Undersea Acoustic Signal Coherence Fluctuations

This paper describes the effects of source motion and multipath propagation in the ocean on the coherence of narrowband acoustic signals at widely spaced sonar receivers. The measure used to evaluate these effects is the magnitude-squared coherence (coherence). The discussion is based on both analytical developments and computer model studies. Definitions of coherence functions are presented. A generalization of the functional relationship between signal-to-noise ratio in a channel and the coherence of the signal is developed. An overview of the computer program for the simulation of the received complex signal envelope with the distortions due to ocean multipath phase, time delays, and motion-induced Doppler is presented. The coherence ambiguity surface formed from time and Doppler differences is introduced for the purpose of addressing peak fluctuations. The fluctuations with time of the time of arrival difference, Doppler difference, and level of the peak coherence are described. Examples of measured and modeled peak coherence phenomena are presented.