GPS radio occultation with CHAMP Jens Wickert, Georg Beyerle, Torsten Schmidt, Christian Marquardt, Rolf K¨onig, Ludwig Grunwaldt, and Christoph Reigber GeoForschungsZentrum Potsdam (GFZ), Division 1, Kinematics & Dynamics of the Earth, Potsdam, Germany, wickert@gfz-potsdam.de Abstract. 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 se- lected 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 feasi- ble. 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. Key words: CHAMP, Remote sensing of Earth’s atmosphere, GPS radio occulta- tion 1 Introduction The CHAMP GPS radio occultation experiment is an important step towards establishing an innovative remote sensing method for sounding of the Earth’s atmosphere and ionosphere. The GPS radio occultation technique is based on precise dual-frequency phase measurements (L-band) of a GPS receiver in a Low-Earth-Orbit track- ing a setting or rising GPS satellite. Combining these measurements with the satellites’ position and velocity information the small phase path increase due to the atmosphere during the occultation event can be derived. It is converted to atmospheric bending angles. Assuming spherical symmetric at- mosphere vertical profiles of refractive index can be determined and converted to atmospheric parameters as pressure, temperature and, using independent knowledge of temperature, also to water vapor within the lower troposphere. The main advantages of the calibration-free GPS occultation technique are global coverage, high vertical resolution and all-weather-capability com- bined with high accuracy. These properties allow various applications in at- mospheric/ionospheric research, weather forecast and climate change detec-