Highlights of Gps Radio Occultation with Champ ( Neutral Atmosphere )

The atmosphere sounding experiment of the German CHAMP (CHAllenging Minisatellite Payload) satellite mission was activated on February 11, 2001. Since then more than 110,000 occultations were recorded as of mid-December 2002. The results of nearly two years CHAMP GPS (Global Positioning System) occultation measurements and data analysis have shown, that significant improvements of the GPS radio occultation technique were reached. CHAMP provides continuously GPS occultation measurements in spite of the activated antispoofing mode of the GPS. The data are analyzed by an operational processing system; the derived vertical atmospheric profiles are provided to the scientific community via the Information System and Data Center (ISDC). Using the operational occultation infrastructure at GFZ near-real-time provision of GPS occultation data and analysis results was demonstrated for the first time. The advantageous consequences for the GPS data processing due to the termination of the Selective Availability (SA) mode of the GPS (possible reduction of the GPS ground station acquisition rate for double differencing and feasibility of single difference processing) were demonstrated with the CHAMP data. Using the radioholographic technique for the data analysis characteristic frequency shifts of direct and reflected GPS signals have been determined in the observations. It was shown that these frequency shifts can be exploited to obtain information on ground elevation at the reflection point and ground-level refractivity.