Integrated atmospheric water vapor estimates from a regional GPS network

Integrated atmospheric water vapor (IWV) estimates from a 15-station-wide network of Global ositioning System (GPS) receivers have been collected continuously since November 1997. The core of this network consists of five stations of the active GPS reference system in the Netherlands. A network with sufficient long baselines was chosen to secure the absolute accuracy of the GPS IWV data. Rapid satellite orbits available 12 to 24 hours after data acquisition are used in the processing of the GPS data, and IWV estimates are available with a typical delay of 1 day. Comparison of the GPS IWV data with data retrieved from a water vapor radiometer and radiosondes shows a good agreement. Different network configurations and processing strategies have been investigated to optimize the network and processing for future near-real-time use. In near-real-time applications, only predicted orbits are available; however, the accuracy of the predicted orbits is, in general, not sufficient for accurate IWV retrieval. We tested whether orbit relaxation, i.e., the simultaneous adjustment of orbit parameters during the processing of the GPS data, could increase the accuracy of the IWV estimates. During an experiment with orbit relaxation applied to predicted orbits a significant improvement of the accuracy of the GPS IWV data was found. The accuracy was comparable to GPS IWV data retrieved with final orbits, the most accurate orbit data available. Results of the experiments and the analysis of operational acquired data are presented