GPS radio occultation with CHAMP

The GPS (Global Positioning System) radio occultation experiment onboard the German CHAMP (CHAllenging Minisatellite Payload) satellite was activated in February 2001. By the end of 2001 about 36,500 occultations were recorded. We review the occultation data processing at the GFZ and discuss selected first results. 1) CHAMP allows for atmospheric sounding with high accuracy despite of the Anti-Spoofing (A/S) mode of the GPS. 2) There are advantageous consequences for the GPS data processing due to the termination of the Selective Availability (SA) mode of the GPS. It is possible to reduce the GPS ground station acquisition rate for double difference processing. The application of space-based single differencing technique for precise occultation data processing became feasible. 3) The state-of-the-art GPS flight receiver onboard CHAMP combined with high-gain occultation antenna allows for atmospheric sounding deep into the lower troposphere. Possible applications and improvements of occultation data analysis in the lower troposphere are discussed.