Page 595 radio acoustic ranging 64 



depth. For areas and seasons where this is proved by test, use the computed velocity for the selected depth. 

 The basis for this is that the increase in the length of the sound wave path is nearly compensated for 

 by the difference between the mean computed velocity and the computed velocity at the selected depth. 

 In certain areas where there are temperature reversals, it has been found that the depth at which a 

 marked change in the temperature gradient occurs will give this agreement. 



Data from R.A.R. survej's along the Pacific Coast and in Alaska, and for some areas on the Con- 

 tinental Shelf of the Atlantic Coast, indicate that for depths not exceeding 300 fathoms a very close 

 empirical relationship exists between apparent horizontal velocity determined by test and bottom 

 velocity, the latter being the average of the velocities computed for the bottom depths along the path. 

 The velocity gradient in such cases will generally be found to fall in the (d) category. Where tests 

 show that such relationship exists, the bottom velocity may be used to plot R.A.R. For depths 

 greater than 300 fathoms, the velocity at 300 fathoms is used. 



In order to obtain satisfactory results from R.A.R., the hydrographer must be fami- 

 liar with the theory of sound propagation; only then will he be able to evaluate the 

 results of tests made to determine apparent horizontal velocity, and to select the most 

 practicable means of determining the correct velocities to use in plotting. In general, 

 seasonal and regional changes of temperature and salinity require changes in the plotting 

 velocity; other influences being constant, an increased depth generally requires a lower 

 plotting velocity; and frequently different values must be used for different distances 

 from R.A.R. stations. 



Regardless of how velocity is used in plotting R.A.R., for the sake of simplicity and 

 in order to eliminate some of the variables, average temperatures and salinities must 

 be assumed to exist throughout an area surveyed during a limited period of time 

 (see 5612). For most areas the errors introduced by these assumptions will be slight. 

 In some areas, however, there are large regional or seasonal changes of temperature and 

 salinity and in these, the regions and periods of time must be hmited to those during 

 which one set of average conditions may be applied. 



64. R.A.R. STATIONS 



Three different typos of receiving stations have been used by the Coast and Geo- 

 detic Survey in the R.A.R. method of control. In the chronological sequence in which 

 they were developed they arc; shore station, ship station, and sono-radio buoy. 

 Sono-ra,dio buoys have entirely taken the place of ship stations and have at least 

 narrowly restricted the use of shore stations. Each type of station has certain advan- 

 tages which make its use particularly advantageous under some conditions, but each 

 has also certain disadvantages which limit its use. 



641. Shore R.A.R. Stations 



The first R.A.R. stations were established on shore, and such stations may still 

 be found advantageous where deep water extends close to the shore in areas where 

 strong currents, prevail. A shore station may be preferable to a sono-radio buoy in 

 many localities on the Pacific Coast and in Alaska, but on the Atlantic Coast, where 

 there are extensive areas of shoal water, their use is generally inexpedient. The 

 selection of shore station sites, and the equipment and its installation are considered in 

 261. Only the advantages and disadvantages are considered here. 



Each shore station must have at least one radio technician to attend the station 

 and at isolated localities two men are generally required. This increases the operating 

 cost but affords several important advantages over an automatically operated station, 

 such as a sono-radio buoy. The technician can adjust the sensitivity of the apparatus 

 to obtain the best reception for the temporary conditions; he can usually repair any 



