ATMOSPHERIC-PRESSURE LOADING EFFECTS ON GLOBAL POSITIONING SYSTEM COORDINATE DETERMINATIONS

Earth deformation signals caused by atmospheric pressure loading are d etected in vertical position estimates at Global Positioning System (G PS) stations, Surface displacements due to changes in atmospheric pres sure account for up to 24% of the total variance in the GPS height est imates. The detected loading signals are larger at higher latitudes wh ere pressure variations are greatest; the largest effect is observed a t Fairbanks, Alaska (latitude 65 degrees), with a signal RMS of 5 mm. Out of 19 continuously operating GPS sites (with a mean of 281 daily s olutions per site), 18 show a positive correlation between the GPS ver tical estimates and the modeled loading displacements, Accounting for loading reduces the variance of the vertical station positions on 12 o f the 19 sites investigated. Removing the modeled pressure loading fro m GPS determinations of baseline length for baselines longer than 6000 km reduces the variance on 73 of the 117 baselines investigated. The slight increase in variance for some of the sites and baselines is con sistent with expected statistical fluctuations. The results from most stations are consistent with similar to 65% of the modeled pressure lo ad being found in the GPS vertical position measurements. Removing an annual signal from both the measured heights and the modeled load time series leaves this value unchanged. The source of the remaining discr epancy between the modeled and observed loading signal may be the resu lt of (1) anisotropic effects in the Earth's loading response, (2) err ors in GPS estimates of tropospheric delay, (3) errors in the surface pressure data, or (4) annual signals in the time series of loading and station heights. In addition, we find that using site dependent coeff icients, determined by fitting local pressure to the modeled radial di splacements, reduces the variance of the measured station heights as w ell as of better than using the global convolution sum.