Loran-Aided GPS Integrity

R. Grover Brown, Paul W. McBurney

Abstract:	GPS Receiver Autonomous Integrity Monitoring (RAIM) has received considerable attention recently, and the problem is much better understood now than it was a few years ago. At first glance it would seem that there would be plenty of redundancy in the planned 24-satellite constellation to provide for self-consistency checking 100 percent of the time. However, this is not so. Even with the robustness of the proposed system, poor detection geometries will exist for a small fraction of the time. This is not fatal to the RAIM concept, but it means that the failure detection system must lean on additional measurement information during these critical times. Both clock-coasting and bare-altimeter aiding have been suggested as a means of riding through short periods of bad detection geometry. The present paper suggests that one-way ranging on Loran-C stations is another way of bringing in additional measurement information. This looks especially attractive with the recent announcement that all Loran-C master stations are to be tightly synchronized to UTC, and thus indirectly to GPS time. This will provide a degree of stability in going from one chain to another that is not present now. The key to obtaining the necessary accuracy from Loran is to calibrate the Loran pseudorange measurements with GPS during periods of good geometry, and then use these additional calibrated measurements during those brief periods when the GPS detection geometry is poor. This self­-calibration mode of operation does lead to some loss in protection during bad geometry periods, but it appears that the protection level would still be adequate for all phases of flight except non precision approach. This paper does not propose a true hybrid GPS/Loran-C mode for navigation purposes. Rather, in the scheln.e envisioned here, GPS would be the primary navigation system and Loran would be used only for checking the integrity of the GPS signals.
Published in:	Proceedings of the International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1988) September 19 - 23, 1988 The Broadmoor Hotel Colorado Spring, CO
Pages:	461 - 466
Cite this article:	Brown, R. Grover, McBurney, Paul W., "Loran-Aided GPS Integrity," Proceedings of the International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1988), Colorado Spring, CO, September 1988, pp. 461-466.
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