Precise Orbit Determination of Low Earth Orbiters with GPS Point Positioning

Precise orbit determination (POD) of low earth orbiters (LEOs) with GPS is becoming a standard practice in the space science community. The need for such information has been growing rapidly due to such scientific applications as radio occultation and ever increasing demands from engineering applications such as spacebased earth sensor positioning. The conventional GPSbased POD strategies rely on data from a network of terrestrial GPS receivers as well as the spaceborne receiver. A complex, lengthy estimation procedure is carried out integrating the GPS data with high-fidelity dynamic models for the LEO. These strategies rely greatly on the GPS measurement strength, especially for low altitude spacecraft. A completely geometric approach based on a kinematic, sequential least-squares filter/smoother has been devised by the authors which does not use dynamic models, but only data from the LEO’s GPS receiver and the International GPS Service (IGS) GPS constellation precise ephemeris and clock data products. Since this approach makes no assumptions regarding receiver motion, it is platform independent. Preliminary static, terrestrial testing with nearly complete modelling of all associated error sources indicates that few decimetre position component r.m.s and few centimetre averaged position component bias are attainable. Initial spaceborne data testing produced sub-metre total displacement r.m.s. This result was however severely weakened by a low LEO receiver data rate. A number of processing and modelling enhancements will be introduced to refine this technique to allow for potential decimetre-level position component precision.