Test Results of a Differential GPS System for NOAA Offshore Surveys

Gregory T. Kremer, Robert P. Denaro, Stephen L. Swale, Timothy Rulon

Abstract:	Under contract to TAU Corporation, NOAA has developed an engineering prototype differential GPS system for use on NOM vessels conducting hydrographic survey, bathymelric survey, and other operations requiring precise positioning. A field demonstration and test was conducted on a NOAA vessel at Norfolk, Virginia, in July 1987. This paper presents the detailed test results. The system consists of three major components: the Differential GPS Reference Station, the Differential Correction Data Link, and the Shipboard Differential GPS Navigator. The shipboard system is designed to interface with onboard data management and recording systems for integration with the sidescan sonar data The Reference Station consists of one or two Trimble Navigation, Ltd., 4000A CIA-Code GPS receivers. Differential Computations are performed in a IBM-PC compatible computer augmented with a NSC 32000 board for additional processing capability. The pseudorange correction generation process consists of independent satellite Kalman filler estimation of pseudo range error, local oscillator compensation, computation of minimum error pseudorange corrections and rate of change, and RTCM message formatting. Several data link options are possible and are envisioned for use by NOAA, ranging from VHF radio to GOES satellite transmission. A particularly interesting time and frequency diversity HF system was tested In this effort, with good success. The navigator at the shipboard end is a PC-based, flexible system. It also interfaces with the Trimble 4000A GPS receiver, and incorporates a mast-mounted dual-axis inclinometer system for mast roll/pitch compensation. The navigation solution is also a Kalman filter, and incorporates automatic modes of conventional GPS (when differential corrections are absent or unreliable) and altitude hold (when only 3 satellites are available) Sea tests were conducted over 3 days on a NOAA launch. Various maneuvers were executed. Results show the preponderance of data under 5 meters error, even using a remote Reference Station 300 miles north at Groton, Connecticut.
Published in:	Proceedings of the 1988 National Technical Meeting of The Institute of Navigation January 26 - 29, 1988 Santa Barbara, CA
Pages:	63 - 71
Cite this article:	Kremer, Gregory T., Denaro, Robert P., Swale, Stephen L., Rulon, Timothy, "Test Results of a Differential GPS System for NOAA Offshore Surveys," Proceedings of the 1988 National Technical Meeting of The Institute of Navigation, Santa Barbara, CA, January 1988, pp. 63-71.
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