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 true range multilateration 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.
Technique
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 , 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 station ships, or 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 – 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 true range multilateration to fix the ship's position.
In deep waters, such as those that prevailed in the Pacific Ocean along the United States West Coast, the Coast and Geodetic Survey could rely upon shore stations to support radio acoustic ranging because the deep water allowed sound to travel to the coast. Along the United States East Coast, where shallower waters prevailed, sound had greater difficulty in reaching the coast, and the Coast and Geodetic Survey relied more heavily on anchored station ships, and later moored buoys, to support radio acoustic ranging.
Chronographs recorded times to the hundredth of a second, and the crew of a ship using radio acoustic ranging could determine their ship's distance from the remote hydrophone stations to within , allowing them to plot their ship's position with great accuracy for the time. With sound waves traveling from the point of the explosion to the distant hydrophones at about , ships occasionally used radio acoustic ranging at distances of over between ship and hydrophone station, and distances of were common.
Development history
Precursors
Radio acoustic ranging had its origins in a growing understanding of underwater acoustics and their practical application during the early decades of the 20th century, and developed in parallel with echo sounding. The first step took place in the early 1900s, when the Submarine Signal Company invented a submarine bell signalling device and a hydrophone that could serve as a receiver of the underwater sounds the bells generated. The crew of a ship equipped with the receiving hydrophone could plot their ship's distance from the submarine bell mechanism and plot intersecting lines from two or more bells to determine the ship's position. The bells were installed at lighthouses, aboard lightvessels, and on buoys along the coasts of North America and Europe, and receiving hydrophones were mounted aboard hundreds of ships. It was history's first practical use of acoustics in an ocean environment.