Numerical Weather Predictions for GPS Positioning

When GPS satellite signals are transmitted through the atmosphere they are affected by the media. In the neutral atmosphere the refraction is a function of pressure, temperature, and humidity along the signal path, and in the GPS positioning process this effect is normally handled by utilising global tropospheric delay models. For high accuracy differential positioning over baselines lengths where the differential effect of the signal delay from the neutral atmosphere is significant, these global models of the signal delay are not sufficiently accurate, and this is especially the case during abnormal weather conditions.This paper describes a new approach where numerical weather predictions (NWPs) are introduced in the GPS data processing instead of global tropospheric delay models. NWPs are predictions of the meteorological conditions for a given area and epoch in time, and can as such be used for estimating the tropospheric delay for a satellite signal by numerical integration along the signal path through the NWP. For the tests described in this paper, the signal delays are determined as a zenith delay through the NWP combined with a mapping function. This approach is useful for kinematic and shorter static GPS applications. The paper describes the theory of the method, and the applicability of the method is evaluated by analysing position accuracies obtained by introducing NWP-derived signal delays in kinematic and static processing of GPS data. Improved position accuracies are obtained for most of the test scenarios, indicating that the method does have a potential.