612 HYDROGRAPHIC MANUAL PaGE 556 



The submarine menace of World War I made urgent the development of a practical 

 method of detection, which led to intensive study of the transmission of sound through 

 sea water in an effort to solve the problem. An organization of the best scientists, 

 working with the military services, added greatly to the knowledge of the subject. 

 Instrumental equipment for, and methods of, transmitting and receiving subaqueous 

 sound were perfected. Instruments for the specific purpose of measuring the time of 

 transmission from source of sound to receiver were also developed. 



After World War I, the United States War Department continued to study the 

 transmission, reception^ and velocity of sound, principally in the waters of Fishers 

 Island Sound. Small bombs, suspended below targets whose positions could be deter- 

 mined, were exploded and the arrival of the sound wave was recorded at shore stations 

 connected by cable to submerged hydrophones at various distances from the source. 

 The results obtained encouraged the Coast and Geodetic Survey to try to modify the 

 method so it could be used to control hydrographic surveys. 



The first experimental work in the development of R.A.R. was performed in collab- 

 oration with the War Department in Fishers Island Sound in October and November 

 1923. The first time-measuring apparatus was based on the equipment developed by 

 the United States Bureau of Standards for the longitude observations of the Coast and 

 Geodetic Survey, modified for use on board ship. In the initial phase of the develop- 

 ment, the Bureau of Standards collaborated in the design of the instrumental equipment, 

 while the Coast and Geodetic Survey developed the practical technique of operation 

 for use in hydrographic surveys. The apparatus was field-tested and the survey 

 routine was developed on the Ship Guide off the coast of Southern California in Feb- 

 ruary and March 1924, and by the end of March the method was in actual use, although 

 improvements continued to be made. 



612. Theory of R.A.R. 



There are several general methods of sound ranging to determine the position of 

 an unknown point: 



(a) The arrival time of a sound signal may be observed at three or more time-coordinated 

 receiving units at known positions. The differences of the arrival times at the various receiving 

 units can be used to derive the position of the source. This is known as the differential method and 

 is in general military use to determine the positions of enemy gun emplacements. 



(6) The travel time of a sound, synchronized with a radio signal at the source, may be observed 

 at two or more shore stations at known positions. The time interval measured at each shore station 

 may be radioed to the survey vessel where the position of the source may be determined. 



(c) A sound signal may originate at the vessel, where the time of its origin is recorded. The 

 arrival times of the sound at two or more receiving stations are transmitted automatically by radio 

 and received and recorded on board the vessel. The time required for the acoustic wave to travel to 

 each receiving station may be scaled from the record and the position determined from these data. 



The accuracy of these methods depends on the position accuracy of the receiving 

 units, the accuracy with which the time intervals are measured, the accuracy with 

 which the velocity of sound is known, and the diagnosis of the path along which the 

 sound wave is propagated in reaching each receiving unit. 



For most efficient use in hydrographic surveying, observations taken for the 

 determination of the ship's position must be controlled on board and the observed data 

 must be available in the shortest possible time after the observations. Because of 

 these requirements the method outlined in (c) is the most practicable for use in R.A.R. 



The subaqueous sound is made by exploding a small TNT bomb in the water near 

 the survey vessel. This explosion is recorded on the vessel on a chronograph tape on 



