Page 639 eadio acoustic ranging 6814 



error may be made (see 6852) . The chronograph attendant should be notified of any- 

 such error as soon as it is discovered at the plotting station and should be requested 

 to check the questionable data, so that appropriate corrections may be made in the 

 plotting and in the Records. 



6814. The Bisectrix 



Between the positions of any two R.A.R. stations there is a line, known as the 

 bisectrix, near which the radio returns from the two stations will be received so nearly 

 simultaneously that it will be difficult to identify the station from which each was 

 received. The officer-in-charge should avoid, so far as possible, obtaining an R.A.R. 

 position near a bisectrix. 



The bisectrices between all R.A.R. stations should be shown on the boat sheet by 

 drawing perpendiculars at the midpoints of the lines joining the stations. When 

 the officer-in-charge knows that a position at the regular time interval would plot near 

 a bisectrix, he should advance or delay it enough to ensure that both of the returns 

 will be received and can be identified. The officer-in-charge should also notify the 

 chronograph attendant in advance which station, of two that will be received almost 

 simidtaneously, should be received first. 



It is possible to reduce the lengths of the radio signals from the R.A.R. stations so 

 that returns from two or more stations may be recorded within a few tenths of a second 

 of one another (see 6432). 



6815. Trial Position 



Trial positions are frequently needed in maneuvering the vessel to the proposed 

 line at the beginning of operations, and are a necessity at the .beginning of operations 

 after drifting at sea during the night. Trial positions where the dead-reckoning position 

 is not even approximately known are sometimes difficult to plot because of the difficulty 

 of identifying the R.A.R. stations. In such a case each R.A.R. distance should be 

 trial plotted from each R.A.R. station, and an intersection found where the sounding 

 at the time of the trial position agrees with the depths on the boat sheet. If only two 

 returns are received at a trial position, and sometimes even where thi'ee or more returns 

 are received, the plotted distances will intersect at two or more positions, of which 

 selection can be made only by a Verification of the depth. 



Trial positions need not be recorded in the Bomb Record, nor in the R.A.R. 

 abstract unless desired, for a permanent record of them is not required. 



6816. Additional Data From Shore Stations 



Where R.A.R. shore stations are used, the radio teclmician on board the vessel 

 should call each shore station by radio after each position to ascertain the intensity of 

 the bomb sound as indicated on the milliammeter in the output circuit of the amplifier. 

 This reading is a comparative measure of the intensity of the sound arriving at the 

 hydrophone and, together with the size of the bomb and distance from the station, 

 forms a valuable record that may be used to determine the sizes of bombs needed under 

 various conditions. These data for each station should be recorded in the ''Remarks" 

 column of the Bomb Record. (See 641.) 



682. Plotting R.A.R. Positions 



The data from which an R.A.R. position is plotted are the elapsed times required 

 for subaqueous sound to travel from the position of the explosion to the several R.A.R. 



