In echo-ranging, the principle of echo-location is used.
It is also called SONAR (SOund NAvigation and Ranging).
In this method, a transmitter installed on the ship transmits ultrasonic sound waves into the sea.
The ultrasonic sound waves reflect back from the sea-bed and reach the receiver installed on the ship.
The time difference is noted.
Since we know the velocity of sound in seawater, the distance travelled by the sound wave in that time is calculated.
The depth of sea is calculated by halving the total distance travelled by the sound wave.