Architecture and performance testing of a software GPS receiver for space-based applications

Space-based GPS technology presents significant challenges over Earth-based systems. These include visibility issues for rotating platforms and tracking of GPS satellites from spacecraft that are in higher orbits than the GPS, realtime resolution of carrier phase ambiguities, and different dynamics during various mission phases. NAVSYS has developed a software GPS receiver that makes use of 3-dimensional digital beam steering technology and inertial aiding to address these issues. This approach offers several advantages including all round visibility for spinning satellites, tracking of weak GPS signals, reduction of multipath, and reprogrammability to accommodate different mission phases. Additionally, a suite of simulation tools based on the NAVSYS Matlab Toolbox and Advanced GPS Hybrid simulation products have been built to allow testing for simulated space environments. The receiver architecture and test tools are described in this paper.