-BS HYDROGRAPHIC MANUAL PaGE 636 



long ring. This signal must be given a sufficient number of seconds in advance of the 

 desired time of position to allow for lighting the bomb and throwing it into the water. 

 This time interval will vary, depending on the length of fuse and the alacrity of the 

 bomber. Just as the bomb strikes the water, the bomber signals with one long ring. 

 This is the official time of the position. The signal is received at the chronograph 

 station where the chronograph is started, and on the bridge where it serves as the 

 "mark" for any observations which are to be made on the position. 



68. R.A.R. OPERATIONS 



681. R.A.R. Positions 



The essential difference between a hydrographic survey controlled by R.A.R. and 

 one controlled by other methods is the manner in which the positions of the survey 

 ship are determined. The positions are not based on sextant angles or other visual 

 observations, but are determined by plotting graphically the distances of the ship from 

 two or more R.A.R. stations whose positions are loiown. The distances are derived 

 from measurements of the travel times of subaqueous sound from a source of sound near 

 the ship to receiving units at the several R.A.R. stations. The various operations in- 

 volved in obtaining an R.A.R. position are necessarily performed at several different 

 places on the ship, and the procedure is, therefore, more complicated than for any of 

 the other methods described in section 33. The positions must be anticipated and 

 preparations made, and the position data are not available for plotting until about 5 

 minutes after the time of the position, all of wliich affect the conduct of the operations. 



6811. Time of Position Data 



The official time of an R.A.R. position, and the time to be recorded in the Sounding 

 Record and Bomb Record, is the exact time that the bomb strikes the water when it 

 is thrown overboard — not the time of its explosion (see 6853 {d)). The position that will 

 be determined is that of the bomb — not that of the vessel at the time of bomb explosion. 



Positions should be obtained at equal intervals and on the minute or half-minute, 

 if practicable. This is not essential, but it facilitates spacing recorded soundings, wliich 

 are generally recorded on the minute or fractions thereof. With experienced personnel, 

 bombs may be timed to strike the water within a few seconds of a desired time without 

 danger, in which case the positions may be recorded on the even time interval. 



The clock at the chronograph station should be synchronized with the sounding 

 clock so the times of positions recorded in the Sounding Record and the Bomb Record 

 will be identical. If they are not automatically synchronized, they should be checked 

 frequently to ensure that the two clocks are always within 2 or 3 seconds of one another. 



The sounding on the position mustl)e obtained prior to the explosion of the bomb, 

 A sonic oscillator must be turned off while the bomb explosion is being recorded, because 

 the oscillator signals would record on the tape. Furthermore, the explosion of a bomb 

 causes a multitude of sound waves which are registered on any echo-sounding instru- 

 ment and wliich obliterate the echo soundings for an appreciable length of time. 



Log readings, revolution counter readings, bearings, sextant angles, and any other 

 supplemental position data are observed at the signal from the bomber which indicates 

 that the bomb has struck the water. These supplemental data must be recorded in the 

 Sounding Record. If the R.A.R. abstract is used (see 8312), the time intervals are 

 recorded in it when they are received from the chi'onograph attendant, and any of the 

 supplemental data needed for plotting on the boat sheet are also recorded in it. 



