CHAMP and SAC-C Atmospheric Occultation Results and Intercomparisons

Hajj, George A. and Ao, C. O. and Iijima, B. A. and Kuang, D. and Kursinski, E. R. and Mannucci, A. J. and Meehan, T. K. and Romans, L. J. and de la Torre Juarez, M. and Yunck, T. P. (2002) CHAMP and SAC-C Atmospheric Occultation Results and Intercomparisons. Submitted to Journal of Geophysical Research.

Abstract

The German CHAMP (CHAllenging Minisatellite Payload) and Argentine SAC-C (Satelite de Aplicaciones Cientificas-C) Earth science missions, launched in 2000, carry a new generation of Global Positioning System (GPS) receivers for radio occultation sounding of the ionosphere and neutral atmosphere. Though the occultation concept for obtaining profiles of atmospheric temperature, pressure and moisture was proven in 1995 with GPS/MET, concurrent measurements from CHAMP and SAC-C present the first opportunity to test three central claims: (1) GPS soundings are effectively free of instrumental bias and drift; (2) individual temperature profiles are accurate to <0.5K between ~5-20 km; and (3) averaged profiles from climate studies can be accurate to <0.1K. These properties imply that a weak climate signal can be detected in less than a decade and studied by different instruments at different times with no external calibration. In this paper these claims are attested by comparing nearby CHAMP and SAC-C profiles. Of 60,000 profiles examined, 60 pairs occuring within 30 min and 200 km of one another were found. Profiles pairs agree to <0.5K (68% confidence interval) and to within 0.1K in the mean between 5-20km altitude, after removing the expected variability of the atmosphere. Individual comparisons show closest agreement near the tropopause, and display finer resolution and substantially different temperatures than numerical weather model analyses from the National Center for Environmental Prediction (NCEP) and the European Center for Medium-Weather Forecast (ECMWF). The inferred error on the analyses is 1-1.5K. Comparisons between CHAMP and SAC-C largely indicated precision; however, several features observed in common, especially near the tropopause, tend also to indicate accuracy. Limitations of previous experiments (e.g. GPS/MET) in probing the lower troposphere are largely overcome with CHAMP and SAC-C, with the majority of profiles (60%) descending to the lowest 0.5 km. This is expected to increase to 90-95% with future system improvements. Examples are selected to illustrate lower-troposphere sensing, including detection of the plantary boundary layer top with a vertical resolution of ~100 m. For the first time, such performance is achieved with GPS Antispoofing encryption on. Daily occultations currently number ~350; this is expected to reach over 1000 in the next year, rivaling the number of semi-daily radiosonde launches. With several new missions in planning this may increase ten-fold in the next 3-8 years, making GPS sounding a vital input to numerical weather prediction and climate research.