ALBERT

Description: The German geoscience satellite CHAMP (CHAllenging Mini-satellite Payload) was successfully launched in mid-2000. Equipped with a GPS (Global Positioning System) receiver provided by the Jet Propulsion Laboratory, CHAMP exploits the signals of the global configuration of GPS satellites for the remote sounding of temperature and humidity in the atmosphere. This innovative remote sensing technique provides globally distributed, calibration-free and weather independent measurements with high vertical resolution. In comparison to the earlier US-American proof-of-concept mission GPS/MET the CHAMP occultation experiment exhibits improved instrument characteristics and aims at quasi-continuous operation. Within the first year of the CHAMP radio occultation experiment almost 58,000 occultation events have been observed. Between 70 to 80% of these were successfully processed to yield atmospheric temperature and humidity profiles. These first measurements indicate that in spite of the activated anti-spoofing GPS mode the state-of-the-art flight receiver allows for global atmosphere sounding with high accuracy. Improvements in signal tracking algorithms result in more than 50% of the occultation events reaching the first kilometer above the ground. At GeoForschungsZentrum (GFZ) Potsdam an operational system has been established to process radio occultation data, satellite orbit data and GPS ground station observations in a quasi-operational manner and to derive atmospheric temperature and humidity profiles. Apart from the standard geometrical optics analysis algorithm advanced wave optical retrieval techniques, such as the backpropagation and canonical transform method, have been implemented. Using variational techniques the derivation of water vapor profiles in the lower troposphere is significantly improved. The observed temperature bias with respect to ECMWF global analyses is less than 1 K above the tropopause and less than 0.5 K between 12 to 20 km at mid and high latitudes. A negative refractivity bias observed at mid and low latitudes is at the focus of current research. Owing to the termination of the selective availability GPS mode in May 2000 a single differencing analysis technique without relying on GPS ground station data could be performed. The results are in excellent agreement with profiles derived by the standard double differencing technique. Finally, radio holographic analyses of the occultation data reveal that contributions from signals reflected at Earth's surface are present in about 20–30% of the data set.