Precise orbit determination for GPS/MET using GPS-SST data

The use of the Satellite-to-Satellite Tracking (SST) between the Global Positioning System (GPS) satellites and a low-Earth satellite for precise orbit determination (POD) has been successfully tested on TOPEX/Poseidon. Recently, this tracking system is used or planned to use in many space missions due to its low cost and high accuracy. GPS/MET (Global Positioning System Meteorology) is an example. The objective of the GPS/MET is to use signals from GPS satellites occulted by the Earth to demonstrate the active limb sounding of the atmosphere. To satisfy this objective as well as other applications (e.g. gravity field improvement), the precise orbit of the GPS/MET is necessary. We have processed the GPS data from the flight receiver on the GPS/MET using the EPOS software, developed at GFZ (GeoForschungsZentrm) Potsdam. The undifferenced pseudo-range and phase observations are used. The orbit accuracy is assessed using a number of tests. These include external comparisons and analysis of data residuals, formal errors and orbit overlaps. In addition, we have estimated empirical acceleration parameters, in order to accommodate unmodeled and/or mismodeled forces in the dynamic orbit determination. This is useful not only for improvement of the orbital fits, but also of the orbit accuracy.