JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 96, NO. B10, PAGES 16,547-16,565, SEPTEMBER 10, 1991 Application of the Global Positioning System to Crustal Deformation Measurement 1. Precision and Accuracy KRISTINB M. LARSON Colorado Center.for Astrod.•tnamicsResearch, Universit.•t of Colorado, Boulder DV•½•,• C. A•ew Institute of Geoph.•tsicsand Planetat&t Ph.•tsics,Scripps Institution of Oceanograph•t,La Jolla, California In this paper we assessthe precisionand accuracy of interstation vectors determined using the Global PositioningSystem (GPS) satellites. These vectorswere between stations in California separatedby 50-450 kin. Using data from tracking the seven block I satellites in campaignsfrom 1986 through 1989, we examine the precision of GPS measurements over time scales of a several days and a few years. We characterize GPS precision by constant and length dependent terms. The north-south component of the interstation vectors has a short-term precision of 1.9 mm + 0.6 partsin 10s;the east-westcomponent shows a similarprecision at the shortestdistances, 2.1 ram, with a largerlength dependence, 1.3 parts in 10s. The verticalprecision has a mean valueof 17 ram, with no clear length dependence. For long-term precision, we examine interstation vectors measured over a period of 2.2 to 2.7 years. When we include the recent results of Davis et al. (1989) for distancesless than 50 kin, we can describelong-term GPS precisionfor baselinesless than 450 kmin lengthas 3.4 mm+ 1.2 partsin 10s, 5.2 mm + 2.8 partsin 10s , 11.7mm + 13 parts in 10s in the north- south, east-west,and verticalcomponents. Accuracy has been determined by comparing GPS baseline estimates with those derived from very long baseline interferometry (VLBI). A comparisonof eight interstation vectorsshows differences ranging from 5 to 30 nun between the mean GPS and mean VLBI estimates in the horizontal components and less than 80 mm in the vertical. A large portion of the horizontal differencescan be explained by local survey errors at two sites in California. A comparison which suffers less from such errors is between the rates of change of the baselines. The horizontal rates estimated from over 4 years of VLBI data agree with those determined with 1-2 years of GPS data to within one standard deviation. In the vertical, both GPS and VLBI find insignificant vertical motion. INTRODUCTION (GPS) satellites.The basicprinciples of this techniqueare similar to VLBI, with some important differences. Thc Many geodetictechniques have been usedto measurecrus- dio signals originate at satellites, rather than quasars, and tal deformation acrossthe North American/Pacific plate GPS receivers make an instantaneous determination of the boundary in California. The oldestdata comefrom triangu- distance between the receiver and satellite, rather than re- lationmeasurements [e.g., Hayford and Baldwin,1908]; more quiring cross correlation of noise signals to determine the recently,precise electronic distance measurement (EDM) length between two sites. From enough measurementsof has provided many details about strain acrossfaults of the signals arriving from different directions the complete vec- plate boundary[e.g., Savage,1983]. Both of these proce- tor baseline between two sites can be computed. Because duresmeasure through the atmosphereand require the mea- the signal strength at the Earth is so much higher than for surement points to be intervisible; they are thus limited to the quasar sourcesused in VLBI and becausethe signal has distancesup to a few tens of kilometers. Both require consid- a known and well-controlledstructure, G PS antennas(and erable skill and expensive equipment to pursue successfully. all other equipment)weigh at most a few hundredpounds In the past decade, measurementsusing extraterrestrial rather than the many tons needed for VLBI. A similar fa- objects,such as satellite lazer ranging (SLR) [Christodoulidisvorable ratio applies to the costsof the two techniques. For et al., 1990;Smith et al., 1990]and very longbaseline inter- all these reasons,GPS is poised to become the method of ferometry(VLBI) [Herring et al., 1986; Clark et al., 1987], choice for crustal deformation geodesyand in many areas have made possible measurements between points hundreds has indeed already become so. to thousands of kilometers apart. This has enabled a di- But since this is such a new technique, it is vital to estab- rect determination of the total contemporary plate motion lish just what its errors are. Judging from past experience acrossthe broad boundary in California. These observations with VLBI and SLR, constructinga formal error budget, are, however, even more expensive to make than the older, while useful, is likely to give an incomplete picture because purely terrestrial, techniques. of the wide variety of semisystematic errors that are little The most recent advance in precise geodetic measure- understood(not to mentionthose that are overlooked).We ments has been the use of the Global Positioning System feel that what is neededis an empirical investigation of the precisionand accuracy of GPS measurements,judged from actual results: the precision being a measure of how exact Copyright 1991 by the American Geophysical Union. the estimate is, and the accuracy a measure of how closethe Paper number 91JB01275. estimateis to the truth [Berington,1969]. Our measureof 0148-0227/91]91-JB-01275505.00 16,547 16,548 LARSONAND AGNEW: GLOBALPOSITIONING SYSTEM PRECISION AND ACCURACY precisionis thus (as for others)the scatterof resultsabout a ography, California Insitute of Technology,University of Cal- mean value; our measure of accuracy is the agreement with ifornia, Los Angeles, and MassachusettesInstitute of Tech- someother technique(VLBI). nology,with substantial help from the U.S. GeologicalSur- This paper is the first of three that describeresults from vey(USGS) and the NationalGeodetic Survey (NGS). When nearly three years of measurementsin central and southern possible, we also included data from the North American California. This paper describes the precision and accu- network of fixed GPS trackers that are part of the Cooper- racy over baselinesfrom 50-450 km. Though some earlier ativeInternational GPS NETwork(CIGNET) [Chin,1988]. work [e.g., Dong and Bock, 1989] has demonstratedsub- Table 1 lists 22 of the GPS sites observed. The stations lo- centimeter precision over these distances,this precisionwas cated in California are shown in Figure 1. The remaining only evaluated from data collectedover a few days. Suches- sites, all located in North America, were used for preciseor- timates are likely to underestimate the long-term precision bit determination, and are discussedin paper 2. While more because a number of error sourcesprobably do not change sites than these were observedduring the 11 experiments, much over this span of time. The only paper that has looked only these 22 were measured at more than one epoch and at long-term precision and accuracy is that of Davis et al. thus can provide a useful estimate of long-term precisionand [1989],but most of the data shownthere werefor baselines accuracy. As noted above, the network formed by those of of 200 m to 50 km, not the longer regional scaleswe discuss the stations in California yields baselinelengths from 50 to here. This suite of measurements also aJlows us to discuss 450 km. the role of orbit determination(fiducial) networks in estab- In each experiment the GPS satellites were tracked at lishinga consistentreference frame [Larsonet al., this issue] eachstation using a TI-4100 dual-frequencyreceiver [lien- (hereafterreferred to as paper 2), and the effectof different son et al., 1985], recordingboth carrier phaseand pseudo- modelingprocedures for the atmosphericdelay (K.M. Lar- range data at 30-s intervals. (Carrier phaseis precisebut son and J.L. Davis, manuscript in preparation; hereinafter ambiguousby an integer number of cycles; pseudorangeis referredto as paper 3). In the next sectionof this paper,we unambiguousbut 2 ordersof magnitudeless precise.) The describe how the GPS data were collected and the analysis intention was to track for the 7-8 hours that several satel- techniques we used to determine the coordinates.of the dif- lites werevisible; for mostof the experiments(all but those ferent stations. The following sectionsdiscuss precision and donein May, June, and September)this meant trackingal- accuracy. most entirely at night. For most experiments the plan was to record data for 4 or 5 consecutivedays, a goal not always EXPERIMENTAL PROCEDURE AND DATA ANALYSIS achieved in practice. Table 2 summarizes the data available from all the sites we have considered. The receiver antenna The data we use were collected during 11 "experiments" was centered over the geodetic monument at each site us- conducted between June 1986 and March 1989. The mea- ing an optical plummet, with a nominal accuracy of about surements in southern and central California were made by 1 mm; the vertical offset between the antenna and the top a four-university consortium, Scripps Institution of Ocean- of the monument was measured by a tape, with perhaps 2- TABLE 1. GPS Stations Station Location Longitude, Latitude, Height, Stamping deg deg m 1, Algonquin Ontario, Canada -78.071 45.958 209 TELESCOPE REF A 2, Blancas Monterey -121.284 35.666 50 none 3, Blackhill Morro Bay -120.831 35.360 201 BLACKHILL 1881 4, Brush Catalina Isl. -118.404 33.409 451 BRUSH 1976 5, Buttonwillow Bakersfield -119.394 35.405 64 A364 1953 6, Center Santa Cruz Island