Page 591 



RADIO ACOUSTIC RANGING 



6352 



6352. Frequency of Tests 



The number of apparent horizontal velocity tests required cannot be specified 

 without a knowledge of the area. A minimum of one or two tests per month should 

 suffice for areas where, and seasons when, temperature and salinity conditions are 

 relatively stable. But where and when temperature and salinity are variable, more 

 frequent tests will doubtless be needed to determine the velocity with the accuracy 

 specified in 6313. 



The hydrographer should not be content with satisfying the minimum require- 

 ments where additional tests can be made in conjunction with other operations, wiihout 

 delaying the progress of the work. During taut-wire measurements apparent hori- 

 zontal velocity tests should be made at every opportunity. Such tests should be 

 distributed regionally as far as practicable throughout the project area and made 

 at frequent intervals during the period of the survey. In buoy-controlled surveys 

 tests should be made at every op- ^ 



portunity over distances that have 

 been measured with taut wire. 



6353. Indirect Measurement of / 



Velocity 



i^/ 



N'?. 



Figure 132. — Velocity of sound determined from time intervals simulta- 

 neously measured to three known stations. 



Near the ofi'shore limits of some 

 R.A.R. sur\^eys it may be difficult 

 or impossible to determine the 

 apparent horizontal velocity of 

 sound from a known distance. It 

 is also possible that appreciable 

 errors may be introduced by plot- 

 ting R.A.R. positions in such 

 areas with an apparent horizontal 

 velocity determined in the inshore 

 areas, where the physical char- 

 acteristics and the depths of the water may be quite difl*erent. An apparent 

 horizontal velocity for use in such areas can be computed, if certain assumptions are 

 made. 



Where elapsed times have been measured from a bomb position to three R.A.R. stations, assuming 

 that the apparent horizontal velocity is the same over the three travel paths, the apparent horizontal 

 velocity may be computed without any knowledge of the three distances. In figure 132, A, B, and 

 C represent three R.A.R. stations, and D represents the position at which three elasped times are 

 measured. The distances o, b, and c between the R.A.R. stations, as well as the angles of the triangle 

 that they form, are known or may be computed. The distances AD, BD, and CD are unknown, 

 but the corresponding elapsed times ti, <2, and tz, required for a subaqueous sound wave to travel 

 from position D to the three R.A.R. stations, are known. From these data the apparent horizontal 

 velocity v may be computed from the following equation: 



,2/2. 



VH, 



.2p (p-^-rs) 2p-yJp^-\-2prs — r'^ — q — qs^ 



