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 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 a short-term precision of 1.9 mm+0.6 parts in 108; the east-west component shows a similar precision at the shortest distances, 2.1 mm, with a larger length dependence, 1.3 parts in 108. 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 108, 5.2 mm+2.8 parts in 108, 11.7 mm+13 parts in 108 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 porition of the horizontal diffrences 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. ¿American Geophysical Union 1991