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44 st / March 2015 www.sea-technology.com off. This could save battery power until a recovery vessel is near at hand. Surface Detection, Recovery On-station methods of identifying a surface vehicle in- clude passive/refective, such as radar refectors, fags, light- refective tape and brightly colored fairings. Smoke bombs and surface water dyes have also been used. Electronic beacon methods include strobe lights and ra- dio direction fnding (RDF) transmitters, such as those made by MetOcean/Novatech, Xeos and others. Similar to the previously mentioned satellite beacons, these are switched on at the surface using pressure-activated switches or capac- itive sensors. Some manufacturers use a light sensor to turn off the strobes during bright daylight hours, saving power. MetOcean provides a remote antenna option with their RDF and a remote strobe head with their fasher. The RDF bea- cons can be located from a surface vessel using Yagi direc- tional antennas, which amplify the signal within a narrow angle centered on the signal source, null meters, shrunken quad antennas or a dual-element switched array antenna, such as the MetOcean/Novatech DF-500N. These receivers determine approximate direction, while range is approxi- mated by signal strength. Operator skill is required in inter- preting the readings as the true direction and a secondary peak may be out by 180°. A ship could be sent directly away from the surfaced UUV due to the ambiguity. Teledyne Benthos (North Falmouth, Massachusetts) TR- 6001 transponders employ a method of near surface acous- tics to locate the sphere on return to the surface. The TR- 6001 is ballasted for transducer "up" at the seafoor and transducer "down" after anchor release. When the sphere reaches the surface, the transducer remains in the water. A three- or four-element hydrophone array on the ship, either hull mounted or towed, can be used to determine time of frst reception of the signal in two axes, port-starboard and fore-aft. The frst hydrophone in a pair to receive the signal is the direction to the transponder. The sphere fip can be used to trigger a tilt switch that would also energize an internal RDF transmitter and xenon strobe light. Transmitting Data, Receiving Location There are creative ideas for building 2-meter transmit- ters and receivers found on websites dedicated to the ama- teur competition called the "foxhunt," or radio-orienteering, where a "fox" hides a 2-meter transmitter that is to be "hunt- ed." Cost, time and limitations associated with building a custom system in place of commercially available units must be considered. Animal tracking websites provide similar information. Designers have the option of housings. The bea- con either has its own housing, or it is integrated into an existing housing. A new option for 10-inch-diameter or larger glass sphere housings is the Beacon Board, a combination GPS receiver and VHF transmit- shore-based operator after the ship has left station. A UUV may have been deployed for a long period of time, and the ship left station. The UUV's ballast weight, however, may become dislodged due to any number of reasons, including damage from a bottom trawl, a defective corrosive link or an incorrectly programmed timer. Onboard beacons utilizing satellite communication networks, such as Argos or Iridium, can alert a shore- based operator of the unexpected surface arrival. Manu- facturers of these beacons include MetOcean (Dartmouth, Canada), Telonics Wildlife (Mesa, Arizona) and Xeos (Dartmouth). A promising addition to the feld is the SPOT 3 satellite GPS messenger. It appears this could be adapted for UUV recovery inside a glass sphere. It's coverage includes large areas of northern oceans. Like the Iridium beacons, the unit determines its position using GPS satellites, sending its posi- tion back through a satellite network to ground stations that deliver the beacon ID and position through cellphone or email. We are giving this system a try in the coming months with a nearshore research project. Beacons are generally activated by an integral pressure- sensitive switch or capacitive sensor. A pressure-activated switch, such as the Nautilus Marine (Buxtehude, Germany)/ Global Ocean Design (San Diego, California) model, can be used with glass spheres or pressure cases. When satellite- based beacons are turned on at the surface, they begin a preprogrammed routine of locating their position and send- ing that data through a data network. There are costs for the beacon, the satellite communica- tion, the data network and possi- bly an Iridium phone for remote locations of the operator. Alert times vary, but can be as fast as under fve minutes. Once alerted, the operator is left with the decision of considering a recovery operation. Some sys- tems, such as Argos, allow the operator to send limited com- mands back to the UUV, such as turning a strobe light on or n s " t e i o n h h o g h o G (Top) A SPOT 3 satellite GPS messenger. (Bot- tom) A pressure-activated switch (Nautilus Marine/Global Ocean Design) may be used directly through a glass sphere or through a pressure case end cap.