A Practical Demonstration of Low Cost Autonomous Orbit Determination Using GPS

Martin Unwin and Martin Sweeting

Abstract:	The GPS Navigation Unit experiment on the PoSAT-1 microsatellite has been operating successfully since the satellite’s launch in September 1993. The primary aims of the experiment are to develop a low-cost and precise means of tracking a satellite as an alternative to the use of published NORAD orbital elements. PoSAT-1 is the first microsatellite to make use of a GPS receiver and the first satellite of any kind to generate autonomously its own orbital elements using GPS. Orbital elements are required both on the ground and in space. Ground-stations require elements for mission operations planning. The spacecraft’s attitude determination and control system (ADCS) also requires knowledge of position. The performance of magnetometer and star-sensor-based ADCS is greatly enhanced when on-board position estimates are improved to the level of 1 km accuracy or better. In all cases, this can be obtained most simply by having an autonomous orbit determination capability on-board the satellite itself. The payloads that satellites carry, from communications to space science, have widely varying position accuracy requirements, but elements generated from GPS data are able to meet most of these needs directly. There are many cases when a small satellite cannot spare the power to operate a GPS receiver continuously. Fortunately, to maintain a reasonable estimate of the orbit, this is actually not necessary. The experiment on PoSAT-1 demonstrates the ability of a small satellite to determine its own orbit autonomously through an intermittently-operated GPS receiver. This paper discusses the system design of an autonomous Navigation Unit suitable for a small satellite based on a GPS receiver. The performance of the Trimble TANS GPS receiver on board PoSAT-1 is discussed, and also the on-board control of the receiver and the processing of the GPS data. The accuracy of the raw GPS data is evaluated, along with the accuracy of the final determined orbit. Comparisons are made to NORAD elements, and to alternative tracking methods. Finally an experimental demonstration of autonomous operation through the implementation of the GPS Navigation Unit is described.
Published in:	Proceedings of the 8th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1995) September 12 - 15, 1995 Palm Springs, CA
Pages:	579 - 587
Cite this article:	Unwin, Martin, Sweeting, Martin, "A Practical Demonstration of Low Cost Autonomous Orbit Determination Using GPS," Proceedings of the 8th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1995), Palm Springs, CA, September 1995, pp. 579-587.
Full Paper:	ION Members/Non-Members: 1 Download Credit Sign In