Radio-Acoustic-Ranging (RAR) navigation, developed within the United States Coast and Geodetic Survey, was the very first navigation system to divorce itself from the necessity to visually see either fixed known objects on land for inshore piloting navigation or astronomic bodies for celestial navigation. As such, it was the revolutionary first step on the 70-year road that has culminated in the development and widespread use of the Global Positioning System for land, sea, air, and near-Earth space navigation.

First discoveries of PIONEER and GUIDE Seamounts. Seamounts discovered by PIONEER and GUIDE.

RAR, like many other technological advances in the marine sciences, had its roots in the sinking of the TITANIC. The invention by Reginald Fessenden of a means to transmit and receive sound waves in the sea was the seed that led to the invention of depth-finding sonars and other types of sonars for looking ahead of a vessel and out to the sides of a vessel. World War I gave impetus to this field of endeavor as German submarines were prowling shipping lanes and sinking American and British shipping at an alarming rate. It was imperative to devise means to discover these menaces and take action to destroy them.

As part of this effort, Commander Nicholas Heck of the Coast and Geodetic Survey was attached to a Coast Artillery Unit in the New York area that was engaged in conducting various experiments utilizing underwater sound for the discovery of undersea objects. Heck, who was the leader in the evolution of wiredrag and wiresweep technology, quickly grasped the power of sonar as an underwater search tool. But additionally, because his background involved the precision navigation of surveying vessels, he saw the possibility of determining the ranges of a sound source from two or more fixed listening devices and fixing the position of that sound source. This was the principle which developed into the RAR method of navigation. The method devised for RAR navigation was to drop a TNT bomb off the stern of the surveying vessel and listen for its explosion at the ship and at two or more fixed hydrophone locations. When the fixed hydrophones received the signal from the explosion, this activated a radio transmitter that would immediately transmit the reception of the signal back to the surveying vessel. Given the distances involved, anywhere from a few miles out to 200 miles, radio transmission time was considered instantaneous. Thus by knowing the time of the explosion at the ship and comparing it to time of radio signal reception from a fixed hydrophone, the travel time of the sound wave from explosion to the fixed hydrophone could be determined. With knowledge of the velocity of sound in the surrounding seawater, wave travel time multiplied by velocity of sound in seawater would give the distance. The intersection of two or more distances (ranges) would fix the position of the surveying vessel at the time of the explosion. This method was first used in 1924 by the Coast and Geodetic Survey Ship GUIDE off the coast of Oregon.

After the initial tests, Heck turned further development of the method over to Robert F. Luce, the commanding officer of the GUIDE. Within two years, RAR reception had extended 206 miles offshore giving the first inkling of the SOFAR layer. RAR as a system was a step in the development of: 1) radionavigation systems; 2) automated offshore telemetering buoys for oceanographic and meteorological studies; 3) further refinement in the understanding of the sound velocity structure of the ocean; and 4) seismic reflection and refraction profiling.

As a navigation method, it was used until the early years of the Second World War when it was superceded by pure radio-navigation methods devised to navigate both Allied and Axis bombers. The 30 photos and diagrams in the Radio-Acoustic-Ranging section of the Coast and Geodetic Survey album commemorate this first non-visual navigation system. RAR should be remembered as a major step forward in the development of electronic navigation systems, as a forerunner of many modern oceanographic telemetering systems, and as a stepping-stone on the path to marine seismic surveys.