APPLICATION OF THE GLOBAL POSITIONING SYSTEM TO CRUSTAL DEFORMATION MEASUREMENT .1. PRECISION AND ACCURACY

In this paper we assess the precision and accuracy of interstation vectors determined using the Global Positioning System (GPS) satellites. These vectors were between stations in California separated by 50-450 km. Using data from tracking the seven block I satellites in campaigns from 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 parts in 10(8); the east-west component shows a similar precision at the shortest distances, 2.1 mm, with a larger length dependence, 1.3 parts in 10(8). The vertical precision has a mean value of 17 mm, 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 distances less than 50 km, we can describe long-term GPS precision for baselines less than 450 km in length as 3.4 mm + 1.2 parts in 10(8), 5.2 mm + 2.8 parts in 10(8), 11.7 mm + 13 parts in 10(8) in the north- south, east-west, and vertical components. Accuracy has been determined by comparing GPS baseline estimates with those derived from very long baseline interferometry (VLBI). A comparison of eight interstation vectors shows differences ranging from 5 to 30 mm 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 differences can 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.