Rapid Science Orbits for CHAMP and GRACE Radio Occultation Data Analysis

The Rapid Science Orbits (RSOs) for the GPS constellation and for Low Earth Orbiting (LEO) satellites (namely CHAMP, GRACE A/B, SAC-C, TerraSAR-X) are generated with very high reliability on a daily basis with 1 day latency to support radio occultation measurements and other on-board experiments. The RSOs are dynamic orbits computed in a two step approach. Separately estimated GPS orbits and clocks are fixed and used thenafter in the LEO orbit determination process. The quality of the GPS orbits is monitored by comparison to the IGS rapid orbit products as well as by orbital fits of Satellite Laser Ranging (SLR) measurements to GPS-5 and -6. Results of the validation allow to conclude, that in results of recent modelling improvements the accuracy of the GPS orbits is 5 cm in all directions (radial, cross- and along-track). The accuracy of the LEO orbits monitored by SLR amounts to 4–5 cm. There exists a 3–4 cm negative bias in the SLR residuals for the GPS orbits, for most of the LEOs considered here the negative bias is on 1 cm level. The origin of the bias is unclear. The Rapid Science Orbits are publicly available also for other scientific applications.