An Improved Solution Algorithm for Self-Calibrating Pseudolite Arrays

Edward A. LeMaster, and Stephen M. Rock

Abstract:	Tasks envisioned for future generation Mars rovers – sample collection, area survey, resource mining, habitat construction, etc. – will require greatly enhanced navigational capabilities over those possessed by the Mars Sojourner rover. Many of these tasks will require cooperative efforts by multiple rovers and other agents, adding further requirements both for accuracy and commonality between users. Although a Mars-based global satellite navigation system analogous to GPS could provide the necessary capability, the cost of such a system with sufficient coverage and accuracy would likely be prohibitively high, at least within the foreseeable future. Precise positioning can be accomplished in the absence of a satellite navigation system, however, by using GPS pseudolites or similar ground-based transmitters to mimic the effects within a local area. Stanford University has developed such a system to aid NASA in its future Mars exploration goals. The greatest difficulty in using pseudolites on the Martian surface is determining the precise locations of the transmitters themselves, a prerequisite for successful navigation to occur. The Self-Calibrating Pseudolite Array (SCPA) developed at Stanford solves this problem by utilizing GPS transceivers for improved geometric observability and limited motion of a rover-mounted transceiver to resolve the carrier-phase cycle ambiguities. This paper presents recent advances in the development of the operational SCPA prototype, expanding upon the preliminary methods and results presented in [1]. It begins by summarizing the overall system requirements and architecture and then discusses the array self-calibration methods themselves, focusing on a new quadratic algorithm for precisely determining the locations of the pseudolite transceivers. This solution method offers a factor of 2-3 improvement in terms of solution convergence over the previous linear algorithm, allowing for the calibration of smaller-baseline arrays in high-multipath environments. When coupled with a stochastic multiplesolution approach, the combined algorithm yields successful self-calibration under most possible array configurations. Finally, the paper reviews the results of field trials performed at NASA Ames Research Center with the K9 Mars rover research platform which validate both the navigation and self-calibration capabilities of the system. By carrying an onboard GPS transceiver, K9 was successfully able to calibrate the system using no a priori position information, and localized the pseudolite beacons to under 5 cm RMS.
Published in:	Proceedings of the 2002 National Technical Meeting of The Institute of Navigation January 28 - 30, 2002 The Catamaran Resort Hotel San Diego, CA
Pages:	562 - 572
Cite this article:	LeMaster, Edward A., Rock, Stephen M., "An Improved Solution Algorithm for Self-Calibrating Pseudolite Arrays," Proceedings of the 2002 National Technical Meeting of The Institute of Navigation, San Diego, CA, January 2002, pp. 562-572.
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