Effects of tidally driven temperature fluctuations on shallow-water acoustic communications at 18 kHz

Abstract

A communications experiment was conducted to investigate the effects of oceanographic fluctuations on high-frequency acoustic transmissions. Source transmissions containing broadband channel probes and phase-shift-keyed (PSK) communication sequences were carried out in 100-m water depth using a stationary 18-kHz source and a stationary 64-hydrophone receive array. Thermistor string data indicate the presence of high-frequency temperature fluctuations generated by the internal tide as it progresses along the continental shelf. Data are analyzed from three periods when the thermal activity is: (1) absent; (2) confined to the lower water column; and (3) confined to the mid-water column. Because of a downward refracting profile, ray paths are most dense near the bottom, which maximizes wavefront interaction with the fluctuating temperature field during period 2. Statistics of the individual ray paths indicate a strong dependence of the temporal correlation on the presence and location of the thermal activity. Ray paths interacting strongly with the portion of the water column exhibiting rapid temperature fluctuations have coherence times of a few tens of seconds, whereas the correlation remain high over the 60-s observation window when the temperature structure is stable. Because of calm sea surface conditions, surface interacting rays are also highly correlated over this time. Spatial coherence estimates and eigenvalue analysis of the array cross-spectral density matrix further indicate that the thermal activity decorrelates the signal in space. Binary PSK (BPSK) data are processed for each of the three periods using two receiver structures: (1) a multichannel decision feedback equalizer and (2) a single-channel decision feedback equalizer preceded by an eigenvector beamformer which projects the data onto the first empirical orthogonal function. Performance of the two receivers is comparable except during period 2. During this period, the eigenvector beamformer performance suffers due to the spatial decorrelation of the signal imposed by the temperature fluctuations.