepeatabilityof2–5partsin108inallthreecom- [6], 15 ponentsforstaticbaselines.Notethat1partin108 correspondsto1mmin100km. 16 Suchhighlyaccuratesolutionsrequiresatellitepositionsofabout1mandbetter.Be- 17 causesuchaccurateorbitswerenotyetavailableatthetime,researcherswereforced Lin 18 toestimateimprovedGPSorbitssimultaneouslywithbaselineestimation.Theneed — 19 for a precise orbital service became apparent. Other limitations, such as the uncer- 0.0 20 taintyinthetroposphericdelayoverlongbaselines,alsobecameapparentandcreated —— 21 aninterestinexploringwatervaporradiometerstomeasurethewetpartofthetro- Lon 22 posphere along the path of the satellite transmissions. The geophysical community PgE 23 requireshighbaselineaccuracyforobviousreasons;e.g.,slow-movingcrustalmo- 24 tionscanbedetectedearlierwithmoreaccuratebaselineobservations.However,the 25 GPS positioning capability of a few parts in 108 was also noticed by surveyors for [6], 26 itspotentialtochangewell-establishedmethodsofspatialreferencingandgeodetic 27 networkdesign. 28 Perhaps the year 1989 could be labeled the year when “modern GPS” position- 29 ing began in earnest. This was the year when the first production satellite, Block 30 II,waslaunched.SeeberandWübbena(1989)discussedakinematictechniquethat 31 used carrier phases and resolved the ambiguity “on-the-way.” This technique is to- 32 day usually called “on-the-fly” (OTF) ambiguity resolution (fixing), meaning there 33 is no static initialization required to resolve the ambiguities. The technique works 34 forpostprocessingandreal-timeapplications.OTFisoneofthemoderntechniques 35 that applies to both navigation and surveying. The navigation community began in 36 1989totakeadvantageofrelativepositioning,inordertoeliminateerrorscommon 37 to co-observing receivers, and to make attempts to extend the distance in relative 38 positioning.Brown(1989)referredtoitasextendeddifferentialGPS,butitismore 39 frequentlyreferredtoaswideareadifferentialGPS(WADGPS).Manyeffortswere 40 madetostandardizereal-timedifferentialGPSprocedures,resultinginseveralpub- 41 licationsbytheRadioTechnicalCommissionforMaritimeServices.TheU.S.Coast 42 Guard established the GPS Information Center (GPSIC) to serve nonmilitary user 43 needsforGPSinformation. 44 TheintroductionofthegeoidmodelGEOID90inreferencetotheNAD83datum 45 represented a major advancement for combining GPS (ellipsoidal) and orthometric heightdifferences.ThemostrecentversionisGEOID99. HISTORICALPERSPECTIVE 7 1 During 1991 and 1992, the geodetic community embarked on major efforts to 2 explorethelimitsofGPSonaglobalscale.TheeffortsbeganwiththeGIG91cam- 3 paign and continued the following year with the International GPS Service (IGS) 4 campaign.GIG91(GPSexperimentforInternationalEarthRotationService[IERS] 5 andGeodynamics)resultedinveryaccuratepolarmotioncoordinatesandearthrota- 6 tionparameters.Geocentriccoordinateswereobtainedthatagreedwiththosederived 7 fromsatellitelaserrangingwithin10to15cm,andambiguitiescouldbefixedona 8 global scale providing daily repeatability of about 1 part in 109. Such results are 9 possiblebecauseofthetrulyglobaldistributionofthetrackingstations.Theprimary 10 purpose of the IGS campaign was to prove that the scientific community is able to 11 producehigh-accuracyorbitsonanoperationalbasis.Thecampaignwassuccessful 12 beyond all expectations, confirming that the concept of IGS is possible. The IGS 13 serviceformallybeganJanuary1,1994. 14 [7] Formanyyears,usersworriedaboutwhatimpactantispoofing(AS)wouldhave 15 on the practical uses of GPS. AS implies switching from the known P-code to the 16 encryptedY-code,expressedbythenotationP(Y)-code.ThepurposeofASistomake 17 Lin theP-codesavailableonlytoauthorized(military)users.TheanxietyaboutASwas 18 considerablyrelievedwhenHatchetal.(1992)reportedonthecode-aidedsquaring — 19 12 techniquetobeusedwhenASisactive.Mostmanufacturersdevelopedproprietary 20 —— solutionsfordealingwithAS.WhenASwasactuallyimplementedonJanuary31, 21 Lon 1994, it presented no insurmountable hindrance to the continued use of GPS and, 22 particularly,theuseofmoderntechniquessuchasOTF.GPSusersbecameevenless PgE 23 dependent on AS with the introduction of accurate narrow correlator spacing C/A- 24 code receivers (van Dierendonck et al., 1992), since the C/A-code is not subject to 25 [7] AS measures. By providing a second civil code on L2, and eventually a third one 26 on L5, and adding new military codes, GPS modernization will make the P(Y)- 27 code encryption a nonissue for civilian applications, and at the same time, provide 28 enhancedperformancetocivilianandmilitaryusers. 29 AmajormilestoneinthedevelopmentofGPSwasachievedonDecember8,1993, 30 whentheinitialoperationalcapability(IOC)wasdeclaredwhentwenty-foursatellites 31 (Blocks I, II, IIA) became successfully operational. The implication of IOC was 32 thatcommercial,national,andinternationalciviluserscouldhenceforthrelyonthe 33 availabilityoftheSPS.Fulloperationalcapability(FOC)wouldbedeclaredJuly17, 34 35 1995,whentwenty-foursatellitesofthetypeBlocksIIandIIAbecameoperational. 36 Teunissen (1993) introduced the least-squares ambiguity decorrelation adjustment 37 (LAMBDA),whichisnowwidelyused. 38 Thedeterminationofattitude/orientationusingGPShasdrawnattentionforquite 39 some time. Qin et al. (1992) report on a commercial product for attitude determi- 40 nation. Talbot (1993) reports on a real-time kinematic centimeter-accuracy survey- 41 ing system. Lachapelle et al. (1994) experiment with multiple (single-frequency) 42 receiver configurations, in order to accelerate the on-the-fly ambiguity resolution 43 by means of imposing length constraints and conditions between the ambiguities. 44 While much attention was given to monitoring the ionosphere with dual-frequency 45 andsingle-frequencycodeorcarrierphaseobservations,Kursinski(1994)discusses the applicability of radio occultation techniques to use GPS in a general earth’s at- 8 HISTORICALPERSPECTIVE 1 mosphericmonitoringsystem(whichcouldprovidehighvertical-resolutionprofiles 2 ofatmospherictemperatureacrosstheglobe). 3 Thesurveyingcommunitypromptlyrespondedtotheopportunitiesandchallenges 4 thatcamewithGPS.TheAmericanCongressonSurveyingandMapping(ACSM) 5 tasked an ad hoc committee in 1993 to study the accuracy standards to be used in 6 theeraofGPS.Thecommitteeaddressedquestionsconcerningrelativeandabsolute 7 accuracy standards. The National Geodetic Survey (NGS) enlisted the advice of 8 expertsregardingtheshapeandcontentofthegeodeticreferenceframe;theseefforts 9 eventuallyresultedinthecontinuouslyoperatingreferencestations(CORS).Orange 10 County(California)established2000plusstationstosupportgeographicinformation 11 systems(GIS)andcadastralactivities.Therearemanyotherexamples. 12 Zumbergeetal.(1998a,b)reportsingle-pointpositioningatthecoupleofcentime- 13 terslevelforstaticreceiversandatthesubdecimeterlevelformovingreceivers.This 14 techniquebecameavailableattheJetPropulsionLaboratory(JPL)around1995.The [8], 15 techniquethatrequiresdual-frequencyobservations,apreciseephemeris,andprecise 16 clockcorrectionsisreferredtoasprecisepointpositioning(PPP).Theseremarkable 17 resultswereachievedwithpostprocessedephemeridesatatimewhenselectiveavail- Lin 18 ability(SA)wasstillactive.Since1998JPLhasofferedautomateddataprocessing — 19 and analysis for PPP on the Internet (Zumberge, 1998). Users submit the observa- 0.0 20 tionfileovertheInternetandretrievetheresultsviaFTPsoonthereafter.Since1999 —— 21 JPL has operated an Internet-based dual-frequency global differential GPS system Nor 22 (IGDG).Thissystemdeterminessatelliteorbits,satelliteclockcorrections,andearth PgE 23 orientationparametersinreal-timeandmakescorrectionsavailableviatheInternet 24 for real time positioning. A website at JPL demonstrates real-time kinematic posi- 25 tioningatthesubdecimeterofareceiverlocatedatJPL’sfacilitiesinPasadena. [8], 26 Finally, during 1998 and 1999, major decisions were announced regarding the 27 modernization of GPS. In 2000, SA was set to zero as per Presidential Directive. 28 Whenactive,SAentailsanintentionalfalsificationofthesatelliteclock(SA-dither) 29 andofthebroadcastsatelliteephemeris(SA-epsilon);whenactiveitiseffectivelyan 30 intentionaldenialtocivilianusersofthefullcapabilityofGPS. 31 32 33 1.2 GEODETICASPECTS 34 35 Thethree-dimensional(3D)geodeticmodelisdefinitelythepreferredmodelforad- 36 justingthree-dimensionalGPSvectorobservationsandcombiningthemwithclassi- 37 calterrestrialobservationssuchasslantdistance,horizontalangle,azimuth,vertical 38 angle,and,withsomerestrictions,leveledheightdifferences.Thethree-dimensional 39 modelisapplicablewithequaleasetothefollowing:smallsurveysthesizeofapar- 40 celorsmaller,largesurveyscoveringwholeregionsandnations,three-dimensional 41 surveysformeasuringandmonitoringengineeringstructures,andthe“pseudothree- 42 dimensional”surveystypicalofclassicalgeodeticnetworksorin“planesurveying.” 43 Application of simple concepts from the theory of adjustments, such as “weighted 44 parameters” and “significance of parameters,” make it possible to use the three- 45 dimensionalmodelinalloftheseapplicationsinauniformmanner.Perhapsthemost

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