Page 557 radio acoustic ranging 613 



which time intervals are recorded. The sound of the explosion travels in all directions 

 and on arrival actuates each receiving unit so that a radio signal is automatically 

 transmitted. The several radio signals are recorded on the vessel on the same tape 

 with the explosion. The travel times to the respective receiving units are scaled from 

 the chronograph tape. These travel times are converted into distances by applying 

 the effective velocity of sound in sea water and, if the positions of the receiving units 

 are known and they are appropriately located, the position of the vessel may be deter- 

 mined from two or more of them. 



613. Practical Use of R.A.R. 



In other methods of horizontal control described in section 33, the survey party 

 must either have good visibility for the angle measurements required to locate the 

 soundings accurately, or be satisfied with relatively poor locations based on dead 

 reckoning, perhaps supplemented by astronomic sights. 



Before the development of R.A.R. it had been generally accepted that soundings 

 could not be located accurately in horizontal position during periods of low visibility, 

 and in areas beyond the visibility of shore signals. Using R.A.R., accurate hydro- 

 graphic surveys can now be conducted, regardless of the visibility, continuously day 

 and night, and if combined with a system of buoy control (see section 25), such surveys 

 can be extended to a distance from shore that appears to be practically unlimited. 



The accuracy of any method used to control hydrographic surveys limits its 

 practical use. In R.A.R., instrumental equipment and field methods have been 

 developed to where the time intervals can be very accurately measured. The velocity of 

 sound, which must be known to compute the distances, is generally difficult to deter- 

 mine and this, combined with an inadequate knowledge of the character of the propa- 

 gation path of the sound in the medium, usually limits the accuracy. The character 

 of propagation may also limit the distance at which R.A.R. is effective, but if a sufficient 

 number of receiving units are used and their sites are carefully selected, the method 

 may be extended to control the hydrographic survey of any ocean area beyond the 

 visible limits of shore signals. 



62. PHYSICS OF SOUND 



The Radio Acoustic Ranging (R.A.R.) method of controlling hj^drographic survej^s 

 is dependent on the transmission of sound through sea water. The propagation of 

 sound through air has been the subject of extensive studies in the past — but its 

 propagation through water has been less extensively mvestigated, because of its com- 

 paratively limited application. The subject of soiuid transmission through water is 

 treated in a general way in a number of publications, but complete information dealing 

 specifically with the horizontal transmission of sound through water is not available 

 in any one of them. 



The recent extensive use of the R.A.R. method of control by the Coast and Geodetic 

 Survey has disclosed the limited knowledge of the subject. A thorough mvestigation 

 of it is difficult because of its complicated nature and would be costly because it would 

 require elaborate instrumental equipment and observations from expensively operated 

 vessels for long periods of time in order to investigate all of its ramifications. With the 

 exception of a few planned experiments, all data on the subject particularly applicable 

 to R.A.R. have been obtained during the survey operations of the Bureau in various 

 offshore localities. A knowledge of it has little practical application except in sub- 



