Attitude Estimation Using GPS

J. Rath, P. Ward

Abstract: Signals from quasars have been used in very long baseline interferometry experiments to study minute changes in the length and orientation of baselines on the earth's surface. In recent years, interferometric methods based on signals originating from the Global Positioning System (GPS) satellites have been used to determine length of baselines to better than one-part-per-million accuracy. Observations central to these experiments are measurements of carrier doppler phase differences between signals originating from the GPS satellites and arriving at the antenna phase centers that define the baseline. With advanced digital receiver and associated software, the accuracy of the phase measurements to within a few degrees have been achievable. An extension of these techniques for the very short baseline between two rigidly mounted GPS antennas results in the determination of the pointing vector between two antenna phase centers. Using three rigidly mounted antennas and the determination of two nearly orthogonal pointing vectors result in the measurement of the attitude of the plane that intercepts the three antenna phase centers. This paper describes a simulation study to establish the validity of an algorithm that uses GPS carrier doppler phase measurements for determining the attitude in three dimensions (heading, pitch, and roll) of a space platform that also is undergoing attitude variation. The space platform was taken to revolve in a circular orbit appropriate for the Space Station and Shuttle. The attitude variation was taken to be periodic. Within each period, the attitude variation had a linear and a quadratic dependence on time and changed direction after achieving the preimposed maximum or minimum value. Carrier doppler phase measurements from a set of three antennas located on the platform to all GPS satellites in a 6 X 3 configuration were generated. The basic measurements used in this study were carrier doppler phase. Sources of measurement errors were identified. It was recognized that the multipath effects are potentially the dominant source of error in carrier doppler phase measurements. A simple model for the multi path phase errors was developed and ways to reduce the effects were studied. Preliminary 1-sigma error in heading, pitch, and roll using a 5-meter antenna separation was 0.015 degree, which compares favorably with state-of-the art determination instrumentation currently used for space applications.
Published in: Proceedings of the 1989 National Technical Meeting of The Institute of Navigation
January 23 - 26, 1989
San Mateo, CA
Pages: 169 - 178
Cite this article: Rath, J., Ward, P., "Attitude Estimation Using GPS," Proceedings of the 1989 National Technical Meeting of The Institute of Navigation, San Mateo, CA, January 1989, pp. 169-178.
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