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

A communications experiment was conducted to investigate the effects of oce anographic fluctuations on high-frequency acoustic transmissions. Source tr ansmissions containing broad-band channel probes and phase-shift-keyed (PSK ) communication sequences were carried out in 100-m water depth using a sta tionary 18-kHz source and a stationary 64-hydrophone receive array. Thermis tor string data indicate the presence of high-frequency temperature fluctua tions 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 lon er water column; and 3) confined to the m id-water column. Because of a downward refracting profile, ray paths are mo st dense near the bottom, which maximizes wavefront interaction with the fl uctuating temperature field during period 2. Statistics of the individual rag paths indicate a strong dependence of the temporal correlation on the presence and location of the thermal activity, Rag paths interacting strongly with the portion of the water column exhibit ing rapid temperature fluctuations have coherence times of a few tens of se conds. whereas the correlation remain high over the 60-s observation window when the temperature structure is stable. Because of calm sea surface cond itions, surface interacting rays are also highly correlated over this time. Spatial coherence estimates and eigenvalue analysis of the array cross-spe ctral density matrix further indicate that the thermal activity decorrelate s the signal in space. Binary PSK (BPSK) data are processed for each of the three periods using tw o receiver structures: 1) a multichannel decision feedback equalizer and 2) a single-channel decision feedback equalizer preceded by an eigenvector be amformer which projects the data onto the first empirical orthogonal functi on. Performance of the two receivers is comparable except during period 2, During this period, the eigenvector beamformer performance suffers due to t he spatial decorrelation of the signal imposed by the temperature fluctuati ons.