ESA GNC Conference Papers Repository

GPS is used successfully for absolute and relative navigation of satellites in Low Earth Orbits. For GEO satellites, GPS has the potential to provide accurate satellite position onboard in real-time and to improve user satellites autonomy in general. GNSS receivers onboard of GEO satellites could simplify satellite operation and reduce complexity of ground infrastructure (tracking antennas) The main innovations of radio-navigation constellations in the next years are: • Increase of the available number of constellations with common features such as frequency bands, BPSK and BOC modulations, • GPS modernization and the introduction of new navigation signals and formats in GPS and Galileo (BOC modulation techniques, introduction of pilot signals, higher chipping and data rates etc.). The main characteristics of the next generation GNSS (GPS III/Galileo) are • Higher Signal-In-Space (SIS) availability through higher number of constellation satellites with larger satellite antenna half beam angles than the ones presented by GPS L1 C/A, • Higher signal power, • Improved User-Range-Error (URE) The uncertainty of using GNSS in GEO altitude has been eliminated by the “GPS Operational Requirements Document (ORD 18th February 2002) that guarantees availability of GPS signals up to approximately geosynchronous altitude (“Space Service Volume, SSV”, which extends from 3000 km to geosynchronous altitude). In the SSV, constellation performance for the GPS signals L1, L2C and L5, is described and specified by the following three parameters: • User Range Error (URE), • Received Power and • Signal availability (determined by half-beam width and number of satellites in constellation. This paper describes the results of an ESA study on the feasibility of using GPS and Galileo in GEO, GTO and SSTO. The suitability of constellations (GPS, Galileo) and signals (L1, L5, and E1, E5a) for navigation of spacecraft in the SSV will be evaluated and compared. Performance during and after station keeping manoeuvres (electrical and chemical thrusters) is included. Important constraints and limitations on performance from user satellite orbit and orientation are discussed, the dependency of performance on the transmit antennas’ gain pattern is demonstrated.