Precise GPS data processing for the GPS/MET radio occultation mission at UCAR

On 3 April 1995 the MicroLab-1 satellite was launched into a 750 km and 70 degree inclination orbit carrying the GPS/Meteorology (GPS/MET) receiver payload. The mission was a proof-of-concept experiment for sensing the Earth’s atmosphere using the GPS limb sounding or radio occultation method. During an occultation, the GPS carrier phase signals received on-orbit are bent and delayed as they graze the Earth’s ionosphere and atmosphere. This delay and bending can be inverted with an Abel integral inversion to obtain accurate profiles of index of refraction of the ionosphere and atmosphere with high vertical resolution. To obtain accurate Abel inversions, it is necessary to precisely measure the L1 and L2 phase delay on the occulting link that is due to the atmospheric delay and bending, a quantity termed excess atmospheric phase delay. This requires precise GPS data processing software with low earth orbit (LEO) determination capability. The University Corporation for Atmospheric Research (UCAR) has developed an automated software system to process ground and space based GPS observations into atmospheric and ionospheric profile data. The orbit determination and excess phase processing are done with MicroCosmÒ , a state-of-the-art satellite orbit and geodetic parameter estimation software package developed by Van Martin Systems, Inc. This paper discusses content and precision of the GPS/MET orbit determination and excess phase processing done at UCAR. The GPS/MET orbit estimation is performed with GPS triple difference measurements (defined as a simple difference of time consecutive double difference measurements) which greatly reduces the processing time and memory requirements over double difference processing. Orbit overlaps between adjacent days suggest that the average GPS/MET LEO position precision is ∼ 5 cm radial root mean square (rms) and ∼ 15 cm 3D rms and the average velocity precision is ∼ 0.15 mm/sec 3D rms. Orbit overlaps with external orbits computed at GFZ show agreement at the 30 cm 3D rms level. Currently, the limiting error source in the excess phase data on the occulting link is not due to LEO orbit error, but due to incomplete correction of small scale ionospheric inhomogeneities which degrade the accuracy of the atmospheric profiles at high altitudes.