Chapter 13Active Sonar: Port/Starboard Discriminationon Very Low Frequency Triplet Arrays1




13.1. Introduction

Very low frequency activated sonars have an operating wavelength close to 1 m and use towed arrays with acoustic lengths of several tens of meters. An example of such a system is displayed in Figure 13.1. In their conventional version, these arrays consist of one line of acoustic sensors (hydrophones) embedded in the center of an elastomer hose. They suffer a limitation due to the symmetry of revolution of the array: the output of a beam steered by compensating the delays of propagation in a given direction is the same on a cone (see Figure 13.3a) whose axis is the array axis. The maximum of the beam response, which indicates the direction of the target, is therefore a cone. For targets of interest, which are at long distances with respect to the depth of water, their direction is given by the intersection of the horizontal plane and the cone, which corresponds therefore to two symmetrical directions with respect to the array. The system cannot discriminate between port and starboard: this is the so-called “port-starboard ambiguity” problem. In active sonar, it is mandatory to solve this ambiguity at each sonar transmission (ping), as the alerted submarine may maneuver between two successive transmissions and may be detected only on one ping.

To solve this problem, one solution is to use two parallel arrays spaced one quarter of wavelength from each other. By recombining ...

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