TRANSMISSION RUNS 



79 



loss pan be determined by measuring the received 

 signal level at the range and depth of the hydro- 

 phone. There are several types of runs. It is possible, 

 for instiinee, to make a vertical transmission run, in 

 which the range between the two ships is kejit very 

 nearly constant and in which the hydrophc ne is 

 slowly raised or lowered, so that the transmissii n loss 

 is determined as a function of depth at a fixed range. 

 In horizontal run.s, the depth of the hydrophone is 

 kept fixed, while the range is changed. Horizontal 



0*85 -L8 WEIGHT 

 Figure 9. Method of suspension of deep hydrophones. 



runs are much easier to carry out than vertical runs. 

 In a vertical transmission run, the depth of the 

 hydrophone can be changed only slightly each time. 

 One or several pings are transmitted while the hydro- 

 phone is kept at a constant depth; and then the 

 hydrophone depth is changed again. Also, whenever 

 the hydrophone is moving through water, the flow of 

 the water past the hydrophone gives rise to noise, 

 which may effectively mask the signal. In a horizontal 

 run, the receiving ship is permitted to drift, or, in 

 shallow water, anchored. The noise due to water 

 current thus is minimized and the hydrophone cable 

 tends to hang straight down. Proper training and 

 control of depth is thus facilitated. The sending ves- 

 sel then runs either toward or away from the receiv- 



ing ship. In this manner the range can be varied con- 

 tinuously by an amount of several thousand yards 

 without ever interrupting the transmission of signals. 

 A supersonic transmission run of the horizontal type 

 takes, on the average, about 20 or 30 minutes. If it is 

 desired to obtain the transmission loss at several 

 depths, two or three hydrophones can be suspended 

 at various depths from the receiving vessel. The over- 

 whelming majority of transmission runs made up to 

 the present have been horizontal runs. 



In all transmission runs, elaborate precautions 

 have always been taken to keep hydrophones at their 

 nominal depth. Because of the wind drift of the re- 

 ceiving vessel, and because of ocean currents going 

 in different directions at different depths, deep 

 hydrophones, which are lowered occasionally as far as 

 450 ft below the surface, will rise to a much shallower 

 depth unless special care is taken to make them hang 

 straight. To this end, a 300-lb weight is suspended 

 from a strong steel cable; the hydrophone hangs 

 down from this weight and is held down by an addi- 

 tional 25-lb weight as shown in Figure 9. The hydro- 

 phone cable carries relatively little weight in this 

 arrangement. 



Horizontal transmission runs can be either ap- 

 proaching runs or receding rims, that is, the sending 

 ship can either close or open the range. In the reced- 

 ing run, the wake of the sending vessel is located be- 

 tween the two ships. Since it has been found that 

 wakes are capable of absorbing sound, the sending 

 ship usually changes its course from time to time in a 



RECEIVING SHIP 



SENDING SHIP 



« TO 12 KNOTS 



Figure 10. Course followed during a receding run. 



manner illustrated in Figure 10 in order that the 

 direct sound path between the two ships may never 

 pass through the wake laid by the sending ship. How- 

 ever, over shallow bottoms where change of course 

 would result in a changing bottom type, this pro- 

 cedure is sometimes not followed. During an approach 

 rim, the sending ship remains between its wake and 

 the recei^^ng ship, and it may, therefore, run along 

 a straight course and pa.ss the receiving vessel at a 



