Page 593 radio acoustic ranging 6631 



For plotting R.A.R. positions, the effective horizontal velocity is required (see 63), 

 and this is more difficult to determine. Sound is rarely transmitted through sea water 

 along a straight horizontal path — it is usually reflected and refracted depending on the 

 velocity gradient (see 623) . The velocity of the sound wave at any instant is a function 

 of the temperature, salinity, and depth of the water at that instant. In a heterogeneous 

 medium, such as sea water, the velocity of the sound wave is continually changing as 

 it is propagated forward. The velocity along the path of the wave is the mean of the 

 different instantaneous velocities. But if this velocity were multiplied by the elapsed 

 time interval measured, the distance obtained would be the length of the path of the 

 wave, whereas what is required in R.A.R. is the horizontal distance. An arbitrary 

 value of velocity must often be used, which when multiplied by the travel time of the 

 sound wave will give the true horizontal distance between the bomb and the R.A.R. 

 station. Such a velocity is termed an apparent horizontal velocity. To determine it 

 for all conditions encountered in R.A.R. is not a simple matter, and empirical methods 

 must often be used. 



Where it is possible to determine an apparent horizontal velocity experimentally, 

 by measuring the travel time of the sound wave over a known horizontal distance, 

 such a velocity can be used if the distances and depths in the area being surveyed are 

 similar to those where the tests were made, and the velocity gradient approximates 

 the original. Otherwise a comparison of the apparent horizontal velocities with the 

 computed theoretical velocities based on the physical characteristics of the water at the 

 time of the tests must be used as a basis for correcting theoretical velocities (see 6343). 

 Sometimes theoretical velocities at selected depths will be found applicable. 



A method is needed by which the elapsed times along the actual sound path may 

 be readily corrected from a knowledge of the velocity gradient and the approximate 

 depths, so that the corrected times may be used with mean computed velocities to 

 plot R.A.R. distances. Until this problem has been satisfactorily solved, empirical 

 methods such as the above must be used to determine velocities for use in plotting 

 R.A.R. distances. 



6361. General Procedure 



The velocities to be used in plotting any R.A.R. survey shall be determined as 

 follows: 



(1) The apparent horizontal velocity shall be accurately determined by tests, during which 

 temperature, salinity, and depth measurements shall be made. 



(2) Velocity-depth curves (see fig. 126) shall be drawn from the data on Form 717 (see 6343) 

 and the velocity gradients shall be studied to determine the most probable path of the sound wave 

 for that season, not only along the track where the tests were made but throughout the area. 



(3) The apparent horizontal velocities determined by test shall be compared with the corre- 

 sponding theoretical velocities computed from the tables (see 6343), to verify the probable path as 

 determined in (2) above, and to determine for what depths to compute theoretical velocities and 

 what arbitrary corrections, if any, to apply to them. 



(4) Apparent horizontal velocities determined by test may be used for plotting where the temper- 

 atures and sahnities vary only slightly from those observed at the time of the test, and for distances 

 and in depths similar to those where the tests were made. 



(5) Computed theoretical velocities, sometimes based on an arbitrarily chosen depth or arbitrarily 

 corrected, shall be used to plot all R.A.R. for which (4) above does not apply (see 6362). 



6362. Specific Rules 



In connection with 6361(5) it is necessary to determine whether to use theoretical 

 velocities computed for an arbitrarily chosen depth; mean velocities from surface 



465382 — 44 39 



