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Abstract

GPS radio occultation measurements on board Low Earth Orbiting (LEO) satellites such as CHAMP (Challenging Minisatellite Payload) provide a new calibration free data source for vertical profiling of atmospheric parameters like temperature and water vapor on global scale. Considering current and planned LEO missions (e.g. COSMIC, EQUARS or METOP), these measurements will provide a valuable data base for climatological investigations and weather prediction in the future. The GPS radio occultation experiment within the CHAMP satellite mission has been activated now for more than 3 years. More than 270,000 occultation measurements are expected as of July 2004. Since the life time of the CHAMP satellite is predicted to last longer than 2007, the first and unique long-term set of GPS occultation data is anticipated. GFZ provides results of an operational occultation data analysis via the Information System and Data Center (ISDC). The results are available at different processing levels: atmospheric excess phase data, bending angles and vertical profiles of refractivity. The temperature profiles provided at ISDC are calculated under dry air assumption. The resulting dry temperature profiles are almost identical with the real temperature at altitudes above 10 km, where the wet component of the refractivity can be neglected. However, vertical profiles of the tropospheric temperature and water vapor can only be derived using ancillary atmospheric information from e.g. meteorological analyses (either temperature or water vapor). Tropospheric water vapor and temperature profiles are derived using the improved version (005) of GFZ’s operational refractivity profiles, which is provided via ISDC since March 2004. Here the Full Spectrum Inversion (FSI) technique is used for data analysis in the lower troposphere, resulting in a significant reduction of the negative refractivity bias in this region. To combine CHAMP refractivity measurements with ancillary meteorological information in a statistically optimal way an operational 1Dvar retrieval code is used for the water vapor retrieval. Even if a unique separation of dry and wet component of refractivity is not possible, especially the initial humidity assumption may be improved provided that refractivity measurement and ancillary temperature are of sufficient quality. We validate the results with radiosonde data and ECMWF analyses. Potentials for global water vapor monitoring are demonstrated.