A GNSS-based Attitude Determination System for Low-Earth Observation Satellites

Pierluigi Silvestrin, Jesus Serrano, Pelayo Bernedo, and Pablo Gonzalez

Abstract:	This paper presents the design of an attitude determination concept based on differential carrier phase measurements. The results of the investigations are illustrated with the example of an application to MetOp, the European Polar Meteorological Satellite, developed by ESA for EUMETSAT. Both nominal and safe modes of operation of the attitude control system are addressed. The paper analyses the drivers of the concept and possible differential multipath mitigation techniques and presents a detailed analysis of the selected technique based on a Kalman filter with solved-for differential multipath parameters. This is compared to the classical scheme in which no estimation of the differential multipath parameters is done. For the safe mode, a suitable approach using the attitude point solution is developed. The null space method (NSM) is selected for solving the integer ambiguity resolution problem (OFAR). Improvements of such a method for speeding up the algorithm are presented. A performance analysis is presented for the three concepts under investigation. The performances of the concept with differential multipath estimation is assessed through a sensitivity analysis with respect to the multipath characteristics, angular rate disturbances, receiver, antenna characteristics and filter update frequency. Furthermore, different configuration of antennas are analyzed. The performance of the selected OFAR technique is assessed in terms of convergence, CPU time, and sensitivity to the initial attitude knowledge error, initial angular velocity, receiver characteristics and multipath effects. Finally, the performance for the safe mode is presented, including the sensitivity to the initial attitude which is the most critical parameter. The study shows that the MetOp nominal pointing mode accuracy requirements can be fulfilled using a Kalman filter which estimates also the differential multipath. The preferred configuration has four antennae located in the vertices of a 0.5 m square. This allows to mount the antennas in a dedicated carbon fibre reinforced structure in order to improve structural stability. The use of narrow FOV antennas is proposed to minimise differential multipath effects. For the OFAR technique, the study shows that the NSM algorithm is very robust giving no failures, even without any attitude knowledge. CPU time for the OFAR depends strongly on the number of satellites considered: starting the computation with a small set of satellites leads to important time savings. For the safe mode, the study shows that a baseline length of 0.2 m is preferable in order to avoid the complexity of OFAR techniques: this would provide a worst case performance of 8”. On the other hand and for an antenna baseline of 0.5 m, the attitude determination accuracy is always better than 3”.
Published in:	Proceedings of the 8th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1995) September 12 - 15, 1995 Palm Springs, CA
Pages:	1775 - 1784
Cite this article:	Silvestrin, Pierluigi, Serrano, Jesus, Bernedo, Pelayo, Gonzalez, Pablo, "A GNSS-based Attitude Determination System for Low-Earth Observation Satellites," Proceedings of the 8th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1995), Palm Springs, CA, September 1995, pp. 1775-1784.
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