Page 555 . 61 



CHAPTER 6. RADIO ACOUSTIC RANGING 

 61. GENERAL STATEMENT 



Radio Acoustic Ranging (R.A.R.) is a method of control used in liydrograpliic 

 surveys for determining the position of the survey vessel by indirect measurement of 

 two or more distances. The distances are derived from the accurate measurement of 

 the times required for a subaqueous sound, originating at the position to be determined, 

 to reach two or more suitably located receiving units whose geographic positions are 

 known. The distance from the source of sound to each receiving unit is determined by 

 measuring this travel tune and multiplying it by the effective velocity of sound. With 

 two or more distances thus determined to receiving units suitably located, the position 

 of the survey vessel is then located on the survey sheet at the intersection of arcs con- 

 structed with these distances as radii. R.A.R. is used in areas beyond the visibility of 

 shore signals and where the use of survey buoys is not practicable or is unnecessary. 

 It is independent of the conditions of visibility and, therefore, may be used at night or 

 during times of reduced visibility. 



611. History of Development 



Subaqueous sound was first used in navigation to determine the direction of an 

 underwater source of sound from a vessel equipped with two hydrophones (subaqueous 

 sound receivers), one on each side of the vessel near the bow. A patent was granted 

 for this device in 1894. The direction was found by varying the heading of the vessel 

 until subaqueous sound of equal loudness was heard simultaneously in earphones, one 

 connected to each hydrophone. Probably the first practical use of subaqueous sound 

 to determine horizontal distances at sea was in connection with the use of the submarine 

 bell as an aid to navigation during fog. Submarine bells, suspended 25 to 30 feet below 

 a lightship, were in general use by the United States Lighthouse Service in 1906. The 

 submerged bell was synchronized to strike simultaneously with the blast of a fog whistle 

 in air, and from the interval of tune between the reception of the subaqueous sound 

 through the water and the reception of the fog whistle through the air a mariner could 

 determine his distance from the lightship. A radio signal w^as successfully substituted 

 for the w^histle signal in experiments conducted in September 1911. The principle 

 upon which those experiments were based is very similar to that utilized in R.A.R. 

 by the Coast and Geodetic Survey, but the method was never extensively used in navi- 

 gation because of the rapid development and use of radio direction finding. 



It is interesting to note that the development of echo-sounding methods to measure 

 depths of water resulted from an attempt to develop a method of measuring horizontal 

 distances by subaqueous sound. Although much experimental work had been done 

 previously (see 512), it was the sinking of the Titanic in 1912 that emphasized the 

 need for a method of determining the existence of icebergs in the track of a vessel and led 

 to the improvement of instruments designed to produce and to detect subaqueous sound. 

 These instruments, however, found their greatest application in measuring depths of 

 water and future development was toward perfecting them for this purpose. 



