Absolute and Relative Orbit Determination for Satellite Constellations

Precise absolute and relative orbit determination, referred to as Precise Orbit Determination (POD) and Precise Baseline Determination (PBD), are a prerequisite for the success of many Low Earth Orbit (LEO) satellite missions. With the spaceborne, high-quality, multi-channel, dual-frequency Global Positioning System (GPS) receivers, typically a precision of the order of a few cm is possible for single-satellite POD, and of a few mm for dual-satellite PBD of formation flying spacecraft with baselines up to hundreds of km. The research in this dissertation addresses and expands methods for computing reliable orbits for not only stable satellite formations such as the US/German GRACE (Gravity Recovery And Climate Experiment) and lower pair of the European Space Agency (ESA) Swarm missions, but also for satellite constellations that include rapidly varying baselines, such as all three Swarm satellites or the combination of the German CHAllenging Minisatellite Payload (CHAMP) and GRACE missions. The POD and PBD solutions are based on an Iterative Extended Kalman Filter (IEKF) that is capable of using relative spacecraft dynamics constraints for enhancing the robustness of the solutions. Moreover, the IEKF allows to iteratively fix the Double-Differenced (DD) carrier-phase integer ambiguities by the Least-squares AMBiguity Decorrelation Adjustment (LAMBDA) method. A subset fixing strategy allowing for partial ambiguity resolution was used instead of the full-set fixing which only accepts ambiguities when all integer ambiguities were fixed for certain epochs. The nominal products of the IEKF are reduceddynamic POD and PBD solutions, but also include the possibility to derive kinematic PBD solutions afterwards. The internal consistency of the reduced-dynamic and kinematic solutions is used as a quality measure in addition to comparisons with POD and PBD solutions by other institutes.Astrodynamics & Space Mission