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.
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.
