Approach and Quality Assessment of Single Difference Processing of GPS Radio Occultation Data at the UCAR CDAAC

The COSMIC Data Analysis and Archival Center (CDAAC) at UCAR is currently processing radio occultation (RO) data from the GPS/MET, CHAMP, and SAC-C missions in preparation for the COSMIC RO mission to be launched in March of 2006. The CDAAC will process data from COSMIC in both post-processing (4-6 weeks after observation) and near real time (< 3 hours after observation on average) modes of operation. Currently, the GPS data processing at the CDAAC is based on the Bernese version 5.0 software package and includes: zenith tropospheric delay and site coordinate estimation, high-rate (30-sec) GPS satellite clock estimation, LEO precision orbit determination, and generation of 50 Hz L1 and L2 excess atmospheric phase delays for each LEO occultation event that utilizes a double-difference processing algorithm with 1-sec GPS ground data. An analysis has been performed to evaluate the effectiveness of generating excess atmospheric phase delays with a single-difference processing algorithm. Single-difference processing removes the effects of GPS satellite clock variations on the LEO radio occultation data by utilizing GPS satellite clock estimates instead of GPS ground data. Spectral analyses of high-rate (1-sec) GPS clock estimates have shown negligible signal power at periods less than 30 seconds. This means that a tracking rate of 30 seconds for GPS ground receivers is sufficient for the removal GPS satellite clocks from LEO radio occultation data. Tracking GPS ground data at 30 second instead of 1 second epochs would result in significant reductions of data transfer and disk storage requirements for the COSMIC mission. Comparisons of one month of CHAMP RO refractivity profiles with ECMWF profiles for double-difference and single-difference (with 30-sec GPS clocks) excess phase post-processing show a reduction of greater than 1.2 % in RMS deviation of fractional refractivity at 30 km altitude for single-difference processing. An assessment of single-difference excess phase processing in a near real time mode of operation is currently being performed.