In-orbit autonomous position determination of satellites using sparsely distributed GNSS measurements : for geostationary transfer orbits, geostationary earth orbit and higher altitudes

The state-of-the-art MosaicGNSS receiver at EADS Astrium is currently a standard product for satellites operating in Low Earth Orbits (LEO). Previous assessments showed that GPS signals taken from the main lobe only result in poor visibility conditions in Geostationary Orbits (GEO). Including the sidelobes of the GPS satellites increases the number of tracked satellites over time. This number however is still very low. The aim of this thesis project is to find alternative solutions to improve the in-orbit autonomous position determination of satellites in GTO, GEO and higher orbits: through the change of the algorithms of the MosaicGNSS receiver, in order to deal with sparsely distributed GNSS measurements. Thus, the proposed topic targets the development and implementation of methods for batch-processing of the acquired signals. The research process started with the development of attitude dynamics capabilities for the EADS Astrium's Space Environment Simulator, which was tuned and verified as compared to observations made during hardware-in-the-loop tests using the Spirent RF Simulator. A representative GTO/GEO mission scenario was created, where analysis of the dynamics and visibility conditions showed that the mean value of tracked satellites was found to be 1 and the maximum 5. In addition, two or more satellites are always in track during 29.8% of the simulation time. In GEO, there are long periods that can last for almost two hours where no satellite can be tracked, and only small periods of some tens of minutes where up to 4 satellites can be tracked. After the choice of a suitable state-of-the-art batch-processing algorithm including system models, optimization criterion (Weighted Least Squares) and optimization approach (Newton-Raphson), the implementation was carried on in a MATLAB environment: and the results in terms of position determination accuracy were compared under different configurations with respect to the results achieved using the state-of-the-art algorithms of the MosaicGNSS receiver, which features a Kalman Filter. The results show that in LEO, the use of 1000 measurements for the estimation provided a good performance, and this number can be collected in approximately 2.7 minutes (~3% of one orbit). For GEO, 4000 measurements provided a good performance, and this number can be collected from an observation period that ranges from 45 minutes (~3%) to 3 hours (~12.5%). In this case, the batch-processing achieved an accuracy of 11.5 m with 1 sigma value of 7.4 m, in contrast to 45 m with 1 sigma value of 35.8 m achieved by the Navigation Module of the MosaicGNSS receiver. Moreover, it is concluded that at least 2 satellites in different positions should be in track during the observation period in order to achieve a fit of an orbital arc during such observation period, where the second satellite not necessarily has to be available during the whole time of the observation period. The thesis extends an ESA project assessing the feasibility of GNSS receivers in GEO and higher altitudes, and supports the activities of EADS Astrium's on-going programs. In the future, the results of this research are expected to be introduced as well in the MosaicGNSS receiver as in EADS Astrium's next generation multi-frequency/multi-constellation receiver, the LION Navigator.Validerat; 20101217 (root