Accuracy Comparison of GPS and an Inverse GPS Locating System

James Watson, Fred Shimabukuro, and David Schwartz

Abstract:	Current studies of the Combat Survivor Evader Locator (CSEL) global rescue radio have stimulated debate over conventional GPS versus an Inverse GPS locating method. The inverse method was investigated for its potential to reduce the hand held radio size. With Inverse GPS the user transmits a burst digital message simultaneously through multiple satellites to a central ground station, and the user location is calculated from the time difference of arrival (TDOA) of these multiple signals. Without a GPS module the user learns his location only by receiving a return message from the ground station. To accomplish this inverse procedure, precision time keeping is done only at the ground station. Many satellites must be quipped with transponders so that multiple satellites are visible to the user and stations. Also the user radio must be quipped to receive as well as transmit. While CSEL may not use Inverse GPS, there are other potential civilian, government, and military, applications which make Inverse GPS of continued interest. This paper examines the location accuracy for inverse and conventional GPS with frquency band alternatives: VHF, UHF, L and S-Band. Satellite transponders are assumed placed on future GPS spacecraft, and location accuracy is evaluated versus: (1) number of modified GPS satellites deployed, and (2) frequency selection. Other related parameters which infhtence accuracy include radio power, chip rate, signal to noise ratio, burst duration, and user radio antenna mask angle. An example not involving satellites is also considered.
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:	841 - 850
Cite this article:	Watson, James, Shimabukuro, Fred, Schwartz, David, "Accuracy Comparison of GPS and an Inverse GPS Locating System," 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. 841-850.
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