A GPS Formation Flying Testbed for theModeling and Simulation of Multiple Spacecraft

Richard J. DeBolt, Patrick A. Stadter, Mark S. Asher

Abstract:	The use of multiple, spatially distributed spacecraft coordinated to achieve common mission objectives has been identified as an important technology for future space missions. Commonly referred to as formation flying, this distributed spacecraft system concept has the potential to revolutionize space operations by restraining system costs through increased autonomy while delivering improved multi-sensor capability and robustness relative to highly complex, single platform systems. Many future missions will consist of configurations of spacecraft in near Earth deployments that must determine, and possibly maintain, a desired formation with minimal ground station involvement. Onboard GPS receivers represent an attractive means to achieve this autonomy, both in terms of absolute orbit determination and relative navigation among the spacecraft in formation. This paper describes a hybrid GPS Formation Flying Testbed (GFFT) that provides a high-fidelity modeling and simulation environment in which to develop, integrate, and test GPS technology and algorithms for formation flying applications. Built around multiple, real-time GPS simulators, the GFFT supplies realistic navigation signals to simulated vehicle models with defined nominal orbital characteristics. While other testbeds have effectively modeled the complexities found in pseudolite environments, their applicability is focussed upon engineering test and analysis rather than operational systems [6]. The GFFT utilizes GPS signals that would be encountered on-orbit, thereby reducing testbed complexity and facilitating development and testing of components and algorithms capable for use in working systems. The GFFT also supports real-time simulation that models the forces experienced by a spacecraft in a low Earth orbit environment and resolves the motion changes caused by the spacecraft control actuator commands. The modular nature of the GPS Formation Flying Testbed provides a means to integrate algorithms developed in other environments, including the pseudolite environment, for performance analysis. In addition to a description of the GFFT model and supporting facilities, the utility of the testbed for formation flying research is demonstrated by detailing the integration, development, and analysis of orbit determination and relative position algorithms for multiple spacecraft. A specific example involving an integrated Kalman filter for absolute and relative navigation of multiple spacecraft in formation is detailed. A focused derivation of the filter is provided and its performance is analyzed using the GFFT. Through this analysis the hybrid nature of the testbed is demonstrated, embodied in the ability to integrate software and hardware-in-the-loop.
Published in:	Proceedings of the 12th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1999) September 14 - 17, 1999 Nashville, TN
Pages:	411 - 420
Cite this article:	DeBolt, Richard J., Stadter, Patrick A., Asher, Mark S., "A GPS Formation Flying Testbed for theModeling and Simulation of Multiple Spacecraft," Proceedings of the 12th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1999), Nashville, TN, September 1999, pp. 411-420.
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