The use of effective phase velocity to decrease the direction finding error for a low-frequency signal in a waveguide

The mode, interference, and phase structures of the low-frequency sound field and the spatial responses of the extended linear array in a waveguide are studied. The wide-angle parabolic approximation and the normal mode method are used to perform calculations. A possibility of approximating the field in the zone of interference maxima by the equivalent plane wave model with the real amplitude and averaged effective phase velocity (EPV) calculated or measured from the phase gradient at the array aperture is investigated. The use of EPV in the zone of interference maxima is shown to decrease substantially the direction finding error. The conclusion is drawn that the array and the transfer function of the waveguide can be approximately matched if the speed of sound in water is substituted with the EPV in the bearing algorithm.