Use of double difference observations in combined orbit solutions for LEO and GPS satellites
The arrival of low Earth orbiting satellites that use GPS for tracking purposes brings the possibility of combined orbit determination processes for these LEO satellites and the full GPS constellation. Such simultaneous solutions have various applications, for instance for enhancing the observability of the GPS orbits, clocks and associated products, or for (near) real-time POD and data validation of the LEO spacecraft. The size of such combined processes is much larger than that of a single POD process either for the GPS constellation without LEO satellites, or for a LEO satellite while holding the GPS orbits and clocks fixed. The main reason for this is the need to use higher data rates in LEO orbit computations, in order to avoid under-sampling of the more variable orbital dynamics. If double differenced GPS measurements are to be used in such solutions, the total number of tracking observations will soon become prohibitive if no special measures are taken. At ESOC’s Navigation Office a new approach to double differenced data processing is being implemented for combined solutions with GPS and LEO satellites. The algorithm enables efficient processing of virtually all double differenced combinations that are geometrically possible between the LEO satellite, a terrestrial ground station, and any two GPS satellites that are in common view. The concepts behind this algorithm will be explained, and it will be shown how such large numbers of double differenced measurements can enhance the stability of the simultaneous estimation process for all involved orbits, clocks and phase ambiguity parameters. Some applications to CHAMP and JASON will be shown.