Test and evaluation of a midwater three dimensional underwater tracking array

A continuing need exists for accurate three dimensional tracking of submerged targets on instrument ranges. The common problems faced by developers of acoustic underwater tracking systems range from launch, recovery, and structure life to telemetry and real time processing of source signals. The Naval Surface Warfare Center controls an underwater test site that ranges from shore to a water depth of 600 meters. To support long term growth, an underwater tracking system capable of operating over the majority of the range with a serviceable life of twenty years was developed and is described. The system design goals included long term compatibility with the marine environment and stringent tracking accuracy requirements. Environmentally compatible materials selection included fiber reinforced plastics for structures, glass spheres for pressure vessels, and titanium for fasteners. Tracking accuracy is achieved using a synchronous tracking system consisting of four hydrophones on an eight meter baseline. Pulse detection and arrival time are determined using a combination of in band and out of band energy detection with replica correlation of the apex channel. The system has single bit digitizing using zero crossing detectors. Differential arrival times for two adjacent channels are determined using cross correlation with the apex channel. This paper contributes an overview of the underwater tracking system and of the Fort Lauderdale facilities and test capabilities. A summary of experimentally obtained data on the tracking accuracy from deployments in 180 and 450 meters is given. Tracking resolutions are given as a function of environmental parameters including range from the array, depth, and ambient sound velocity profiles. Array performance characteristics are extrapolated over usable water depth based on evaluation of the arrays at the two test sites