Aircraft Tracking of Underwater Vehicles Equipped with Optical Beacons

Abstract

During shallow water exercises, the performance of acoustic tracking and telemetry systems is degraded by severe multipath interference. The feasibility of an optical source attached to the underwater vehicle (UV) and a tracking aircraft-based receiver was theoretically established. Supporting water absorption and surface interaction experiments were also performed. The limiting case was the tracking of an unmanned underwater vehicle (UUV). The requirements of daylight operation, atmospheric visibility, limited space and weight, self-contained power, exercise duration, sample rate, optimum search area, robustness in varying scattering and sea states, non-cooperating (except for low-data-rate communications of information such as depth) source and receiver, and relative simplicity, lead to two optimum candidate systems. One system uses a commercially available 5 megawatt q switched and double laser diode pumped YAG laser operating at 532 nm and 1 Hz rep rates. The second system uses a pulsed (2 μsec) zenon flash tube. Both systems satisfy the robustness constraint by intentional beam spreading. A performance constraint of 10:1 signal to noon solar upwelling shot noise ratio was imposed. This constraint can be met for water depths of 10 and 5 absorption lengths, respectively, for the laser and incoherent systems. An optimum search diameter of approximately 700 meters (m) at an optimum aircraft elevation of 3,000 meters is calculated for both systems. The 4-inch diameter F/1 wide-angle light pulse detection system gates a 4-inch diameter F/1 intensified charged coupled device (CCD) imaging system that locates the light surface penetration point. Another candidate receiver that performs both functions is a positive sensitive photomultiplier tube with crossed wire anodes. A supporting night-time experiment has been designed and is under construction.