Page 145 control and signal building 2574 



where subaqueous distances may be reliably measured, but its accuracy depends pri- 

 marily on the accuracy with which the velocity of sound in sea water may be deter- 

 mined (see 635). In an area of moderate and uniform depths, where the velocity may 

 be determined within 2 meters per second, positions of the control stations sufficiently 

 accurate for the survey of an offshore area may be obtained by this method. 



The scheme, as illustrated in figure 41, may be compared to a system of triangu- 

 lation quadrilaterals in which the lengths of all sides and diagonals are measured, 

 instead of the angles. Instead of being planned for intervisibility between stations, the 

 scheme must be planned so that the distances between stations in the same quadrilateral 

 are considerably less than the maximum expected to be reliably measured by subaque- 

 ous sound ranging. 



--. 5 



y^---. 6 



20 30 



Approximate Scale 



Figure 41.— Sono-radio buoys located by acoustic distances. 



Sono-radio buoys are anchored at stations A and 5 at a suitable distance offshore 

 and located by subaqueous distances from successive ship positions 1 to 5 determined 

 by three-point sextant fixes on shore stations (see 2534). This establishes a base AB 

 whose length is further checked by subaqueous distances measured at both ends of the 

 line (see 2533), and from which the scheme is extended into the offshore area. Addi- 

 tional sono-radio buoys are anchored to form quadrilaterals with sides of slightly unequal 

 lengths, so that, when measured, the bomb returns will not be recorded simultaneously 

 on the chronograph tape (see 6814). It is desirable that the time intervals be meas- 

 ured over each fine in both directions in order to obtain a double determination of the 

 distance. This will generally be possible, but it will require at least six sono-radio 

 buoys, so that one may be placed at each station of two adjacent quadrilaterals. After 

 the required time intervals have been measured in adjoining quadrilaterals, the two 

 inshore sono-radio buoys may be replaced by ordinary buoys and anchored at the 

 next offshore stations. If stable velocity conditions exist, which are likely to worsen 

 later, this procedure should be continued until all of the stations which will be required 

 in the area have been determined. 



It is usually neither necessary nor practicable to connect both ends of a scheme of 

 this type to shore stations for it generally extends into deep water where less accuracy 

 may be tolerated. If it is feasible to do so, however, the scheme should be extended 

 to another shore connection, in which case a proportional adjustment of the closing 

 error should be made. It might be possible to extend the scheme in tlie same general 

 direction to a land area with control stations computed on the same datum; or it might 



465382—44 11 



