An acoustic device that surveys the water column horizontally. In fisheries, it is used to detect a concentration of fish away from the vessel and to determine the distance to them.

     The main elements of the sonar are an underwater transmitter of a powerful acoustic signal and a sensitive receiver, responding even to weak reflections of this signal from submerged objects in the water. 

Both of these elements are designed in such a way that their arrangement in the locator provides determination of the direction to the reflecting object and the distance to it. On submarines and surface vessels, sonar is the primary, and often the only, means of gathering information about conditions and circumstances beneath the sea surface. 

On nuclear-powered submarines, special sonar is used as a navigational aid. Large ships are equipped with echo sounders - sonars that measure the depth of the ocean. Many fishing vessels use sonar to detect schools of fish, and marine animal biologists use sonar to study the sounds made by marine life. Underwater, an acoustic beam, like a searchlight or radar beam in the air, is aimed at a target, and the sound energy reflected from it enters the receiver. 

From the sonar, as from the radar, the radiation is emitted in short pulses. The distance to the target is determined as the product of the speed of sound in the water by half the time interval between the pulse emission and the arrival of its echo. Because the sonar's receiving antenna has a sharp radiation pattern, the bearing of the target is determined by rotating the microphone when it is tuned to the echo. 

In practice, the operator follows the light marks on the panoramic screen, which correspond to the detected objects, and this greatly facilitates their location. Range of the sonar is limited to a radius of a few kilometers. 

The speed of sound in water is approximately 1.5 km/s, so sonar searches are much slower than radar or light searches in the atmosphere. Because of the relatively long wavelength of sound, sonar has rather poor spatial resolution: where the eye would discern every rivet on a ship's hull, sonar will "see" only the entire vessel as a single spot. Besides, sea water is far from being an ideal medium for sound propagation. 

The idea of sonar is not new. As far back as World War I, hydrophones were used on surface ships and submarines to detect enemy ships using passive sound detection methods. Later, piezoelectric transducers and electronic amplifiers of sound range signals were developed, which led to the development of active sonar systems. 

Since then, many types of advanced devices have been developed, including all-around sonar, variable depth sonar, etc. 

Hydrolocators are divided into two main types: active (emitting a signal and receiving its reflection) and passive (receiving noise produced by the target).