Overview

The goal of both passive and active sonar systems is reliable long- range detection and ranging capability, but the basic considerations that influence performance of the two types of sonar are quite different. The receiving array in passive systems such as towed arrays or wide aperture ranging arrays must be able to detect signals with unknown frequency content, and therefore must operate over a frequency band much greater than the band of a typical active system. And they must do so in the presence of interfering noise. Chap. 4 shows that there are many ways to design hydrophones with adequate broadband sensitivity that are small, lightweight, and inexpensive compared to the high-power projectors needed for active sonar. But the main problem in passive sonar is control of the interfering noise, especially in ship-mounted arrays.

In small active sonar systems, such as depth sounders or fish finders, it is quite feasible to use the projector array as the receiver, since the frequency of the received echoes is known to be the same as the transmitted frequency or only slightly Doppler shifted. But in many naval active sonar systems where high performance is critical there is much to be gained by use of a separate receiving array. A separate receiving array, either hull mounted or towed, can be made larger than the projector array and, therefore can have more directionality to aid in bearing determination and noise discrimination; it can also be designed to include noise reduction measures which may not...