Analysis of Global Positioning System (GPS) data from two sites separated by a horizontal distance of only ∼2.2 m yielded phase residuals exhibiting a systematic elevation angle dependence. One of the two GPS antennas was mounted on an ∼1‐m‐high concrete pillar, 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° to 25°. We performed simulations based on a simple (ray tracing) multipath model with a single horizontal reflector which demonstrated that the results from the elevation angle cutoff tests and the pattern of the residuals versus elevation angle could be qualitatively reproduced if the reflector were located 0.1–0.2 m beneath the antenna phase center. We therefore hypothesized that the elevation‐angle‐dependent error was caused by scattering from the horizontal surface of the pillar, located a distance of ∼0.2 m beneath the antenna phase center. We tested this hypothesis by placing microwave absorbing material between the antenna and the pillar in a number of configurations and by analyzing the changes in apparent position of the antenna. The results indicate that (1) the horizontal surface of the pillar is indeed the main scatterer, (2) both the concrete and the metal plate embedded in the pillar are significant sources of scattering, and (3) the scattering can be reduced greatly by the use of microwave absorbing materials. These results have significant implications for the accuracy of global GPS geodetic tracking networks which use pillar‐antenna configurations identical or similar to the one used for this study at the Westford WFRD GPS site.

We have observed variations at the several centimeter level in estimates of the vertical coordinate of site position. The estimates are obtained from our analysis of data acquired from the Swedish permanent Global Positioning System (GPS) network. The observed variations are strongly correlated with changes in the indirectly inferred accumulation of snow, which we assume collects on the radomes and pillars; the GPS sites could not be observed directly due to their remoteness. Numerical simulations which assume a simple geometry for the snow cover are used to study the effects of snow accumulation on GPS phase observables and hence on estimates of the vertical coordinate of site position obtained from these observables. Our results indicate that the variations in the vertical coordinate of site position can be fully explained by reasonable accumulations of snow which retard the GPS signals and enhance signal scattering effects.

This paper presents results from a study of elevation‐angle‐dependent systematic effects on estimates of relative site positions within the Swedish permanent Global Positioning System (GPS) network. Two months of data from 16 sites have been analyzed with three different elevation cutoff angles, namely, 10°, 15°, and 20°. We present offsets between these solutions and demonstrate that estimates of the vertical component of several baselines strongly depend on the minimum elevation angle (elevation cutoff angle) of the data analyzed. Offsets of 22.3 ± 1.6 mm in the vertical component are evident when the elevation cutoff angle is changed from 10° to 20°. We investigate these offsets and conclude that a significant part is due to differential phase errors caused by scattering from structures associated with the mounting of the antenna to the pillar and with the pillar itself. The horizontal components of baseline are less affected. We found, however, that the offsets in the horizontal components increase with baseline length. For the longest baselines (∼1500 km) offsets of more than 5 mm are evident in the north component when the elevation cutoff angle is changed from 10° to 20°. These offsets are most likely due to differential phase errors caused by nonuniform antenna phase patterns; an effect that presumably increases with baseline length and which also might increase because of scattering from the pillars and the antenna mounts. We identify the scattering structure and reduce associated errors in the vertical component of baseline to a significant degree on one of the sites by using microwave‐absorbing material. The results presented are of importance for those analyzing data from existing networks and for those who intend to establish permanent GPS geodetic networks.

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