Atmospheric gradients estimated by GPS compared to a high resolution numerical weather prediction (NWP) model

The estimation of horizontal atmospheric gradients, in addition to zenith delays, is a strategy now commonly used in geodetic Global Positioning System (GPS) positioning. This strategy compensates for inhomogeneities in the atmospheric water vapor distribution above GPS sites, and has shown to increase the positioning precision, e.g. in geodynamic networks. While the zenith delay has been successfully related to the pressure at the GPS site and the water vapor above the site, the relation of the GPS estimated horizontal gradients to atmospheric quantities remains unclear. To get a better understanding of the nature of these gradients inferred by GPS, this study compares GPS tropospheric observations from the MAGIC permanent network on the NW side of the Mediterranean Sea with simulations based on the high resolution NWP model ALADIN (Meteo France). To verify the model performance, we use meteorological measurements from the FETCH ship campaign in the Gulf of Lyon in March-April 1998. For this study, five stations of the MAGIC network close to the Golf of Lyon have been selected. Results from two periods, representing two different weather situations occuring within the FETCH observation campaign, are presented. (C) 2001 Elsevier Science Ltd. All rights reserved.