Page 123 control and signal building 2534 



bombs should not he iisod because of possible damage to the ship's hull. Bombs 

 containing one-cpiarter pint of TNT have been used while lying-to, without apparent 

 damage to the ship. Larger bombs should be used only while underway. 



Acoustic positions are determined underway while steaming on a line between the 

 two R.A.R. stations, or near the station to be located. In the latter case a figure-eight 

 course is steered toward and away from the station, passing it on the down-current 

 side at a distance of 15 to 25 meters and dropping the bomb near the buoy each time 

 the bombing station on board comes abeam of the buoy. 



Wlien the data have been obtained by this method, a graphic analysis of the dis- 

 tances is almost mandatory, especially if the velocity of sound is not accurately known or 

 is likely to be variable. In this analysis the distance arcs, determhied by multiplying the 

 mean R.A.R. time intervals by the velocity, are plotted with a beam compass on an 

 accurate projection on a nearly distortionless sheet (see 713). In practice all the dis- 

 tance arcs will rarely intersect at a point, and considerable judgment is necessary in 

 analyzing them before deciding on the most probable location of the station. All other 

 factors being eciual, the distances should be weighted according to their relative lengths, 

 the shorter being considered the more accurate. The copies of Form. 715 on which the 

 time intervals were recorded should be referred to when determining the relative relia- 

 bility of the distances. That series of R.A.R. time intervals which shows good agree- 

 ment should be given more weight than results which are erratic. A study of a series 

 of erratic time intervals will often disclose certain returns which may be rejected to 

 change the mean so that the distance will plot correctly. Such rejections should be 

 made with caution, for a series of time intervals which are in agreement may possibly 

 be in error for some unsuspected reason. This might result in changing the mean of a 

 series of returns to bring it into agreement with an erroneous series, which would result 

 in the selection of an incorrect location. 



After the graphic analysis the geographic position of the station may be deter- 

 mined graphically or by computations. The latter is generally not necessary unless 

 the scale of the graphic determhuition is smaller than that of the survey, in which case 

 there would be a loss ui accuracy in transferring the position to the sheet with the 

 larger scale. 



2534. Subaqueous Distances From Three-Point Fixes 



The position of an R.A.R. station, within sight of shore stations, may be deter- 

 mmed by a series of simultaneous three-point sextant fixes and short subaqueous 

 distances at successive positions encircling the station. Four to six sextant fixes should 

 be taken Iji to 2 miles distant and in different directions from the R.A.R. station so as 

 to furnish a good intersection of distance arcs at the station. 



This method is especially useful to locate shore station hydrophones anchored at 

 positions close inshore where strong sextant fixes are not available. 



The position of an R.A.R. station so located should be determmed by a graphic 

 plot on an aluminum-mounted sheet. Each three-point sextant fix is plotted and from 

 that position as a center the distance arc to the R.A.R. station is drawn after it has 

 been, computed from the velocity of sound. The velocity should be based on the sur- 

 face or near-surface temperature and salinity, for the sound will hav^ traveled this 

 short distance along a direct path near the surface (see 622). If the correct velocity 

 has been used and all other data are accurate the distance arcs will intersect at a point, 

 which will be the position of the R.A.R. station. If an erroneous velocity has been 

 used the distance arcs will not intersect, but the true position of the station will be 



