Sonar - active sonar device sound detect underwater objects. Previously used the name "asdik" (English. ASDIC, abbreviation of Allied Submarine Detection Investigation Committee).
Sonar (sonar) are divided into two fundamentally different types: side-scan sonar and sonar circular or sector review.
Side-scan sonar have a narrow radiation pattern in the horizontal plane (0,1 ° - 2 °), and a wide acceptance angle in the vertical plane (40 ° - 60 °), which allows to obtain the radiation pattern of the antenna receiving and transmitting "knife-like" shape with the direction its' perpendicular to the line of the vessel. Side-scan sonar need to work at greater depths and for the detection of large submerged objects.
Sonar Omnidirection or "scan sonar," in contrast to the side-scan sonar is static, not moving the device itself, but only the working part is located at the end of the cylindrical body. The device is lowered on a cable-rope into the water from a ship or boat in the summer or under the ice, at work in the winter time. Operating frequency omnidirectional sonar is generally in the range of from 200 to 1000 kHz. Frequency range 600 - 1000 kHz can detect small enough objects, which is useful for detection of fish. A small size allow sonar to use it even in small fishing boats.
Sounder - a highly specialized sonar device for the study of the bottom relief of the water basin. Typically uses an ultrasonic transmitter and receiver, as well as a computer for data processing and rendering topographic map of the bottom.