Introduction to Active Sonar Detection. Basic Notions

On May 4, 1918, in the Toulon harbor, the Messidor submarine was detected at a distance of around 500 m by a new type of device: active sonar. A century on, performance has steadily increased, the field has diversified and today, several million active sonars operate in Earth's waters, most of them for fishing and navigation. This article covers the basics of how these devices work. Readers will then be able to delve deeper into the subject with the help of specialized books. From a functional point of view, this presentation stops at the primary detection stage under simple conditions (noise alone): tracking algorithms, echo classification and reverberation are not covered here. Unless otherwise stated, detection is considered to be monostatic (transmission and reception take place in the same place). Technologically speaking, the various performance-limiting factors such as those due to reverberation, cavitation and anechoic coatings will be dealt with in greater depth in the second article on advanced concepts.

The first section of this article compares active and passive underwater detection. First, the differences between active and passive non-acoustic detection techniques are discussed. Applications are then described: emitting sound pulses has many civilian uses (sounding, telecommunications, imaging, etc.), unlike passive listening, which is largely military in scope. Of course, active sonar is also used by navies to detect submarines and other threats: this is the main focus of this article. Their range is determined by the active sonar equation, which will be presented and compared with the passive equation. This equation serves as the framework for the presentation: each of its terms will be detailed in a succession of specific sections.

The second section presents three examples of active sonars and their range at sea (taken from the open literature). In the final section, these will be compared with those obtained using the sonar equation, under certain assumptions. The 1 ^er example, dating from the Second World War, is that of the detection of U-Boats by Allied active sonars (ASDICs). The 2nd example is that of a powerful bow sonar fitted to US-Navy surface ships, one of whose missions is to protect aircraft carriers from submarines. The 3rd concerns the detection of a diver in the vicinity of a ship at anchor or dockside.

The third section deals with acoustic emission in water, starting with the definition of acoustic level. Explosive sources, but above all the widespread piezoelectric transducers and their common characteristics, are presented. The operation of three common types of transducer is described in detail, illustrating the above general considerations.