GPS radio occultation with CHAMP: First results and status of the experiment

The GPS (Global Positioning System) radio occultation experiment onboard the German CHAMP (CHAllenging Microsatellite payload) satellite was successfully started on February 11, 2001. By the end of July 2001 about 16,000 occultations were recorded. More than 80% of the measurements could be processed to derive vertical profiles of atmospheric temperature and specific humidity. Throughout the measurement period GPS anti-spoofing mode was enabled. On average, about 230 globally distributed occultations per day were recorded. Their distribution shows a characteristic latitudinal dependence with maximum occultation density at polar regions and minimum density in the equatorial region. A statistical comparison of dry temperature profiles, derived from CHAMP measurements, and corresponding meteorological analyses shows excellent agreement between 8 and 30 km altitude. For the Northern hemisphere the mean deviation is <0.5 K with a standard deviation of <1 K between 12 and 25 km. A single difference technique was applied to the data analysis. The temperature difference for an individual profile compared to the corresponding data, retrieved by double differencing, is <0.1 K below 20 km and <0.5 K below 35 km altitude.