Autonomous Navigation for COMET with GPS

James W. Chaffee, Khalil Zare, Joseph Pojman

Abstract:	An algorithm for completely autonomous navigation of the low earth orbit (LEO) COMET spacecraft using C/A code GPS is described. The algorithm is based on a Kal- man filter with dynamics linearized about a nominal ref- erence which accounts for geopotential terms and drag and is initialized on orbit. The measurements are proc- essed in a two-stage approach which uses point estimates of position and bias based on pseudoranges from all satel- lites in view. Propagation about the nominal orbit is done in closed form using an extension of the classical method of Clohessy-Wiltshire. This method, due to K. Zare, is good for elliptic orbits of any eccentricity. The algorithm was tested using simulated data from a model with higher fidelity than the filter model. Noise sources included re- sidual ionospheric error, user clock, receiver noise and SA. The COMET mission required a particularly accurate es- timate of a de-orbit vector. With SA errors and no differ- ential corrections it was considered necessary to employ smoothing to obtain the de-orbit vector. However, the fil- ter provided sufficient accuracy in real-time to accom- plish the mission. Though the algorithm was designed for a specific mis- sion, it is sufficiently general to work with any LEO sat- ellite. Moreover, with the use of smoothing the method will lend itself to more complex on-orbit maneuvers than de-orbit, including station-keeping and docking.
Published in:	Proceedings of the 6th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1993) September 22 - 24, 1993 Salt Palace Convention Center Salt Lake City, UT
Pages:	293 - 300
Cite this article:	Chaffee, James W., Zare, Khalil, Pojman, Joseph, "Autonomous Navigation for COMET with GPS," Proceedings of the 6th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1993), Salt Lake City, UT, September 1993, pp. 293-300.
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