SONAR stands for SOund Navigation And Ranging.
It is a technique that uses sound propagation to navigate, measure distances (ranging), communicate with or detect objects on or under the surface of the water, such as other vessels.
SONAR systems work by sending out sound waves and then detecting the echoes that return from objects.
The waves used in SONAR are ultrasound waves, which have frequencies higher than the upper audible limit of human hearing (typically above 20 kHz).
Ultrasound is used because its shorter wavelength provides greater resolution and detail in detection, and it is not disturbed by or does not disturb audible marine life or human activities.