In-orbit autonomous position determination of satellites using sparsely distributed GNSS measurements : for geostationary transfer orbits, geostationary earth orbit and higher altitudes
Abstract: The state-of-the-art MosaicGNSS receiver at EADS Astrium is currently astandard product for satellites operating in Low Earth Orbits (LEO). Previousassessments showed that GPS signals taken from the main lobe only result inpoor visibility conditions in Geostationary Orbits (GEO). Including thesidelobes of the GPS satellites increases the number of tracked satellitesover time. This number however is still very low.The aim of this thesis project is to find alternative solutions to improvethe in-orbit autonomous position determination of satellites in GTO, GEO andhigher orbits: through the change of the algorithms of the MosaicGNSSreceiver, in order to deal with sparsely distributed GNSS measurements. Thus,the proposed topic targets the development and implementation of methods forbatch-processing of the acquired signals.The research process started with the development of attitude dynamicscapabilities for the EADS Astrium's Space Environment Simulator, which wastuned and verified as compared to observations made duringhardware-in-the-loop tests using the Spirent RF Simulator. A representativeGTO/GEO mission scenario was created, where analysis of the dynamics andvisibility conditions showed that the mean value of tracked satellites wasfound to be 1 and the maximum 5. In addition, two or more satellites arealways in track during 29.8% of the simulation time. In GEO, there are longperiods 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 canbe tracked.After the choice of a suitable state-of-the-art batch-processing algorithmincluding system models, optimization criterion (Weighted Least Squares) andoptimization approach (Newton-Raphson), the implementation was carried on ina MATLAB environment: and the results in terms of position determinationaccuracy were compared under different configurations with respect to theresults achieved using the state-of-the-art algorithms of the MosaicGNSSreceiver, which features a Kalman Filter.The results show that in LEO, the use of 1000 measurements for the estimationprovided a good performance, and this number can be collected inapproximately 2.7 minutes (~3% of one orbit). For GEO, 4000 measurementsprovided a good performance, and this number can be collected from anobservation period that ranges from 45 minutes (~3%) to 3 hours (~12.5%). Inthis case, the batch-processing achieved an accuracy of 11.5 m with 1 sigmavalue of 7.4 m, in contrast to 45 m with 1 sigma value of 35.8 m achieved bythe Navigation Module of the MosaicGNSS receiver. Moreover, it is concludedthat at least 2 satellites in different positions should be in track duringthe observation period in order to achieve a fit of an orbital arc duringsuch observation period, where the second satellite not necessarily has to beavailable during the whole time of the observation period.The thesis extends an ESA project assessing the feasibility of GNSS receiversin GEO and higher altitudes, and supports the activities of EADS Astrium'son-going programs. In the future, the results of this research are expectedto be introduced as well in the MosaicGNSS receiver as in EADS Astrium's nextgeneration multi-frequency/multi-constellation receiver, the LION Navigator.
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