GPS radio occultation with CHAMP : Initial results

Fig. 1. The German CHAMP satellite (view from antivelocity direction with GPS occultation antennas) Abstract: The German CHAMP (Challenging Minisatellite Payload) satellite (launched on July 15, 2000) is a geoscience mission aiming for the determination of Earth’s gravity and magnetic field and for GPS-based atmospheric sounding. The atmospheric profiling experiment onboard CHAMP was started in February 2001 and is considered as an important step to improve the GPS (Global Positioning System) radio occultation technique as a tool for global limb-sounding of Earth’s atmosphere. CHAMP carries the latest generation of Jet Propulsion Lab’s GPS flight receiver and has a more directional occultation antenna compared to the GPS/MET “proof of concept” experiment. By the end of May 2001 more than 7000 occultations were analysed. First statistical comparison of a set of 438 vertical profiles of dry temperature, derived from CHAMP measurements, with corresponding global weather analyses were performed. The observed temperature bias is less than ~1 K above the tropopause and less than 0.5 K in the altitude interval from 12 to 20 km at latitudes >30°N. About 55 % of the compared vertical profiles reached the last kilometer above Earth’s surface. Analysis of GPS radio occultation signal in the lower troposphere applying wave optics techniques indicates contribution of a secondary component, which is interpreted as reflected GPS signal from Earth’s surface.