Attitude Determination For Space Transfer Vehicles Using GPS

Kevin D. Keierleber and Stanley C. Maki

Abstract:	During prolonged periods of operation a Space Transfer Vehicles (STV) inertial measurement system may build up excessive attitude errors. Using GPS the STV attitude can be updated in order to meet mission requirements. Four different methods of attitude determination were considered; Thrust vector determination, Differential and GPS, Phase time triple difference, and carrier phase attitude determination. Due to close proximity with the space station, antenna separation limitations on an STV, and the lengthy integration times for the Phase time triple difference solution, the carrier phase attitude determination method was selected. Using the Centaur upper stage fro teh STV configuration and a carrier phase measurement accuracy of .2cm the paper shows that the initial misalignment errors using the phase time triple difference solution would be tolerable for most mission functions. By integrating GPS with a good quality Inertial Navigation System (INS) the attitude determination process can be imporved, while at the same time the INS can assist the GPS receiver carrier trackign loops in maintaining carrier phase lock. Attitude determination for the STV using the carrier phase attitude determination method requires the availiability of at least four satellites. A geometrical approach is presented for selecting a good satellite geometry. In summary, the procedure is to select a key satellite that has the highest elevation angle. Then select satellites #2 and #3 at low elevation angles on opposite sides of the antenna baseline, and finally select satellite #4 away from the other three but preferably in a phase more perpendicular to the baseline than a plane containing teh baseline. The goal is to select satellites with a good geometric distribution so as to proivde good antenna baseline azimuth'elevation sensitivities, and good distribution of the unit vector differences (GDOP). A descriptio of how to mathematically callculate the azimuth/elevation sensitivities for a group of satellites once the key satellite has been selected is given. These sensitivities and the GDOP are then inserted into a composite rankintg equation to determine the best group of four satellites. The attitude is then calculated from these four satellites by adapting carrier phase position determination algorithms that have been developed by Magnavox.
Published in:	Proceedings of the 1991 National Technical Meeting of The Institute of Navigation January 22 - 24, 1991 Sheraton San Marcos Hotel Phoenix, AZ
Pages:	85 - 101
Cite this article:	Keierleber, Kevin D., Maki, Stanley C., "Attitude Determination For Space Transfer Vehicles Using GPS," Proceedings of the 1991 National Technical Meeting of The Institute of Navigation, Phoenix, AZ, January 1991, pp. 85-101.
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