Sien Wu

GPS Geodesy: A Status Report

Accuracies of a centimeter or better in earth baseline measurements are crucial for many investigations in crustal dynamics. This implies measurement accuracies of the order of a few parts in 108 of baseline length for regional geodesy. In recent years, techniques exploiting the Global Positioning System (GPS) have advanced rapidly toward this accuracy goal. The latest GPS techniques developed at JPL have resulted in centimeter-level agreement with independent measurements made by very long baseline interferometry (VLBI) for baselines up to 2000 km in length. Techniques that have shown promise for GPS-based geodesy include: (1) carrier phase ambiguity resolution, (2) multi-day GPS orbit determination, (3) stochastic estimation of the zenith tropospheric delay, and (4) simultaneous use of carrier phase and pseudorange. The order of importance depends upon the scale of the network, and the approaches are often synergistic. For example, ambiguity resolution can depend upon the ability of the other techniques to improve precision. These results have been acheived despite a partial GPS constellation, regional tracking only, and pseudorange data plagued by multipath. A full GPS constellation and a permanent global tracking network will be in place in the early 1990’s, and a new receiver/antenna prototype at JPL is now producing smoothed pseudorange observables accurate to better than 5 cm.