| A daily study spanning a month of the shallow water acoustic channel was conducted to estimate the environmental influence on performance of an underwater acoustic communications system. An automated acoustic modem transmitted phase-coherent modulated sequences of identical data with 186 dB re 1muPa source level, at coded rates from 4000 to 16000 bits/s with 4 or 8 kHz symbol bandwidth, three times daily for a month. A 64 channel Mills-Cross receiver array was used with horizontal and vertical beams each containing 32 and 33 elements respectively, spaced 0.03 meters apart, with a sampling frequency of 72 kHz. Source and receiver were deployed at depths of 20 meters respectively, with a 720 meter separation range. Environmental measurements of wind velocity and direction, surface wave activity, current and sound velocity profiles, and tidal measurements were performed. Results demonstrate reliable achievement of high data-rate shallow water acoustic communications using phase-coherent modulation techniques. |
