AN INTRODUCTION TO THE GLOBAL POSITIONING SYSTEM AND SOME GEOLOGICAL APPLICATIONS T. H. Dixon JetPropulsion Laboratory Divisionof Earthand PlanetarySciences Pasadena,California Abstract.Receivers equipped to measuredual frequency data acquisitionand analysisare having a significantim- carrierphase signals from satellitesof the GlobalPosition- pact on studies of near-fault crustal deformation and ing System(GPS) havebeen capable, under special condi- earthquakeprocesses, until recently the province of con- tions, of determiningrelative horizontalpositions among ventional terrestrialgeodetic techniques.The enhanced stationsseparated by oneto a few hundredkilometers with satelliteconstellation, improved models, and establishment a precisionof one to severalmillimeters since the early of global tracking networks have extendedseveral mil- 1980s. The major obstaclesto making this capability limetershorizontal positioning capability to stationsepara- routine,extending it to all partsof theglobe, and extending tionsof 1000km or morein virtuallyall partsof the world. it to longerstation separations, have been equipment cost, This enablesstudy of new classesof tectonicproblems that limitations in the GPS satellite constellation,arduous data previouslywere difficult to attackwith any geodetictech- analysis,uncertainties in satelliteorbits, uncertainties in nique.Examples include a completekinematic description propagationdelays associated with variabletropospheric of ongoingcrustal deformation in broad, complexconti- watervapor, and difficultiesin resolvingcarrier phase cy- nentalplate boundaryzones, and measurementof relative cle ambiguities.Recent improvements have occurred in all plate motionat convergentboundaries where global mod- theseareas. The increasingease and reducedcost of GPS els maybe poorlyconstrained. INTRODUCTION extendingthe range and accuracyof GPS measurements are emphasized.I have aimed for broadcoverage of most With the adventin the early 1980sof a satellite-based of therelevant topics and an intuitiverather than complete navigationsystem known as the Global Positioning System or rigorous treatment, giving more derailed references (GPS) operatedby the U.S. Departmentof Defense it whereappropriate. becamepossible for a user with the properreceiver to obtain almost instantaneousthree-dimensional position informationaccurate to severalmeters. With the comple- A BRIEF HISTORY OF THE GPS PROGRAM tion of the satelliteconstellation in the early 1990s this capabilitywill be extendedto virtually all parts of the The spacesegment of GPS is a constellationof satellites globe.This is a remarkableachievement and buildson a in highEarth orbit equipped with powerfulradio frequency number of technologicaladvances in the last several transmittersand highly stable atomic clocks. Easton [1978] decades. Even more remarkable is the fact that. with careful reviewsthe majordevelopments leading to thiscapability. attentiontoexperiment configuration anddat• analysis it is In 1967 an early prototypeof a GPS satelliteknown as possible to obtain relative position data 3 orders of Timation 1 was launchedinto low Earth orbit (-900 km magnitudemore precise than the design level of the altitude) as part of a military test program in satellite system.This enhancedperformance allows for measure- navigation.Weighing about 40 kg and consumingonly 6 ment of crustalstrain and fault motionrates in just a few W of power,it carrieda UHF transmitterslaved to a stable )rears. quartzclock, with a frequencydrift of several parts in 10• This paper reviews fundamentalprinciples of GPS, per day. Additional proof-of-conceptsatellites followed, discussessome geologicaland geophysicalapplications culminating10 yearslater in the NavigationTechnology and their accuracyrequirements, and considersimplica- Satellite (NTS) 2, very similar to subsequentGPS tions for GPS experimentdesign. Recent developments satellites.NTS-2 was launched into a 20,300-km-altitude Copyright1991 by the AmericanGeophysical Union. Reviewsof Geophysics,29, 2 / May 1991 pages249-276 8755-1209/91/91RG-00152 $05.00 Papernumber 91RG00152 ß 249 ß 250 ß Dixon: THE GLOBAL POSITIONING SYSTEM 29, 2/REVIEWS OF GEOPHYSICS orbit, weighed440 kg, consumed400 W of power, and oneanother in thesky provide correlated (redundant) range transmittedtwo L band (-1.2 and 1.5 GHz) timing and information,an effect known as geometricdilution of ranging signals based on a sophisticatedcesium clock, precision(GDOP). If observationsare limited to four witha frequencydrift of lessthan two parts in 10•3 per satellitesby receiverdesign, these geometric effects can be day. One year after NTS-2, the first "Block 1" GPS minimized by choosingsatellites which maximize the satellitewas launched,part of the operationaltest phase of volumeof a tetrahedron,defined by the pointsof intersec- the GPS program. By 1990 the Block 1 constellation tion on a unit spherecentered on the user, of vectors includedsix functioningsatellites launched between 1978 betweenthe satelliteand the groundreceiver. and 1985. It is alsopossible to obtaindistance information (strictly It hasbeen recognized for sometime thathigh-precision speaking,the changein distance)through phase measure- geodetic measurementscould be made by exploiting mentson the carrier signal itself, keepingtrack of the signalsfrom artificial satellites[e.g., Preston et al., 1972; number of cycles after signal acquisition.Assuming MacDoran, 1979; Counselmanand Shapiro, 1979]. The perfectclocks, and ignoringpropagation effects, Block 1 constellationhas provensatisfactory for develop- ing andrefining experiment design and analytical concepts and for initiating a number of high-precisiongeodetic monitoringprograms. The first Block 2 satellite,with a = (v,/œ)(n + 0)) (2) number of improvementsrelative to its forebears,was wheren is the numberof integercarrier wavelengths at launchedin February1989. As of thiswriting, a totalof 10 signalacquisition (initially unknown),t• is the phasein Block 2 satellitesare in operation.A total of 21 Block 2 satellitesplus three sparesare plannedto be in operation cycles,)• isthe wavelength, f is the frequency, and v, isthe by the end of 1992. They will orbit at an altitudeof about phasevelocity (the importanceof distinguishingv, and 20,000 km in six orbital planes with 12-hour periods, groupvelocity, vg, will becomeapparent). Since the wavelengthof the carrieris considerablyshorter than that enablingsimultaneous observation of four or more GPS of the lower frequencycode modulations(Table 1), the satellitesin virtuallyall partsof the globe. resulting"distance" measurement, though ambiguous by the initial numberof wavelengths,is considerablymore POINT POSITIONING WITH GPS ,,,-ooiootho,, o pseudorangemeasurement and is one of the keys to high-precisionGPS measurements[Bossler et al., 1980; Counselmanand Gourevitch,1981; Remondi, 1985]. RangeMeasurement Carrier phase is not measureddirectly, as this would An observeron Earth can uniquelylocate his position requirevery high samplingrates. Rather,the signalis by determiningthe distancebetween himself and three mixed ("heterodyned")with a signal generatedby the satelliteswhose orbital positionsare alreadyknown. With receiver's internal clock (local oscillator) and, after GPS, distanceinformation is based on the travel time x of a band-passor low-pass filtering, the resulting lower satellite signal, obtained by measuringthe difference frequency"carrier beat phase"is sampled.Most current betweenthe transmit (t•) andreceive (tr) times at theGPS generationreceivers accomplish this "downconversion" receiverof a specialranging code, describedin the next with electronicsthat includeextensive analog circuitry. section.If we ignore transmissionmedia effects on the Some newer generationreceivers have largely digital speedof light c and any timing(clock) errors, then the true architecture,reducing production cost, size, and power rangep betweensatellite and receiver is just C(tr -- rs). consumptionand enablingdigital samplingof the carrier Errorsin receiveror satelliteclocks are present in therange phasesignal with only minimalpreprocessing [Melbourne, estimate,which for this reasonis referredto as pseudo- 1990]. rangeR, definedmore precisely as TABLE 1. Summaryof GPS SignalCharacteristics R = p + c(Atr - Ats+ Atp) (1) Carriers Code Modulations C/A whereAt r is thereceiver clock offset from "true" (GPS sys- L1 L2 P (L1 only) tem) time (we ignore any other receiver-inducederrors), At•is thesatellite clock offset, and At is thedelay associ- Frequency(carrier) 1.57542 1.2276 10.23 1.023 atedwith all other error sources, rn•ainly due to atmos- or chiprate GHz GHz MHz MHz (codemodulation) phericpropagation effects. Information from a fourthsatel- Wavelength 19.0cm 24.4 cm ~30 m ~300m liteallows a first-orderclock correction (Atr - Ats),and ap- proachesdiscussed below can be appliedto estimateand correct for At, enabling meter-levelpositioning under idealconditionS. For analysispurposes we considerthe pseudorangeor Observationgeometry affects the quality of theresulting phaseparameters in termsof what the receiveractually three-dimensionalposition. Satellites that appearclose to sees(the "observable"),explicitly accounting for major 29, 2 / REVIEWSOF GEOPHYSICS Dixon' THE GLOBALPOSITIONING SYSTEM ß 251 error sources.For example,in simplifiedform the phase wherem• andm 2 are the angularfrequencies (m = 2•rf) observable, sometimes called integrated Doppler or associatedwith the L1 andL2 carriersand A• andA c are accumulateddelta