Physical Problems of Direction Finding in the Deep Sea

The methods of direction finding for an immobile point source have been analyzed to obtain initial data for constructing acoustic distance measuring and tomography algorithms as applied to the deep sea. It is found that, to obtain reliable bearing estimates in the near- and far-field acoustic illumination zones (NFAIZ and FFAIZ), both in summer and in winter, it is sufficient to use the values of effective phase velocity (EPV) or effective group velocity (EGV) of sound, which are close to the measured speed of sound in water. However, in the shadow zone (SZ) under summer conditions, the effective velocities differ significantly from the speed of sound in water, and their values depend on distance, complicating additionally the solution of this problem. Therefore, to estimate the EPV and EGV, one must have information about the distance to the source. It is shown that application of vertically oriented antennas makes it possible to estimate the distance in the SZ and calculate independently the EPV and EGV values for each distance, which is necessary for direction finding. Thus, under summer conditions, conventional signal direction finding is performed in acoustic illumination zones, whereas in the SZ, in the case of simultaneous application of horizontal and vertical antennas, one must previously determine the distance to the source for calculating the bearing. The shadow zone is abscent in winter; thus, to phase a horizontal antenna on almost all distances, one can use the average speed of sound in water, but the antenna range must be determined.