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Abstract

GPS (Global Positioning System) radio occultation aboard the German CHAMP satellite (CHAllenging Minisatellite Payload) was activated in February 2001. It brought significant progress for establishing this innovative remote sensing technique as a standard method for precise monitoring of the Earth’s atmosphere on a global scale. We characterize the operational occultation data analysis at GFZ, review the results obtained so far and focus to future prospects. CHAMP provides continuously up to 200 vertical profiles of refractivity, temperature and water vapour daily. Since the experiment is currently expected to last until 2007, the first long-term set of GPS occultation data is expected. The measurements are analysed by an automatically working processing system, results are provided to the scientific community via the CHAMP data center at GFZ (ISDC, Information System and Data Center). Beginning in January 2003 a continuous transmission of analysis results for the purpose of data assimilation for numerical weather prediction is tested. An average delay of ~5 hours between each measurement and arrival of the data at the assimilation center (MPI Hamburg) is reached continuously. Data from meteorological analyses (ECMWF) and radio sondes are used to permanently validate the vertical atmospheric profiles from CHAMP to ensure high data product quality. Between 10 and 35 km the derived refractivity is nearly bias-free in relation to ECMWF e.g. for a set of ~60,000 vertical profiles, measured between March 2002 and 2003. The standard deviation is less than ~1%. This is confirmed by comparisons with ~ 10,000 coincidencing radio sonde measurements. In the middle and lower troposphere a negative refractivity bias (in relation to the analyses and radio sonde data) is observed in the Geometrical Optics (GO) retrievals. A maximum value of 5% is reached in the Tropics in the vicinity of the Earth’s surface. This bias is observed also by other analysis centers and was found to be caused by multipath propagation, receiver tracking errors or signal loss due to critical refraction. It also results in dry-biased water vapour profiles. In order to reduce the bias improved techniques for the data analysis in the middle and lower troposphere are applied (e.g. Canonical Transform sliding spectral, CTss or Full Spectrum Inversion sliding window, FSIsw). These techniques reduce the refractivity bias by a factor of more than 2 and are foreseen for the implementation to the operational processing system. Recent results of the analyses of global tropopause parameters and stratospheric temperatures, derived from 2.5 years of CHAMP measurements, are presented and will be discussed. Special attention is given to the tropical tropopause.