Boosting Shipboard RAIM Availability

Joseph Spalding, James LaMance and Alison Brown

Abstract:	Differential GPS (DGPS) is becoming widely used for a variety of precise positioning applications. Test results have demonstrated the capability to provide real-time navigation data to an accuracy of 2 meters. This exceeds the accuracy requirement of 10 meters (2 dRMS) established for the majority of radionavigation applications. However, on occasion, transient errors are observed in the DGPS solution where, for a short period of time, the navigation error exceeds 10 meters. These errors can be caused by transients in the satellite signals (e.g. selective availability), errors in the reference station (e.g. multipath), or errors in the DGPS receiver itself. Many of the applications for DGPS require a highly reliable solution. One such application is navigation buoy positioning. Currently, buoys are located by the USCG using DGPS. DGPS can provide the capability for locating buoys in all weather conditions with a high level of reliability. However, current methods do not always include an integrity measure on the DGPS solution. In addition to the real time integrity monitoring, the Coast Guard desires that data be archived and the integrity status be regenerated if the need arises (e.g. post-mission validation for litigation considerations). This paper describes an autonomous DGPS Fault Detection and Isolation (FDI) algorithm which can be implemented in the user’s DGPS receiver. The algorithm makes use of redundant measurements to test the validity of the DGPS solution. An additional redundant measurement can be obtained through the use of a tide model and altitude aiding the FDI algorithm. With altitude aiding, the algorithm can be implemented in any DGPS receiver with 6 or more channels. The DGPS FDI algorithm was tested on board the US Coast Guard Cutter RED WOOD and at the Research and Development Center in Groton, CT to demonstrate the ability of the algorithm to detect navigation accuracy failures and the contribution of the altitude aiding to reduce false alarms. The tests were conducted underway from December 11-14, 1994 and on land from February 24 through March 1, 1995. During the tests the tide models December 11-14, 1994 and on land from February 24 through March 1, 1995. During the tests the tide models were shown to reliably add information to the FDI process and reduce the percentage of yellow warning alarms. During the land tests the contribution of high accuracy diffrential corrections was analyzed. The FDI algorithm has previously been demonstrated to reliably detect failures through simulation and field testing. With the improvements of the altitude aiding and high accuracy differential corrections, false alarms and warnings have been virtually eliminated.
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:	907 - 912
Cite this article:	Spalding, Joseph, LaMance, James, Brown, Alison, "Boosting Shipboard RAIM Availability," 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. 907-912.
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