DGPS Harbor Entrance and Approach Requirements Analysis

Peter Morris, James Doherty, Daniel McGaffigan

Abstract:	The work described in this paper was organized to determine whether vessels with DGPS as their primary means of navigation can safely transit navigationally challenging waterways in the U.S., given that the vessels' backup, or secondary source of navigation, utilizes one of the following systems: · Loran-C · Inertial Navigation Systems / Inertial Measurement Units · GPS without selective availability. In this work it is assumed that the three DGPS backup systems listed above are coupled to the primary DGPS receiver so that the accuracy of the combined primary-secondary system is that of DGPS whenever DGPS signals are available and the receiver is properly functioning. As a stand-alone backup to any of the secondary systems, marine radar is assumed to be part of the vessel's suite of navigation equipment. The results of this investigation indicate that only Loran-C integrated with DGPS was able to satisfy the target level of safety for all scenarios considered. The high performance of the DGPS/Loran-C integrated system is primarily due to the excellent accuracy-preserving characteristics of the processed Loran-C signals as well as the high service reliability due to multiple redundant stations. The results for a DGPS system coupled with INS/IMU were less favorable primarily because of the exponential error growth of the INS/IMU following loss of DGPS. This error growth leads to a rather rapid transition to the second backup system - marine radar -which has a high incident rate for a narrow channel in zero visibility. Although its accuracy as a backup system is superior to INS/IMU, GPS without SA is roughly equivalent to the low-end IMU backup systems in terms of incident probability. The principal reason for this behavior is the relatively low MTBF (due to the occurrence of poor satellite geometry) of GPS and the failure mode (GPS service) common to both the primary and secondary systems.
Published in:	Proceedings of the 55th Annual Meeting of The Institute of Navigation (1999) June 27 - 30, 1999 Royal Sonesta Hotel Cambridge, MA
Pages:	63 - 67
Cite this article:	Morris, Peter, Doherty, James, McGaffigan, Daniel, "DGPS Harbor Entrance and Approach Requirements Analysis," Proceedings of the 55th Annual Meeting of The Institute of Navigation (1999), Cambridge, MA, June 1999, pp. 63-67.
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