GPS clock correction estimation for near real-time orbit determination applications

Precise orbit determination for low Earth orbiting (LEO) satellites using the global positioning system (GPS) is usually done in a post-processing mode introducing GPS satellite orbits and clock corrections. Depending on the mission objectives, the latency and required accuracy of precise LEO orbit information can vary significantly. Near real-time (NRT) orbits for certain geodetic missions (e.g. atmospheric sounding or altimetry) may require accuracy down to 10 cm (radial) or 0.1 mm s(-1) (along-track) and with a latency of less than 2-3 hours after the GPS measurements have been taken on the satellite. In order to fulfill such a demand in NRT, it is crucial that GPS orbit and clock products are available within the desired time frame and with adequate accuracy. In response to the demanding NRT processing for future geodetic satellite missions (i.e. ESA's Sentinel series), the Astronomical Institute of the University of Bern (AIUB) and Deutsches Zentrum fur Luft- and Raumfahrt (DLR) have made an effort in finding an alternative approach to obtain high-rate NRT GPS orbit and clock solutions by way of GPS clock corrections estimation. A realistic NRT orbit determination (OD) scenario has been simulated in this joint-study assuming a radial orbit accuracy of 10 cm and of 3 hours latency for the orbit product. A summary of all currently available NRT GPS orbit and clock products and a description of the generation of NRT GPS clock corrections are given in detail in this paper. The estimated GPS clock corrections are validated with a kinematic precise point positioning (PPP) for static ground stations. For direct evaluation of this innovative approach on the LEO orbits, the resultant merged high-rate NRT GPS product was tested using GPS data from the GRACE (Gravity Recover And Climate Experiment) mission. The results from the NRT simulation show promising aspect of such a concept to achieve unprecedented LEO orbit accuracy in NRT. (C) 2009 Elsevier Masson SAS. All rights reserved.