Page 637 radio acoustic ranging 6812 



6812. Frequency of R. A. R. Positions 



The frequency of positions in R.A.R. surveys, like surveys controlled by other 

 methods, depends on a number of conditions which are discussed in 3313. The max- 

 imum frequency of E.A.R. positions is limited by the time required to obtain the R.A.R. 

 distances from the chronograph tape and to plot the position on the boat sheet, this 

 generally requiring from 3 to 5 minutes depending on the distances involved and the 

 adroitness of the personnel. Nothing is gained by taking R.A.R. positions faster than 

 the data can be computed and plotted. 



Depending on the current, R.A.R. surveys on the Atlantic and Gulf Coasts can 

 be satisfactorily controlled by positions every 5 or 6 minutes in areas of moderate depths 

 when surveyed on a 1 : 80,000 scale or larger, and every 8 or 10 minutes in deeper offshore 

 areas surveyed on a scale of 1:100,000 or smaller. Positions are often taken still les& 

 frequently in offshore areas where large cast-iron bombs are required or when it is 

 necessary to conserve bomb materials. On the Pacific Coast and in Alaska waters, 

 offshore surveys have been satisfactorily controlled with positions obtained at intervals, 

 as long as 30 minutes, but the fixed positions must be supplemented by adequate dead- 

 reckoning data. 



Accurate dead-reckoning data must be recorded for use in conjunction with R.A.R. 

 surveys and it is frequently advantageous to identify some of the intermediate points, 

 between bombs as positions (see 3312). In addition to those at regular intervals, where 

 practicable, R.A.R. positions should be taken at the following times: 



(a) At the beginning and end of each sounding Hne, except at a 180° change in course where the 

 beginning of the new line is so close to the end of the previous line that time does not permit two 

 R.A.R. positions so close together. In the latter case the position at the end of the first line should 

 be determined by dead reckoning so that an R.A.R. position can be obtained at the beginning of the 

 new line. This fixes a position on the new line as soon as possible so that the hydrographer may 

 change the course, if necessary, to make the track of the vessel coincide with the proposed line. 



(b) At all major changes in course. In general, changes in course of 3° or less need be merely 

 recorded in the Sounding Record with the time of the change, changes in course from 3° to 10° should 

 be recorded as positions and given numbers, but changes in course greater than 10° should be fixed 

 by R.A.R. positions. 



(c) At all changes in speed. 



6813.. Obtaining an R.A.R. Position 



Assuming that an R.A.R. position is desired at 14:18, either because of the regular 

 interval between positions ordered by the officer-in-charge or because he has specifically 

 notified the chronograph attendant that a position is desired at that time, the following 

 sequence of events occurs (see also 3442): 



At 14:17, one minute before the position is desired, the bomber is signaled by bell 

 to indicate the size of bomb desired. The chronograph attendant then tests the 

 chronograph to make sure that it will operate satisfactorily and that sufficient chrono- 

 graph tape remains on the roll, and sees that the radio technician has the receiver turned. 

 on and properly tuned. 



The bomber acknowledges the above signal with a two-bell signal which is received' 

 at the chronograph station and on the bridge. This indicates that the signal has been 

 received and understood by the bomber and notifies the bridge personnel that an 

 R.A.R. position is to be expected on the next even minute. The bomber selects a bomb 

 of the size desired, inserts the fuse, and prepares it for lighting and, with bomb and push 

 button in hand, stands by the lighting element ready to light the bomb when the next 

 signal is received. 



