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Clearly Superior (Top) A 250-meter-long AUV sub-bottom profle across the shallow buried channel (vertical scale is referenced to the water surface using the AUV depth sensor). imaging SONAR SYSTEMS Horizontal layering (H) and dipping refectors (R) resolved in a 350-meter SBP line collected in the channel between Greenwich Bay and Patience Island, Rhode Island. system, the small footprint from a lowfying AUV, combined with the strong returns, results in positional accuracy for small man-made features, such as buried pipelines. Confguring the AUV The modular Gavia Offshore Surveyor AUV consists of a nosecone, battery, control unit and propulsion unit. In commercial survey work, the Offshore Surveyor AUV is normally confgured with an inertial navigation system (INS) assisted by a Doppler velocity log (DVL) and differential-ready GPS receiver. Sensor modules are chosen dependent on the data desired, with the vehicle automatically detecting their presence on start-up. The vessel can be confgured with one or two lithium-ion battery modules that can be exchanged in the feld in a few minutes once depleted, with the two-battery confguration allowing missions up to 12 hours at 4-knot speed. For the Greenwich Bay SBP missions, the build was: nosecone with obstacle avoidance sonar and imaging camera, battery module, Kearfott Corp. (Little Falls, New Jersey) T-24 INS with Teledyne RD Instruments (Poway, California) WHN1200 DVL, control module with GPS, Teledyne Benthos ATM-900 acoustic modem, Iridium Communications Inc. (McLean, Virginia) A3LA-X telemetry, side scan sonar, SBP and propulsion unit. In this confguration, the AUV was 2.9 meters long and weighed 88 kilograms in air. Mobilization The Gavia Offshore Surveyor was assembled and tested at the Teledyne Benthos offces then transported by road to Allens Harbor Marina near North Kingstown, Rhode Island. Operations were conducted from the University of Rhode Island RV Shanna Rose, a 42-foot lobster boat adapted for nearshore survey. It has a low back deck suitable for manual AUV deployment in calm seas and an enclosed survey offce for mission planning and data processing. The AUV was manually carried onto the vessel in its cradle and strapped down on the back deck for transit to the survey site. Spare battery modules enabled a full day of operations without recharging. Once at the survey site, the AUV was manually deployed over the stern of the vessel and manually piloted away using the Wi-Fi link, and the mission execute command sent. During the mission, the status of the vehicle was monitored using the integrated acoustic modem. Regular updates on the vehicle position, depth and speed were tracked on a laptop. Multiple missions 45 minutes to two hours long were run over the two survey days in two separate areas of the bay. SBP Data Collection Initially, the AUV did a series of test missions to determine the optimum system settings and fy height for the refector defnition and penetration in Greenwich Bay. With an AUV-mounted SBP, the same survey line can be repeated accurately with data collection settings changed in each run, enabling tests of the effects of changing parameters, such as fy height, pulse length, transmit power and receiver gain. The frst set of missions consisted of several lines across Greenwich Bay to fnd a suitable sub-bottom feature for www.sea-technology.com SiDe SCan SonarS SuB-BoTTom proFilerS BaTHymeTry SySTemS The Leader in Underwater Technology info@edgetech.com uSa 1.508.291.0057 FEBRUARY 2013 / st 47