Radio acoustic ranging

Radio acoustic ranging, occasionally written as "radio-acoustic ranging" and sometimes abbreviated RAR, was a method for determining a ship′s precise location at sea by detonating an explosive charge underwater near the ship, detecting the arrival of the underwater sound waves at remote locations, and radioing the time of arrival of the sound waves at the remote stations to the ship, allowing the ship′s crew to use triangulation to determine the ship′s position. Developed by the United States Coast and Geodetic Survey in 1923 and 1924 for use in accurately fixing the position of survey ships during hydrographic survey operations, it was the first navigation technique in human history other than dead reckoning that did not require visual observation of a landmark, marker, light, or celestial body, and the first non-visual means to provide precise positions. First employed operationally in 1924, radio acoustic ranging remained in use until 1944, when new radio navigation techniques developed during World War II rendered it obsolete.

To fix their position using radio acoustic ranging, a ship′s crew first ascertained the temperature and salinity of sea water in the vicinity of the ship to determine an accurate velocity of sound through the water. The crew then threw a small TNT bomb off the ship′s stern. It exploded at a depth of about 100 feet (30 meters), and a chronograph aboard the ship automatically recorded the time the explosion was heard at the ship. The sound traveled outward from the explosion, eventually reaching hydrophones at known locations – shore stations, anchored manned station ships, or unmanned moored buoys – at a distance from the ship. Each hydrophone was connected to a radio transmitter that automatically sent a signal indicating the time its hydrophone detected the sound. At the distances involved – generally less than 200 nautical miles (370 km) – each of these radio signals arrived at the ship at essentially the same instant that each of the remote hydrophones detected the sound of the explosion. The ship′s chronograph automatically recorded the time each radio signal arrived at the ship. By subtracting the time of the explosion from the time of radio signal reception, the ship′s crew could determine the length of time the sound wave required to travel from the point of the explosion to each remote hydrophone and, knowing the speed of sound in the surrounding sea water, could multiply the sound′s travel time by the velocity of sound in sea water to determine the distance between the explosion and the hydrophone. By determining the distance to at least two remote hydrophones in known locations, the ship′s crew could use triangulation to fix the ship′s position.

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