Development of a GPS Receiver for Reliable Real-Time Attitude Determination in Space

E. Glenn Lightsey, Clark Cohen and Bradford Parkinson

Abstract:	A 3 pound, 4.5 watt GPS receiver capable of performing differential carrier phase measurements, developed by Trimble Navigation and modified at Stanford University, was launched on board the Air Force sponsored RADCAL (RADar CALibration) satellite into an 800 km polar orbit in the summer of 1993. At ION GPS-93, the authors presented the first known reduction of differential phase measurements to determine spacecraft attitude. At the time of this writing the RADCAL satellite continues to provide regular GPS measurements which may be postprocessed for attitude determination. The solution accuracy is estimated to be 0.5 deg RMS for a half meter antenna separation. Efforts have continued at Stanford to modify the existing GPS receiver software to provide reliable real-time attitude solutions in space at a bandwidth of 1 hz or higher, suitable for use as a sensor in a closed loop spacecraft attitude control system. A number of new algorithms have been developed and are ready for incorporation into the receiver customized for space operation. Some of these modifications include: improved GPS line of sight determination, computation of orbit elements, improved carrier phase integer ambiguity resolution in space, line bias and baseline estimation from in-flight data, and an in- flight cold-start tracking acquisition algorithm. The result is a more robust GPS sensor designed to operate in the space environment. These algorithms are presented along with the operational considerations involved in using a GPS receiver as the sensor for a real- time attitude control loop. Space missions which will fly the new receiver design in late 1994 and early 1995, such as the NASA Spartan GADACS mission, are also described.
Published in:	Proceedings of the 7th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1994) September 20 - 23, 1994 Salt Palace Convention Center Salt Lake City, UT
Pages:	1677 - 1684
Cite this article:	Lightsey, E. Glenn, Cohen, Clark, Parkinson, Bradford, "Development of a GPS Receiver for Reliable Real-Time Attitude Determination in Space," Proceedings of the 7th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1994), Salt Lake City, UT, September 1994, pp. 1677-1684.
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