224 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1944 
data are recorded thereon and become available to the hydrographer 
in the shortest possible time. This method avoids errors made in 
transmission, which are always possible where the data are received 
elsewhere and radioed to the vessel. 
R. A. R. OPERATION 
To determine a position by R. A. R., the following data must be 
known: The receiving stations (that is, the hydrophones) must be 
established at known positions. The travel times of the subaqueous 
sound from its origin to its receipt at each receiving station must 
be measured with an accuracy of about 0.01 second. The effective 
horizontal velocity of sound must be known. (The travel path of the 
sound is not necessarily a straight line, as is explained later, but to 
determine horizontal distances by R. A. R., it is obvious that the 
velocity of sound that is required is the horizontal distance divided 
by the travel time.) 
A sheet with a projection is prepared on which the positions of the 
receiving stations are plotted. The measured travel times are multi- 
plied by the effective horizontal velocity of sound to obtain the hori- 
zontal distances between the sound source and the receiving stations. 
The position of the sound source is then at the intersection of the arcs 
drawn from the stations with the computed distances as radii. 
The following description illustrates briefly how R. A. R. is used by 
the Coast and Geodetic Survey: A subaqueous sound is produced by 
the explosion of a TNT (trinitrotoluene) bomb thrown overboard 
from the survey ship while under way. A hydrophone (subaqueous 
receiving unit) in the hull of the ship, and a radio receiver on the ship 
are connected to a chronograph. The receiving station consists of a 
submerged hydrophone connected to a radio transmitter which oper- 
ates automatically when the hydrophone is actuated by a subaqueous 
sound. 
In addition to the ordinary survey personnel, certain specialists are 
required in R. A. R. One officer is in direct charge of all operations; 
he plots the ship’s positions as determined from the R. A. R. data. 
A chronograph attendant is in charge of the chronograph and oversees 
its functioning during the time from the bomb explosion to the receipt 
of the radio signals. A radio technician is in charge of the instru- 
mental equipment on the survey ship; he attends to the proper tuning 
of the radio receiver and assists the chronograph attendant in identify- 
ing the radio signals from the receiving stations. An explosives expert, 
called a bomber, is in charge of the explosives and the preparation of 
the bombs; he lights the bomb and throws it overboard when instructed 
to do so by the chronograph attendant. 
