Analysis of Atmospheric Delays and Asymmetric Positioning Errors in GPS

Wednesday, 17 December 2014
Kathryn Materna and Thomas Herring, Massachusetts Institute of Technology, Cambridge, MA, United States
Error in accounting for atmospheric delay is one of the most significant limiting factors in the accuracy of GPS position determination. Delay due to tropospheric water vapor is especially difficult to model, as it depends in part on local atmospheric dynamics. Currently, the delay models used in GPS data analysis produce millimeter-level position estimates for most of the stations in the Plate Boundary Observatory (PBO) GPS network. However, certain stations in the network often show large position errors of 10 millimeters or more, and the key characteristic of these errors is that they occur in a particular direction. By analyzing the PBO network for these asymmetric outliers, we found that all affected stations are located in mountainous regions of the United States, and that many are located in the Sierra Nevada Mountains. Furthermore, we found that the direction in which the asymmetric outliers occur is related to the direction of local topographic increase, suggesting that topography plays a role in creating asymmetric outliers. We compared the GPS time series data with several forms of weather data, including radiosonde balloon measurements, numerical weather models, and MODIS satellite imagery. The results suggest that GPS position errors in the Sierra Nevada occur when there is strong atmospheric turbulence, including variations in pressure and humidity, downwind of the mountain crest. Specifically, when GPS position errors occur in the Sierra Nevada, lee waves are likely to be observed over the ridge; however, not all lee wave events produce position errors. Our results suggest that GPS measurements in mountainous regions may be more prone to systematic errors than previously thought due to the formation of lee waves.