GPS radio occultation with CHAMP and GRACE: A first look at a new and promising satellite configuration for global atmospheric sounding

CHAMP (CHAllenging Minisatellite Payload) and GRACE (Gravity Recovery And Climate Experiment) formed a satellite configuration for precise atmospheric sounding during the first activation of the GPS (Global Positioning System) radio occultation experiment aboard GRACE on 28 and 29 July 2004. 338 occultations were recorded aboard both satellites, providing globally distributed vertical profiles of refractivity, temperature and specific humidity. The combined set of CHAMP and GRACE profiles shows excellent agreement with meteorological analysis. Almost no refractivity bias is observed between 5 and 30km, the standard deviation is between 1 and 2% within this altitude interval. The GRACE satellite clock stability is significantly improved in comparison with CHAMP. This allows for the application of a zero difference technique for precise analysis of the GRACE occultation data.