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Issue link: http://sea-technology.epubxp.com/i/477967
www.sea-technology.com March 2015 / st 43 I t was only in the middle of the 20th century that unteth- ered free vehicles became possible with advances in fo- tation materials. Free vehicles operate independently from any surface vessel and, more recently, have been referred to as UUVs. These include AUVs, midwater foats, gliders and benthic landers. UUVs operate by managed control of their buoyancy. A positive buoyancy (fotation) is overcome by larger negative buoyancy (ballast weight), making the ve- hicle's net density greater than seawater and, therefore, able to sink to a position at mid-depth or on the seafoor. For recovery, the ballast weight is released from the fotation, or displacement increased, making the UUV less dense than seawater. It foats to the surface, carrying with it physical samples, recorded data or both. Because of ocean currents and waves, its own propul- sion or imperfect streamline, the UUV may have moved some distance from its initial drop point. Add limited vis- ibility due to darkness, fog or storms, a low surface profle, and problems locating the small, but expensive, UUV start to compound. Biological samples carried from the depths may be sterilized by the warm surface waters in a matter of 30 minutes or less. Ship costs can run more than $40,000 per day, making search patterns expensive. Weather may change, and the sun will certainly set. It is mandatory to locate and recover the UUV in the shortest period of time possible. While the UUV does not present a large target in itself, there are ways to amplify its presence on the surface to facilitate location and recovery. Onboard Beacons Selecting the most appropriate recov- ery beacons requires assessment of ship proximity to the UUV, overall system cost and complexity, depth and dura- tion. Ship proximity determines if the beacon is to aid a ship on site or alert a Surface Detection For Recovery of UUVs Using Beacons to Locate a UUV on the Surface By Kevin Hardy • Brock Rosenthal An unmanned undersea vehicle, a lander built for James Cameron's historic dive to the Marianas Trench, returns to the surface, presenting a low profle to the recovery vessel. The recovery fag, EdgeTech (West Wareham, Massachusetts) acoustic transducer, MetOcean/Novatech RDF radio trans- mitter and strobe lights, both external and inside the sphere, are seen. (Top Right) Examples of self- contained recovery beacons from MetOcean/No- vatech: (left) strobe light, (center) RDF transmitter and (right) Iridium beacon.