Precise Real-Time Low Earth Orbiter Navigation With GPS

Willy Bertiger, Bruce Haines, Da Kuang, Michael Lough, Stephen Lichten, Ronald J. Muellerschoen, Yvonne Vigue, and Sien Wu

Abstract:	Technology is currently available to support real-time on-board knowledge of the position of a low earth orbiter at the 5-15 meter level using the civilian broadcast GPS signal with sophisticated models and filtering techniques onboard the spacecraft. Without these techniques, the standard positioning service yields 50-100 meters with the current level of SA. Proposed augmentations and/or enhancements to the GPS system will make RMS accuracies of 10 centimeters to a few decimeters available to the real-time on-board user. Presently, near real-time processing of GPS tracking data can routinely provide low-Earth orbit determination accuracy at the level of 5 cm. Such processing systems can, in fact, be fully automated: recent results from the Jet Propulsion Laboratory (JPL), where ongoing daily processing of low-Earth GPS tracking data has been undertaken for several years, are presented in this paper showing orbit determination accuracies at the sub-10 cm level. At the present time, such solutions can be produced with about 10 hrs delay after real-time, but recent improvements in JPL's processing system will soon enable turn around at the 1-hr level or better for such precise orbit determination. We anticipate that orbit determination at the 1-cm accuracy level will be demonstrated, with some refinements to the current system, in the not too distant future. Continuing enhancements in the automation of data retrieval and precise orbit processing will result in continuing decreases in latency for ground-based generation of precise orbit products for Earth orbiters. Such ephemerides can be propagated ahead slightly to provide real-time knowledge. However, there are advantages to an onboard, real-time orbit determination capability. These include unique mission requirements (military, strategic, scientific), as well as the potential to dramatically lower navigation operations costs through the enabling of a fully autonomous spacecraft. JPL has been actively involved in the development of technology to enable a fully autonomous spacecraft in low-Earth orbit. This paper includes recent results of analysis of actual and simulated GPS data collected in space which demonstrate that a 10-cm (or better) real-time onboard orbit determination capability is presently technologically feasible. In addition to space-based data, present-day tests in real time of Wide Area Differential GPS (WADGPS) on aircraft in real time, show upper bounds for space based users with a global WADGPS at the level of 30 cm RMS horizontal and 60 cm RMS vertical. The paper describes several alternative technology roadmaps which can be followed to make such a capability routinely available to a wide range of low-Earth orbiters. The discussion will include the use of Wide Area approaches as well as non-WADGPS approaches for achieving this capability. In addition to supporting a sub-10 cm real-time onboard positioning capability in Earth orbit, this system could also support a few decimeters real-time kinematic positioning for ground, sea, and air users globally.
Published in:	Proceedings of the 11th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1998) September 15 - 18, 1998 Nashville, TN
Pages:	1927 - 1936
Cite this article:	Bertiger, Willy, Haines, Bruce, Kuang, Da, Lough, Michael, Lichten, Stephen, Muellerschoen, Ronald J., Vigue, Yvonne, Wu, Sien, "Precise Real-Time Low Earth Orbiter Navigation With GPS," Proceedings of the 11th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1998), Nashville, TN, September 1998, pp. 1927-1936.
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