DEPENDENCE ON RANGE 



403 



o 20 



10 



o 



BOW 



30 



60 90 



BEAM 

 ASPECT ANGLE IN DEGREES 



ISO 



180 

 STERN 



Figure 16. Theoretical dependence of target strength on range for the U570 (HMS/M Grayh). 



23.4.1 



Theory 



In the theoretical target strength studies described 

 in Section 20.5,^ plans of the Graph were employed 

 in calculating target strengths.^* This submarine has 

 a maximum radius of curvature of about 560 yd. 

 Since target strengths vary with range primarily for 

 ranges less than the maximum radius of curvature of 

 the submarine, the target strength of the Graph 

 would be expected to approach a limiting value as 

 the range is increased to 500 or 600 yd. 



Actually, the target strength of the Graph is very 

 near this limiting value at ranges beyond only a few 

 hundred yards, especially at beam aspect. Target 

 strengths have been computed for ranges of 8, 12, 

 16, 200, and 1,000 yd on the assumption of a non- 

 directional source of sound and are plotted against 

 aspect angle in Figure 16. The shorter ranges are the 

 distances from the projector to the nearest part of 

 the submarine. It is apparent from Figure 16 that the 

 variation of target strength with aspect changes 

 markedly as the range is reduced from 200 to 16 yd; 

 no intermediate ranges were used. 



These calculations were based on a nondirectional 

 source; therefore the results neglect the effect of 



limited coverage of the submarine by a directional 

 beam at close ranges. Such an effect is not important, 

 however, if the reflection is primarily specular and 

 comes from a small area amidships, as discussed in 

 the preceding section. 



23.4.2 



Observations 



Verification of a dependence of target strength on 

 range has not been possible in the direct measure- 

 ments because the transmission loss has not been 

 known accurately. Instead, it has been assumed, for 

 the ranges used during the target strength measure- 

 ments — from 200 to 1,000 yd — that the target 

 strength at near-beam aspects remains constant. 

 Such a.n assumption as necessary in order to calculate 

 the transmission loss; in some of the measurements 

 at San Diego, a constant target strength was assumed 

 at constant aspect, and the transmission loss was 

 computed from a plot of the echo level E plus 40 log 

 r against the range r as the range is opened or closed 

 (see Section 21.5.1). The most recent measurements 

 at San Diego have employed a nondirectional hydro- 

 phone mounted on the submarine to measure the 

 transmission loss; this method, if practical, might 



