What is $SONAR$? For what is it used? Explain its working in brief.

(N/A) $SONAR$ stands for Sound Navigation and Ranging. It is a technique used to determine the depth of the ocean and to locate underwater objects such as reefs,submarines,shipwrecks,etc.
Working:
In this method,a strong ultrasonic wave is sent from a transmitter installed on a ship towards the bottom of the ocean. This ultrasonic wave is reflected back after hitting the ocean bed and is received by a detector (receiver) on the ship.
The time interval $t$ between the transmission and reception of the ultrasonic wave is recorded.
If $V$ is the speed of sound in seawater,then the total distance $2d$ covered by the sound wave (going to the bottom and coming back) is given by $2d = V \times t$.
Therefore,the depth $d$ of the ocean is given by $d = (V \times t) / 2$.