Use of DGPS For an In Water Acoustic Survey For the PTS SWIMSS

Robert Reid

Abstract:	The Portable Tracking System (PTS) is being designed to support the U.S. Navy’s requirement to perform open-ocean testing in diverse environmental conditions, including shallow water. The Shallow Water In-line Multiplexed Sensor System (SWIMSS) is a prototype system used to track the position of in-water vehicles that are performing naval operations. SWIMSS is designed to be deployed in water depths up to 2000 feet. It consists of 10 in-line sensor hydrophones that time-division multiplex (TDM) acoustic data over a single-mode fiber-optic cable. An installation and testing of SWIMSS was conducted 10-l 8 September 1994, 120 miles off the coast of Newport, RI, at 39” 53.5’N and -72” 07.2W in approximately 300 feet of water. In order to perform this kind of in-water tracking operation, each hydrophone sensor must be acoustically surveyed in a geodetic coordinate system. The survey of the sensor nodes requires a mobile survey ship to “ping” at various key range points. Aboard the survey vessel is a Differential Global Positioning System (DGPS) receiver and UHF RP telemetry equipment, as well as an underwater acoustic projector. The survey vessel received differential corrections from the Montauk Point, Long Island, New York Coast Guard Station. The SWIMSS cable is terminated to a moored surface ship where the acoustic pings are detected. The receive vessel is logging the DGPS position of the survey vessel via the RF link on a SUN Workstation. Personnel on the receive vessel vector the survey vessel to the various survey waypoints for data collection. The underwater projector, hull- mounted to the survey vessel, transmits a “ping” that is synchronized with DGPS time. The ping is received by the SWIMSS sensor, multiplexed on the fiber, and demultiplexed by shipboard electronics aboard the receive vessel. The time delay caused by the transition of the ping through the water is measured by an acoustic digital signal processor interfaced to the shipboard demultiplexer and DGPS. The signal processor in tum is interfaced to the SUN, and the transit times through the water are displayed and stored. A ‘YP course is run over the SWIMSS sensor by the survey vessel. Based on the known time of the transmission, the position of the survey vessel acquired via DGPS, and the RP link, the transit time of the ping through the water and the velocity of sound in the water, the latitude, longitude, and depth of the SWIMSS hydrophone sensor can be calculated. The new innovation here regards the fact that shallow water is a harsh environment for the transmission of acoustic pings. In the past, with deep-water acoustic tracking range surveys, several hydrophones would be making detections on a direct-path transmission from pinger to hydrophone. This technique ties the hydrophones together as they relate to each other. This is not possible in shallow water because the direct-path acoustic transmission is shorter, necessitating the requirement of a single hydrophoue survey. This is the case because hydrophones may not be receiving the same acoustic paths. DGPS was the tool that makes possible hydrophone position in shallow water.
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:	919 - 823
Cite this article:	Reid, Robert, "Use of DGPS For an In Water Acoustic Survey For the PTS SWIMSS," 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. 919-823.
Full Paper:	ION Members/Non-Members: 1 Download Credit Sign In