Volume 82 (2004) Issue 1B Pages 381-395
The operational data analysis of the GPS radio occultation experiment aboard the German CHAMP (CHAllenging Minisatellite Payload) satellite mission is described. Continuous Near-Real-Time processing with average time delay of ∼5 hours between measurement and provision of analysis results is demonstrated. A delay of less than 3 hours is reached for individual events. This is made possible by using an operationally operated ground infrastructure, consisting of a polar downlink station, a globally distributed fiducial GPS ground network, a precise orbit determination facility, an automated occultation processing system and an advanced data center (the Information System and Data Center at GFZ, ISDC). The infrastructure was installed within the CHAMP and the German GPS Atmosphere Sounding Project (GASP). More than 120,000 globally distributed occultation measurements were automatically analysed during 2001 and 2002. A set of∼46,000 vertical profiles of refractivity, temperature and water vapor is validated with meteorological analyses from the European Centre for Medium-Range Weather Forecasts (ECMWF) and data from the global radiosonde network. The mean temperature bias in relation to the analyses is less then 0.4 K between 10 and 35 km, the mean deviation of the refractivity is <0.5%. A height dependent standard deviation of ∼1 K at 10 km and ∼2 K at 30 km is observed. This result is confirmed by comparing ∼6,000 CHAMP occultations with corresponding radiosonde measurements. A negative bias of the refractivity in relation to the analyses up to ∼5% in the Tropics is found in the lower troposphere. It corresponds to mean meridional dry biases of the specific humidity up to ∼30%. It is shown, that the application of a heuristic retrieval method, based on the Canonical Transform method and the sliding spectral approach, reduces the refractivity bias on average by a factor of ∼2. The corresponding bias in the specific humidity is reduced by a factor of ∼3. In mid-latitudes almost no more refractivity bias out of the planetary boundary layer is observed. This is shown by a comparison of CHAMP refractivity and water vapor profiles with radiosonde data. More than 50,000 globally distributed electron density profiles were automatically derived during 2001/2002. A validation study including 1,004 comparisons with corresponding ionosonde data yields a bias of 0.18 MHz and 13.4 km for foF2 and hmF2 respectively. The standard deviation is 1.28 MHz (foF2) and 46.8 km (hmF2).