Servicing and Positioning Aids-to-Navigation with DGPS

Joseph W. Spalding, Stephen E. Flynn, Frank van Diggelen

Abstract:	Servicing and positioning the over 50,000 federal short range aids-to-navigation in U.S. waters has long been one of the most challenging maritime jobs perfomred by the U.S. Coast Guard. The buoy tender fleet must routinely operate in close proximity to the very hazards the aid is there to mark. After servicing the aid, it must be put back precisely on its charted position. To date, buoy tenders have relied on the slow, labor intensive, and weather dependent method of shooting horizontal sextant angles to fix the vessel’s position as it maneuvers to come alongside, pick up, and set a buoy. Recently, tenders assigned to the northeast began using differential GPS with spectacular results. The computer monitor has replaced the positioning team of four individuals; provides real time information to the conning officer, thereby reducing the positioning evolution from an average of one hour to under five minutes; and allow the ship to service aids no matter how poor the visibility. But, concerns that the receivers can occasionally give a grossly incorrect position and provide no warning of this has slowed a full-scale adoption of DGPS. Some senior Coast Guard officials worry that without an alamr, commanding officers who rely on DGPS may place the tender in harms way, particularly if there is insufficient visibility to backup the system with traditional navigational techniques. Others worry that without the ability to prove that the DGPS receiver is fully operational at the time the buoy is set, the possibility of DGPS positioning error could leave the service vulnerable in legal cases where the plaintiff claims that a maritime accident resulted from an aid that was not placed on its charted position. In July and August 1993, an integrity monitoring program developed by the U.S. Coast Guard R&D Center and NAVSYS Corporation was tested aboard the Coast Guard Cutter RED WOOD. The system proved to be extremely reliable. In both simulated and actual operational circumstances where the DGPS receiver was required to perform in marginal signal conditions, the alarm consistently sounded when the navigation solution was outside the accuracy limits.
Published in:	Proceedings of the 6th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1993) September 22 - 24, 1993 Salt Palace Convention Center Salt Lake City, UT
Pages:	669 - 676
Cite this article:	Spalding, Joseph W., Flynn, Stephen E., van Diggelen, Frank, "Servicing and Positioning Aids-to-Navigation with DGPS," Proceedings of the 6th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1993), Salt Lake City, UT, September 1993, pp. 669-676.
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