GPS Expedition To Tangier Island

Lloyd C. Huff and Benjamin W. Remondi

Abstract:	The National Ocean Service (NOS) is investigating the application of GPS vertical positioning of its waterborne hydrographic survey platforms to the measurement of depths directly on the NOS chart datum of Mean Lower Low Water (MLLW). One of the more difficult problems that the NOS has encountered during exploratory testing of the GPS vertical positioning of survey vessels is the acquisition and maintenance of resolved phase ambiguities (i.e., integers). As a result of poor satellite geometry, sky blockage, the number of visible satellites, and incorrect ambiguity integers, the NOS has found that GPS vertical positions of suitable accuracy are typically available only 85% of the time. Working with The XYZ’s of GPS, methods have been developed that positively impact the robustness of GPS vertical solutions and the allowable separation between the moving survey platform and the reference station(s). During the summer of 1999, a GPS Expedition to Tangier Island in the Chesapeake Bay was conducted to obtain GPS phase observables from a moving survey platform that could be processed in conjunction with phase data from reference stations located at distances ranging from 1 to 100 km. The vessel used for the expedition, the NOS S/V BAY HYDROGRAPHER, was outfitted with two dual frequency receivers (Ashtech Z-12); one antenna was mounted on the bow and the other was mounted near mid-ship on a horizontal bar, well above the cabin. During the GPS Expedition to Tangier Island, investigations were conducted for mapping the geoid, testing an underwater towbody, measuring the effect of vessel speed on the hydrodynamic interaction between the vessel and the water surface, and the robustness of a newly developed algorithm. The paper discusses the several investigations and concludes that the NOS is now closer to being able to fruitfully rely on GPS vertical positions in its hydrographic survey work.
Published in:	Proceedings of the 2000 National Technical Meeting of The Institute of Navigation January 26 - 28, 2000 Pacific Hotel Disneyland Anaheim, CA
Pages:	323 - 332
Cite this article:	Huff, Lloyd C., Remondi, Benjamin W., "GPS Expedition To Tangier Island," Proceedings of the 2000 National Technical Meeting of The Institute of Navigation, Anaheim, CA, January 2000, pp. 323-332.
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