2533 HYDROGEAPHIC MANUAL PaGE 122 



difficult to determine and, in addition, depends on an accurate measurement of the 

 time element (see 6352 and 685) . Because of the uncertain knowledge of the velocity, 

 distances should be determined from three or more R.A.R. stations and frequently it 

 is necessary to supplement the method by taut-wire measurements. 



The time intervals may be measured from a sound originating at the buoy w^hose 

 position is desired or at three or more stations of known positions. In the former, 

 R.A.R. stations are at the known positions and an ordinary buoy is at the position 

 to be determined and all three distances are measured simultaneously. In the latter, 

 an R.A.R. station is at the position to be determined and time intervals are measured 

 successively at each of the known positions. The latter method is advantageous when two 

 or more stations are to be located, the number that can be located being limited only 

 by the number of returns that can be recorded on a chronograph tape without confusion. 



In addition to the above methods, if both are R.A.R. stations the time interval 

 between them may be measured in other ways. The source of the sound may be located 

 at (1) a point directly on line between the tw^o stations or (2) at a point on the exten- 

 sion of the line betw^een stations. The first method is not recommended unless frequent 

 positions of the ship can be determined from other R.A.R. stations to ensure that the 

 ship is kept on the line. It is to be noted also that any error in the ship's run correction 

 is doubled. If the time intervals are so measured the distance between the stations 

 will be derived from the sum of the two time intervals. The second method is not 

 recommended for use unless experience has proved that short acoustic distances may 

 be accurately determined, although it does eliminate any error from the ship's run 

 correction. The acoustic position in the second case will probably be close to one of 

 the R.A.R. stations, and accurate measurement of short distances by acoustic methods 

 has been generally difficult (see 7636f/). If this method is used the desired time 

 interval between stations will be the difference between the two recorded time intervals. 

 To use either of these methods, fairly accurate dead-reckoning positions of the two 

 stations are reciuired, so the acoustic position may be located as nearly as possible in 

 line with the stations. 



In measuring a time interval in order to determine a subaqueous distance, bombs 

 must not be exploded at stations occupied by sono-radio buoys. A bomb explosion 

 near the hydrophone may cause the armature of the electromagnetic unit to become 

 displaced so that it is no longer balanced between the pole pieces (see 6565) and the 

 unit becomes inoperative. Before bombing at a station occupied by a sono-radio buoy, 

 it must be replaced with an ordinary buoy or, if a relieving buoy is used, the sono-radio 

 buoy must be removed and the bombs exploded at the relieving buoy. 



Regardless of the method used to determine the time interval, a series of three to 

 five measurements should be made to provide for the elimination of erratic results. If 

 reliable returns are difficult to obtain from a required station, it may be necessary to 

 obtain a much longer series. A single determination is seldom to be depended on because 

 of the imcertainties in the process. As each measurement is made, the chronograph 

 tape is scaled and the results are tabulated on Form 715, Abstract of Bombed Distances, 

 and compared to ascertain when there is a sufficient number of satisfactory determina- 

 tions. In these measurements bombs containing the same quantity of explosive should 

 be used for an entire series. A change in the size of bomb will frequently affect the 

 time interval slightly, but enough for the result to appear erratic when compared with 

 the results obtained from bombs of a different size. 



The determinations may be made while the ship is lying-to at the proper position, 

 or they may be made while underway. If they are made from the ship lying-to, large 



