410 



SUBMARINE TARGET STRENGTHS 



23.6.3 Indirect Measurements 



Visible light from a motion picture projection bulb 

 was used in the optical measurements at MIT. Since 

 the frequency, or the band of frequencies, was not 

 properly scaled to correspond with usual echo- 

 ranging frequencies, it was not practical to investi- 

 gate the dependence of target strength on frequency. 

 However, since improperly scaled light was used to 

 give results for comparison with direct and other in- 

 direct measurements, four frequency effects should 

 be remembered in interpreting the results of the op- 

 tical measurements.'^ 



First, at certain aspects when the insonified sur- 

 face of the actual submarine subtends only a few 

 Fresnel zones at 24 kc, the surface of the model il- 

 luminated by visible light subtends many zones, 

 since the wavelength of the light was much shorter 

 compared with the dimensions of the model than was 

 the wavelength of the sound used in echo ranging 

 compared to the dimensions of an actual submarine. 



As a result, for the optical measurements, the ex- 

 pressions for the target strength due to the effects of 

 a group of Fresnel zones approached their asymp- 

 totic values, especially for surfaces with at least one 

 large radius of curvature, such as cylinders and 

 planes. On submarines this effect might apply to the 

 conning tower, keel, and top deck, so that in the 

 optical measurements the effect of the conning tower, 

 and the effect of the deck at an altitude angle of 90 

 degrees, might be overemphasized. 



Secondly, nonspecular reflection is too small by a 

 factor of the square root of the ratio of the actual 

 wavelength used to the properly scaled wavelength. 

 In the optical measurements, this factor is about 45, 

 or about 17 db. In other words, nonspecular reflec- 

 tion "measured optically is about 17 db too low. 

 This factor may account for the very low target 

 strengths obtained optically at off-beam aspects 

 where nonspecular reflection may be more important. 



Thi rdly, diffuse reflection or scattering may be too 

 great optically because the wavelength may be much 

 smaller than the surface irregularities. An attempt 

 was made to minimize this error by using glossy 

 black surfaces. 



Finally, where two or more specular reflections 

 occur, the light beams do not interfere, as sound 

 beams do, because they are incoherent. Since the 

 incoherent sum is an average of the interference pat- 

 tern, this smn may actually be more interesting than 

 the detailed interference pattern itself, as the aver- 



age is more significant, in most applications to prac- 

 tical echo ranging at sea, than the exact pattern. 



Thus the results of the optical measurements, 

 though suggestive of what might be encountered in 

 practical echo ranging, cannot be compared directly 

 with the other measurements unless these effects of 

 the wavelength are considered and accounted for. 



In the acoustical measurements at Mountain 

 Lakes, no long-term systematic variation with fre- 

 quency was observed for full-scale frequencies from 

 about 1 to 35 kc. Figure 23 shows relative echo level 

 plotted against frequency for the Mountain Lakes 

 measurements on the Graph at beam aspect, at a 

 full-scale range of about 15 yd. The peaks and dips 

 evident in this illustration are largely the result of 

 interference phenomena arising from two specular 

 reflections from the hull and conning tower, as the 

 frequency is changed, and of the response character- 

 istics of the system. Interference phenomena result- 

 ing from multiple reflections from several surfaces 

 on the submarine are clearly shown in Figure 24, 

 where the relative beam target strength is plotted 

 against altitude angle for a full-scale frequency of 

 26 kc. The nearly smooth curves at altitudes of and 

 90 degrees result from direct specular reflection from 

 the deck and hull respectively. The intricate interfer- 

 ence pattern at altitude angles between 10 and 50 

 degrees and 270 and 350 degrees results from path 

 differences in the sound doubly reflected from the 

 hull and from the blister tank; similar patterns are 

 evident for soimd reflected from the bottom of the 

 submarine model. 



23.7 OCEANOGRAPHIC CONDITIONS 



Target strength measures the reflecting character- 

 istics of a target and is computed from the echo level, 

 source level, and transmission loss from equation (6) 

 in Chapter 19. Since it depends only on the target 

 itself, it is theoretically independent of the medium 

 and its characteristics, independent of the transmis- 

 sion characteristics of soimd in water, and therefore 

 independent of oceanographic conditions insofar as 

 they affect transmission. 



In practice, however, reported target strengths 

 have been found to depend markedly on the prevail- 

 ing oceanographic conditions in cases where the 

 transmission loss has not been accurately known. 

 Since target strength is computed from the echo 

 level, source level, and transmission loss, improp>er 

 appraisal of the transmission loss will appear as an 

 error in the target strength values reported. 



