Azimuth-dependent Mapping Functions From Numerical Weather Models in VLBI Analysis for CONT02

In the past few years a significant improvement of VLBI analysis has been obtained by the use of numerical weather models (NWM) for the determination of elevation-dependent mapping functions for the troposphere delays. For the VLBI campaign CONT02, which covers 15 consecutive 24 hour sessions with eight stations in the second half of October 2002, the operational analysis pressure level data from the European Centre for Medium-Range Weather Forecasts (ECMWF) is used to provide the hydrostatic and wet mapping functions not only once per station and epoch but also every 30 degrees in azimuth. The determination is based on a 3D ray-trace program which uses the following properties of the NWM: 20 deg x 20 deg grids with 50 km increments around each of the eight stations, 21 levels (from 1000 hPa to 1 hPa) vertical resolution of the profiles and six hours time intervals. The application of these azimuth-dependent mapping functions in the VLBI analysis of CONT02 shows that the hydrostatic gradients are well determined with this approach, whereas the accuracy of the wet gradients from a NWM as described above still contains some deficiencies. The azimuth-dependent hydrostatic mapping functions are compared to the tilting of the 200 hPa pressure levels, and the residual wet gradients estimated in the VLBI analysis are compared to those derived from the NWM. It is also discussed whether standard gradient models with north and east gradients can properly account for asymmetries of the troposphere. Thus, the presentation describes what presently available numerical weather models can do in terms of asymmetric modeling of the troposphere delays. The results of the VLBI analysis provide a reliable estimation how big the effect will be for GPS.