In-Orbit Results From a Space-borne GPS Attitude Experiment

Stuart M. Duncan, M. S. Hodgart,Martin J. Unwin and Richard Hebden

Abstract:	This paper describes the current progress on the continuing development of a GPS receiver for use as an attitude sensor on small satellite missions in Low Earth Orbit. The SGR-20 Space GPS Receiver has flight heritage as an attitude sensor having flown on a number of missions since 1999. Previously published work [1], [2] demonstrated the feasibility of using the SGR-20 as an attitude sensor. The work presented here builds upon the previously published results by improving the robustness of the integer ambiguity resolution using a new algorithm developed by the authors. The algorithm we have developed utilises a least squares model augmented by statistical checks based on the geometry of the antennas and the noise environment experienced during our in-orbit experiments. It reliably provides a validated integer ambiguity solution using only five double-difference measurements and a singlefrequency per baseline. From our preliminary testing using both simulations and in-orbit data the correct integer ambiguity solution is uniquely identified in around 86% percent of epochs. The presence of multipath and/or antenna phase centre variation has minimal effect on the success rate. We give examples of the noise environment experienced by the SGR-20 on our microsatellites, and a discussion on the possible sources of error. Some results from simulation then show the expected effect of the different error sources on our new integer ambiguity algorithm. Finally, using the results from two of our in-orbit experiments conducted on the TopSat microsatellite we give an indication of the attitude performance achievable, using simply a point solution for the attitude, given the demonstrated microsatellite noise environment.
Published in:	Proceedings of the 20th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2007) September 25 - 28, 2007 Fort Worth Convention Center Fort Worth, TX
Pages:	2412 - 2423
Cite this article:	Duncan, Stuart M., Hodgart, M. S., Unwin, Martin J., Hebden, Richard, "In-Orbit Results From a Space-borne GPS Attitude Experiment," Proceedings of the 20th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2007), Fort Worth, TX, September 2007, pp. 2412-2423.
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