Acoustic Impulse Response Fluctuations and Coherent Underwater Acoustic Communications in Shallow Water under Autumn Conditions

The article presents the results of a full-scale experiment aimed at assessing the temporal variability of the impulse response of a hydroacoustic channel and efficiency of coherent underwater acoustic communications using bottom-mounted transmitters and receivers at frequencies of ~10 kHz on the Black Sea shelf in autumn. Three prominent maxima of variable amplitude were observed in the impulse response structure throughout the experiment (~36 h). The range of variability of the root-mean-square decoding error was ~11 dB; the bit error ratio varied from 0 to 0.10. A strong relationship was found between the values of decoding errors and the amplitude of the maximum arrival in the structure of the reference impulse response, corresponding to a group of rays with one reflection off the surface, as well as the coefficient of variation of high-frequency fluctuations of the amplitude of this arrival in the instantaneous estimate of the impulse response. Numerical modeling was used to confirm the hypothesis that in autumn conditions, characterized by the absence of a pronounced seasonal thermocline, the main hydrophysical cause of the variability of the amplitude of the main arrival, and, as a consequence, the effectiveness of underwater acoustic communications, consisted in an insignificant (fractions of a degree) change of temperature in the upper layer of sea water.