Temporal coherence and time spread of ocean surface scattered high-frequency acoustic signals

Abstract

The influence of the ocean environment on the temporal coherence and time spread of high-frequency, surface scattered acoustic signals over intervals ranging from a few seconds to tens of hours have been investigated experimentally. Pulses transmitted every 128 ms reveal persistent micro-path arrivals subsequent to the main surface reflected signal. The observed structure has implications for channel equalization update rate requirements and optimal range resolution for target detection. The measurements were made during the (BBCoh/PasSAS) joint experiment with the Marine Physical Laboratory, Scripps Institution of Oceanography conducted in Fall 2000 off the coast of San Diego in water depths of 200 m to 700 m. Signals transmitted include 0.1-ms pure-tone pulses and 8-ms 13-kHz bandwidth linear frequency modulated (LFM) sweeps centered at 18, 32, and 46 kHz. The projector was suspended from the Floating Instrument Platform (FLIP) at a water depth of 170 m while the receive hydrophones were suspended from a moored buoy at two different ranges, 640 m and 2350 m, with water depths of 200 m and 700 m, respectively. Environmental measurements included water temperature, current speed and direction, directional wave height, and wind speed. [Work sponsored by ONR Code 321US under the ARL Program.]