Using GPS Satellites to Measure Star Positions

James W. Sinko and Allen B. Takahashi

Abstract:	The technique described here that has been used for measuring the absolute position of stars using inexpensive and commercially available equipment consists of imaging a Global Positioning System (GPS) satellite against a background of stars. The satellites are sufficiently faint (11 th to 13th magnitude) that a moderate-sized telescope and a Charge-Coupled Device (CCD) are required to obtain the images. The telescope is held in a fixed position to avoid drive errors for roughly 15 seconds so that the images of both the stars and the satellite are trailed. A GPS-clock-controlled shutter is used to insert timing marks in the trails. The start and stop position of each timing mark for both the satellite and star trails are determined by using a least-squares algorithm. These positions and times, along with the precise positions of the earthbound observer and the GPS satellite, are used to determine the celestial coordinates of the star. To date, accuracies of the order of one arcsecond have been obtained using a lo-inch diameter telescope. The technique is relatively insensitive to errors introduced by atmospheric effects because light from both the star and the satellite are passing through almost the same atmosphere. For the equipment used in this experiment the accuracy of this technique is limited primarily by image signal-to-noise ratios and by simplifying assumptions made in the processing algorithms. With a larger telescope the accuracy should improve significantly, and other possible limiting factors can perhaps be countered. The most promising aspect of this technique is that all positions obtained by this method are tied to the GPS satellites whose positions are accurately known relative to an inertial reference frame.
Published in:	Proceedings of the 9th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1996) September 17 - 20, 1996 Kansas City, MO
Pages:	1269 - 1274
Cite this article:	Sinko, James W., Takahashi, Allen B., "Using GPS Satellites to Measure Star Positions," Proceedings of the 9th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1996), Kansas City, MO, September 1996, pp. 1269-1274.
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