People at sea use sonar to measure distance with echoes. Sonar sends sound pulses through water and measures how long they take to bounce off an object or the seabed and return.
The system calculates range from the travel time and the speed of sound in seawater, which averages about 1,500 metres per second. Because the pulse travels to the target and back, the measured distance is based on half the total journey. Sonar can locate vessels, submarines, rocks, ice, fish schools and underwater structures.
A closely related device is an echo sounder, a type of active sonar used especially to measure water depth. It points a sound pulse downward and calculates the seabed’s distance from the vessel; this process is also called echo sounding. Alexander Behm received a German patent for an echo-sounding device in 1913, while early active sonar research accelerated during World War I as navies sought ways to detect submarines.
A common mix-up is radar: radar uses radio waves in air, whereas sonar uses sound waves underwater. Passive sonar only listens to sounds made by other objects, so measuring distance with returned echoes specifically involves active sonar.