GPS Satellite Surveying

399 Pages·2004·3.414 MB·399\399

by  Leick, Alfred

GPS SATELLITE
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ALFRED LEICK
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LibraryofCongressCataloging-in-PublicationData:
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36 Leick,Alfred.
37 GPSsatellitesurveying/AlfredLeick.—3rded.
p. cm.
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Includesbibliographicalreferencesandindex.
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ISBN0-471-05930-7(cloth)
40 1.Artificialsatellitesinsurveying. 2.GlobalPositioningSystem. I.Title.
41 TA595.5.L45 2004
42 526.9'82—dc21 2003049651
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PrintedintheUnitedStatesofAmerica
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14 [1]
15 INTRODUCTION
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21 Anewandexcitingeraofpositioningonland,onthesea,andinspacebeganwith Sho
22 thelaunchofthefirstglobalpositioningsystem(GPS)satelliteonFebruary22,1978.
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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
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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
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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
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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
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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.
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30 1.1 HISTORICALPERSPECTIVE
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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
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1978 LaunchoffirstGPSsatellite
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4 1982 PrototypeMacrometertestingatM.I.T.
5 Geodeticnetworkdensification(Eifel,Germany)
1983
6 PresidentReaganoffersGPStotheworld“freeofcharge”
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Geodeticnetworkdensification(MontgomeryCounty,Pennsylvania)
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1984 EngineeringsurveyatStanford
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Remondi’sdissertation
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PrecisegeoidundulationdifferencesforEifelnetwork
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Codelessdualbandobservations
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1985 KinematicGPSsurveying
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Antennaswapforambiguityinitialization
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FirstinternationalsymposiumonprecisepositioningwithGPS
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16 Challengeraccident(January28)
1986
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18 1987 JPLbaselinerepeatabilityteststo0.2–0.04ppm —
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LaunchoffirstBlockIIsatellite
20 OTFsolution ——
21 1989 WideareadifferentialGPS(WADGPS)concepts Nor
22 U.S.CoastGuardGPSInformationCenter(GPSIC) PgE
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1990 GEOID90forNAD83datum
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25 NGSephemerisservice [4],
1991
26 GIG91experiment(January22–February13)
27 IGScampaign(June21–September23)
28 Initialsolutionstodealwithantispoofing(AS)
1992
29 NarrowcorrelatorspacingC/A-codereceiver
30 Attitudedeterminationsystem
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Real-timekinematicGPS
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ACSMadhoccommitteeonaccuracystandards
33 OrangeCountyGIS/cadastraldensification
34 1993 Initialoperationalcapability(IOC)onDecember8
35 1–2ppbbaselinerepeatability
36 LAMBDA
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IGSservicebeginningJanuary1
38 Antispoofingimplementation(January31)
39 RTCMrecommendationsondifferentialGPS(Version2.1)
40 1994 NationalSpatialReferenceSystemCommittee(NGS)
41 Multiple(single-frequency)receiverexperimentsforOTF
42 Proposaltomonitortheearth’satmospherewithGPS(occultations)
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Fulloperationalcapability(FOC)onJuly17
44 1995 Precisepointpositioning(PPP)atJPL
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HISTORICALPERSPECTIVE 5
1 TABLE1.1 (Continued)
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1996 PresidentialDecisionDirective,firstU.S.GPSpolicy
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4 VicepresidentannouncessecondGPScivilsignalat1227.60MHz
1998
JPL’sautomatedGPSdataanalysisserviceviaInternet
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6 VicepresidentannouncesGPSmodernizationinitiativeandthirdcivilGPSsignal
7 1999 at1176.45MHz
8 IGDG(Internet-basedglobaldifferentialGPS)atJPL
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Selectiveavailabilitysettozero
10 2000 GPSJPObeginsmodificationstoIIR-MandIIFsatellites
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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,
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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.
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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
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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
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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
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serviceformallybeganJanuary1,1994.
14 [7]
Formanyyears,usersworriedaboutwhatimpactantispoofing(AS)wouldhave
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on the practical uses of GPS. AS implies switching from the known P-code to the
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encryptedY-code,expressedbythenotationP(Y)-code.ThepurposeofASistomake
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theP-codesavailableonlytoauthorized(military)users.TheanxietyaboutASwas
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considerablyrelievedwhenHatchetal.(1992)reportedonthecode-aidedsquaring —
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techniquetobeusedwhenASisactive.Mostmanufacturersdevelopedproprietary
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solutionsfordealingwithAS.WhenASwasactuallyimplementedonJanuary31,
21 Lon
1994, it presented no insurmountable hindrance to the continued use of GPS and,
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particularly,theuseofmoderntechniquessuchasOTF.GPSusersbecameevenless PgE
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dependent on AS with the introduction of accurate narrow correlator spacing C/A-
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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
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on L5, and adding new military codes, GPS modernization will make the P(Y)-
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code encryption a nonissue for civilian applications, and at the same time, provide
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enhancedperformancetocivilianandmilitaryusers.
29
AmajormilestoneinthedevelopmentofGPSwasachievedonDecember8,1993,
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whentheinitialoperationalcapability(IOC)wasdeclaredwhentwenty-foursatellites
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(Blocks I, II, IIA) became successfully operational. The implication of IOC was
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thatcommercial,national,andinternationalciviluserscouldhenceforthrelyonthe
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availabilityoftheSPS.Fulloperationalcapability(FOC)wouldbedeclaredJuly17,
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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
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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
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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