A constant beam width towed sonar array

It is well known that the beam width of a linear equally spaced array changes with the frequency of the signal being received. This affects the bearing accuracy with which a target can be located and the resolution between two targets. Also, the assessment of the frequency spectrum of the target signal is distorted unless the target is exactly aligned with the main response axis of the array's beam. Systems which have constant beam width within the frequency band of the receiver enable the bearing of a target to be determined to a fixed accuracy with a constant resolution. They overcome the deficiencies associated with conventional arrays. This paper presents a review of four different methods of achieving constant beam width. It is shown that the homothetic replica array has a simple analytic solution and shows potential for applications requiring bandwidths greater than one octave. This is demonstrated in the design and implementation of a twenty element hydrophone array with a constant beam width from 400 Hz to 3200 Hz. The experimental results are in agreement with those predicted by computer simulations. The main contribution of this paper is to show that the SHA technique proposed in 1970 by Smith, Hixon and Au is still the optimum choice in a practical sonar array system.