The word 'SONAR' stands for Sound navigation and ranging.
The figure given below shows the principle of a SONAR in which, ultrasonic waves are sent in sea water in all directions, from the ship. These waves are received after reflection from an obstacle such as the enemy submarine, iceberg, sunken ship etc. Hence, the distance between the ship and the obstacle can be determined.
To find the distance of obstacle from the ship the time interval (t) between the instant when waves are sent and the instant when waves are received, after reflection from the obstacle is measured.
The distance d of the obstacle from the source is then —
where, V is the speed of ultrasonic wave in water.
The depth of sea can also be found by this method. The process is then called echo depth sounding.