(N/A) $SONAR$ stands for $Sound$ $Navigation$ $And$ $Ranging$.
It is a device that uses ultrasonic waves to measure the distance,direction,and speed of underwater objects.
Working: $A$ sonar device consists of a transmitter and a detector installed in a ship or boat. The transmitter produces and transmits ultrasonic waves. These waves travel through water and strike the object on the seabed. After striking,the waves are reflected back and are sensed by the detector. The detector converts the ultrasonic waves into electrical signals which are appropriately interpreted.
Calculation: If the time interval between transmission and reception of the ultrasound signal is $t$ and the speed of sound through seawater is $v$,then the total distance $2d$ traveled by the wave is given by $2d = v \times t$,where $d$ is the depth of the object.
Applications: $1$. Used to determine the depth of the sea. $2$. Used to locate underwater hills,valleys,submarines,icebergs,and sunken ships.