GEODESY USING THE GLOBAL POSITIONING SYSTEM - THE EFFECTS OF SIGNAL SCATTERING ON ESTIMATES OF SITE POSITION

Analysis of Global Positioning System (GPS) data from two sites separa ted by a horizontal distance of only similar to 2.2 m yielded phase re siduals exhibiting a systematic elevation angle dependence. One of the two GPS antennas was mounted on an similar to 1-m-high concrete pilla r, and the other was mounted on a standard wooden tripod. We performed elevation angle cutoff tests with these data and established that the estimate of the vertical coordinate of site position was sensitive to the minimum elevation angle (elevation cutoff) of the data analyzed. For example, the estimate of the vertical coordinate of site position changed by 9.7+/-0.8 mm when the minimum elevation angle was increased from 10 degrees to 25 degrees. We performed simulations based on a si mple (ray tracing) multipath model with a single horizontal reflector which demonstrated that the results from the elevation angle cutoff te sts and the pattern of the residuals versus elevation angle could be q ualitatively reproduced if the reflector were located 0.1-0.2 m beneat h the antenna phase center. We therefore hypothesized that the elevati on-angle-dependent error was caused by scattering from the horizontal surface of the pillar, located a distance of similar to 0.2 m beneath the antenna phase center. We tested this hypothesis by placing microwa ve absorbing material between the antenna and the pillar in a number o f configurations and by analyzing the changes in apparent position of the antenna. The results indicate that (1) the horizontal surface of t he pillar is indeed the main scatterer, (2) both the concrete and the metal plate embedded in the pillar are significant sources of scatteri ng, and (3) the scattering can be reduced greatly by the use of microw ave absorbing materials. These results have significant implications f or the accuracy of global GPS geodetic tracking networks which use pil lar-antenna configurations identical or similar to the one used for th is study at the Westford WFRD GPS site.