DEFINITION OF TARGET STRENGTH 



347 



10 20 30 50 70 100 



RADIUS IN YARDS 

 Figure 2. Target strength of a sphere. 



200 300 



500 700 1000 



the two-way transmission loss is more conveniently 

 represented by the more general function 2U than 

 by 40 log r. Equation (4) then becomes 



10 log /, = 10 log A; -f 10 log T?" - 2H. (5) 



The total transmission loss 2H cannot be predicted 

 or estimated very reliably because of widely varying 

 oceanographic conditions. Instead, it must actually 

 be measured during the course of the experiment. 



19.1.3 Fundamental Definition 



By defining the target strength T as 10 log k, the 

 echo level £' as 10 log /,, and the source level S as 

 10 log F, equation (5) becomes 



r = E - S + 2H (6) 



where 2H is the total transmission loss in decibels 

 from the source out to the target and back to the 

 source again. Equation (6) is the fundamental defini- 

 tion of target strength. This equation is always used 

 in the computation of target strengths measured at 

 sea since it involves only directly measurable quanti- 



ties — that is, echo level, source level, and transmis- 

 sion loss from the source out to the target and back 

 to the source. 



Since h and /^ are measured in the same units, it is 

 evident from equation (1) that A; has the dimension 

 of an area and the value of T will depend on the units 

 which are used. Since the yard is used in range- 

 prediction work as the unit of length, the source level 

 is defined in terms of the intensity at 1 yd, and the 

 transmission loss, which enters twice into equation 

 (6), is defined in terms of the intensity drop from a 

 range of 1 yd out to the range of the target. Conse- 

 quently k in equation (3) may be expressed in square 

 yards. 



Equation (6) was derived from physical concepts 

 in order to express as a sum of separate terms the 

 effects on the strength of the received echo of (1) the 

 size, shape, and orientation of the target; (2) the 

 intensity of the source; and (3) the range of the 

 target. This separation can be realized only at long 

 ranges, where the sound reflected from the target be- 

 haves as if it were emitted from a point source and 

 the target strength becomes independent of the 



