GPS ground station data for CHAMP radio occultation measurements

The role of GPS ground station data for the CHAMP Atmospheric Profiling experiment is discussed. The profiling activities at GeoForschungZentrum Potsdam (GFZ) are briefly described and first results of the occultation processing system are presented. The atmospheric excess phase of the occultation link between the CHAMP and the GPS satellite is derived by a double difference method using ground station data. The influence of ground station data on the accuracy of the derived atmospheric profiles is discussed. A relationship between the temperature errors at several heights resulting from errors in atmospheric excess phase is given, e.g. an error of 1mm/s in the time derivative of the excess phase results in a temperature error of 1 K at 30 km. Furthermore, a study was performed to estimate the required data rate of the ground station measurements. In particular the influence of the Selective Availability (SA) termination on May 2, 2000 is emphasized. We conclude that the acquisition rate can be reduced from 1 Hz to 0.1 Hz without loss of information. Simulations of the CHAMP occultation experiment show that the joint Jet Propulsion Laboratory, Pasadena (JPL) & GFZ “high rate & low latency” network enables the processing of globally distributed occultation events with a highly redundant number of ground stations. Latency aspects are studied in view of the future task of providing atmospheric occultation data for operational weather forecast.