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In SONAR (sound navigation and ranging), ultrasonic waves are sent in all directions from the ship and they are then received on their return after reflection. If there is some obstacle such as an enemy submarine, iceberg, a sunken ship etc. , its distance from the source can be calculated by measuring the time interval t between the instant when waves are produced and the instant when waves are received after reflection. The distance of the obstacle from the source is then d = Vt/2 where V is the speed of ultrasonic waves in water. The depth of the sea can also be found by this method.
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