Monitoring the Water Vapour Content of the Atomsphere using the Austrian GNSS Reference Station Network

GPS has become an important tool both in navigation and in precise point positioning. One of the nuicance parameters limiting the accuracy of point determination is the water vapor content of the troposphere. On the other hand meteorologists are interested in the wet component of the troposphere as a valuable tool for Numerical Weather Prediction and Climate Studies. Therefore GPS offers a low cost monitoring of water vapor with high temporal resolution. We make use of continuous measurements of the GPS/GLONASS reference station network in Austria, which currently consists of about 27 sites with a mean distance of 50 km. The total zenith path (TZPD) delays for 2 weeks in 2002 (May and August 2002) with an anticipated huge difference in the water vapour content were calculated. Subsequently the results are compared to contributions of different processing centers of the COST-716 project (Exploitation of Ground Based GPS for Climate and NWP) and with zenith path delay estimates provided by the IGS. Based on the observed atmospheric pressure field the Zenit Wet Delay has been separated from the TZPD estimates. As meteorologists need the water vapor within less than two hours, special attention is paid to the availability, reliability and especially on the quality of the satellite orbits used for the network calculations. For this reason we try to use rapid ephemeris instead of the IGS final orbits, whereby we make a quality control of the rapid orbits. If one or more satellites show poor quality or if one satellite is missing at all we include the broadcast ephemeris information instead.