TRANSMISSION LOSS 



371 



_i 



Ul 



ffi 

 o 



Ul 



o 



o 



o 



+ 



I 

 UJ 



30 



25 



20 



10 



500 1000 1500 



RANGE IN YARDS 



Figure 6. Target strength plot. 



2000 



2500 



In Figure 5, a amounts to about 4.5 db per kyd, at 

 24 kc. It may be that actually 



A = ar -\- b 



(4) 



where 6 is a constant. Present data indicate, however, 

 that 6 is probably negligible.'^ 



By definition, the transmission anomaly includes 

 the effects of reflection from the ocean surface. So 

 little is known about surface reflection with any de- 

 gree of certainty, however, that no attempt is made 

 to include in equation (4) an additional term to take 

 it into account. If surface reflection is appreciable, it 

 may cause the constant b in equation (4) to be nega- 

 tive, in effect decreasing the transmission anomaly 

 and therefore the transmission loss itself, as described 

 later in Section 21.5.4. 



21.5.1 Methods of Measurement 



Transmission loss may be measured in three ways. 

 First, during an opening or closing run, the echo level 

 in decibels above the source level may be corrected 

 for geometrical divergence by adding 40 log r; then, 

 the result may be plotted as a function of the range, 

 as long as the submarine maintains a constant aspect. 

 A typical plot of this nature is illustrated in Figure 6. 

 Then the slope of the points represents twice the 

 attenuation coefficient, and the intercept at zero 

 range corresponds to the target strength. Such a de- 



termination presupposes that the target strength 

 does not change over the ranges used. 



Second, before and after each run on a submarine, 

 a transmission run may be made with an auxiliary 

 surface vessel, in the usual manner, as described in 

 Section 4.3.2. 



Third, the signals transmitted by the echo-ranging 

 vessel may be received by a hydrophone mounted on 

 the submarine, amplified, and measured. The trans- 

 mission anomaly and attenuation coefficient may 

 then be determined readily by correcting the level of 

 the echo above the source for simple geometric 

 divergence, and measuring the slope of the plot of 

 the echo level against the range. 



But measuring the transmission loss in any one 

 of these three ways is difficult. Aspects and speeds 

 must be carefully maintained and measured, a pro- 

 cedure particularly difficult for a submerged sub- 

 marine. For reasons of safety, the echo-ranging vessel 

 is advised not to approach the submarine closer than 

 about 300 or 400 yd, and poor sound conditions often 

 limit echo ranges to 1,000 yd or even less, especially 

 off the coast near San Diego. When a transmission 

 run is made with an auxiliary surface vessel, hori- 

 zontal temperature gradients may result in a trans- 

 mission loss between the projector and the hydro- 

 phone suspended from the surface vessel which is 

 different from that between the projector and the 

 submarine. Another disadvantage of measuring the 



