The military has developed an advanced underwater winged glider based on the air force's Flying Wing design, the Liberdade XRAY (see Figure 11). This vehicle is "being developed as a part of the Navy's Persistent Littoral Undersea Surveillance Network (PLUSNet) system of semi-autonomous controlled mobile assets. PLUSNet uses unmanned underwater vehicles (UUVs) and autonomous underwater vehicles (AUVs) to monitor shallow-water environments from fixed positions on the ocean floor or by moving through the water to scan large areas for extended periods of time" (ONR, 2006).
The XRAY was develop primarily with the aid of the Marine Physical Laboratory at Scripps Institution of Oceanography and the University of Washington's Applied Physics Laboratory, and also with the following institutes, universities and corporations: University of Texas at Austin's Applied Research Lab, Applied Research Lab at Penn State University, MIT, Woods Hole Oceanographic Institute, Harvard University, SAIC, Bluefin Robotics, Metron, Heat, Light, and Sound (HLS) Research, and the Space and Naval Warfare (SPAWAR) Systems Center in San Diego.
The vehicle is the largest of all of the underwater gliders (6.1 meter wing span), which is an advantage in terms of hydrodynamic efficiency and space for energy storage and payload. The glider's primary function is to track quiet diesel-electric and the new fuel cell submarines operating in shallow-water. According to military doctrine it can "be deployed quickly and covertly, then stay in operation for a matter of months. It can be programmed to monitor large areas of the ocean (maximum ranges exceeding 1000 km with on-board energy supplies). The glider is very quiet, making it hard to detect using passive acoustic sensing" (ONR, 2006).
The vehicle was designed for easy and rapid deployment and retrieval, as well as payload carrying capability, cross-country speed, and horizontal point-to-point transport efficiency which is better than existing gliders. Liberdade XRay's first major ocean test was performed in August 2006 in Monterey Bay, California, where it reported real-time via an 3.0 to 8.5 kHz underwater acoustic modem as well as with an Iridium satellite system while on the surface. The vehicle had an array of 10 kHz bandwidth hydrophones located in the SONAR dome and across the leading edge of the wing. The XRay exceeded a 10 to 1 glide slope ratio (D'Spain et al., 2007). Later deployments were in the Philippine Sea, near Hawaii, and in Monterey Bay using the hydrophone array "to detect low frequency source signals, marine mammals (blue and humpback whales), and ambient ocean noise" (APL, 2007). The XRay glider is hoped to achieve 1-3 knot cruise speeds, have a 1200-1500 km range, and be able to remain on-station up to 6 months in partial buoyant glides.
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