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. More than a century after this premiere, 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. On the technological front, the various performance-limiting factors due to cavitation and the presence of anechoic coatings are presented in the article [RAD 6 715] .

The first section (§ 1 ) of this article compares active and passive underwater detection. First, the differences between active and passive non-acoustic detection techniques are discussed. Then the applications are described; unlike passive listening, which is largely for military purposes, emitting sound pulses has many civilian uses, notably for sounding and telecommunications, two areas we will briefly present. Of course, active sonar is also used by navies to detect submarines and other threats, which is the focus of this article. The detection range of this equipment is determined by the active sonar equation, which will be presented and compared with the passive equation [RAD 6 712] .

The second section (§