Autonomous Deterministic Spacecraft Positions Using the Magnetic Field and Attitude Information

Eleanor Ketchum

Abstract:	Smaller smarter spacecraft are becoming more the norm than the exception, particularly as budgets for all space platforms continue to drop. As a result, autonomous spacecraft position determination is evolving from a dream to a requirement. This is particularly true for the SMall Explorer (SMEX) Program at the Goddard Space Flight Center, which is currently planning its future missions, so called SMEX lite, to be as autonomous as possible to reduce overall program costs. The Global Positioning System provides a free signal for very accurate position finding, but the small spacecraft user ends up paying dearly in the end for qualified equipment. Also, many low earth orbiting spacecraft, particularly stellar and solar observers, need position information to no better than the 50 - 100 km required for communications with ground stations. Further, even those spacecraft with fine positioning needs using GPS could benefit from a real-time coarse position estimate to reduce their time to first fix. It would consequently be useful to be able to determine position onboard the spacecraft with inexpensive, readily available, space qualified hardware, even if the solution is only accurate to 100 km. Two groups have successfully determined spacecraft orbits by filtering magnetic field measurements. While both bodies of work are interesting and prove the observability of the problem, the fact that the filters take several orbits to converge and require large amounts of computer memory render these efforts, impractical for the same small spacecraft they are presumably intended to serve. This paper presents a technique for solving for spacecraft position deterministically using attitude and magnetometer data. The computer memory and speed required to solve for and correct the point solutions are presented. The paper also discusses results achieved using Upper Atmospheric Research Satellite flight data; aside from singularities which exist at the magnetic equator, the spacecraft position is determined using this method on average to better than 20 km.
Published in:	Proceedings of the 1996 National Technical Meeting of The Institute of Navigation January 22 - 24, 1996 Loews Santa Monica Hotel Santa Monica, CA
Pages:	517 - 526
Cite this article:	Ketchum, Eleanor, "Autonomous Deterministic Spacecraft Positions Using the Magnetic Field and Attitude Information," Proceedings of the 1996 National Technical Meeting of The Institute of Navigation, Santa Monica, CA, January 1996, pp. 517-526.
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