418 



SUBMARINE TARGET STRENGTHS 



BEAM 



Figure 33. Effect of conning tower on acoustical 

 measurements on U570 (HMS/M Graph). 



high-speed camera at San Diego, the echoes appear 

 to be-a group of fine spikes or peaks, although some 

 evidence points to peaks which are found in the same 

 places in successive echoes. A more complete discus- 

 sion of the detailed structure of off-beam echoes and 

 their origin is postponed to the next section. Here the 

 more general features of off-beam echoes are dis- 

 cussed. 



The beginning and end of an echo at off-beam 

 aspect are not clearly defined; usually the amplitude 

 builds up and dies away gradually, blending into the 

 background at either end, so that precise measure- 

 ment of the echo length is impossible. However, an 

 examination of off-beam oscillograms shows that 

 these echoes are longer than the signals and there- 

 fore suggests an extended target.-' 



If the entire length of the submarine is effective in 



reflecting sound, as seems indicated for off-beam 

 echoes, the lengths of these off-beam echoes should 

 vary with the aspect angle, depending on the length 

 of the submarine in the direction of the sound beam. 

 In other words, if a submarine is an extended target 

 and scatters soimd throughout its length, the length 

 of the echoes which it returns should depend on the 

 aspect which it presents to the sound beam. 



Accordingly, the lengths of these off-beam echoes, 

 diminished by the length of the signal used, have been 

 measured or estimated as accurately as possible, 

 then plotted against the cosine of the aspect angle 

 which accounts for the foreshortening of the sub- 

 iriarine.^^ Figure 34 illustrates the results of this 

 analysis, where the broken curve connects the meas- 

 ured points, and the solid curve is a polar plot of the 

 cosine of the aspect angle, modified at bow aspect to 

 account for the "shadow" which the forward section 

 casts on the stern section. A similar dip is included at 

 stern aspect, where the after part of the submarine 

 shadows the forward part. 



The maximum elongation — echo length minus 

 signal length — was found to occur at quarter as- 

 pects, roughly 15 degrees from the bow and stern 

 on either side, and amounted to about 85 yd.^" The 

 actual length in the direction of the sound beam of a 

 fleet-type submarine 300 ft long, at aspects 15 de- 

 grees from bow and stern, is about 96 yd, which con- 

 firms the suggestion that the entire surface of the 

 submarine scatters sound. At an aspect angle of 135 

 degrees, when the target had an extension of 49 yd 

 in the direction of the soimd beam, the elongation 

 amounted to about 38 yd. 



At bow aspect, this elongation was reduced to 

 50 yd. This reduction is attributed to the shadow 

 cast by the forward section on the after section. A 

 similar drop, however, was not observed at stern 

 aspect. 



These elongation phenomena are apparently inde- 

 pendent of signal length and echo-ranging frequency 

 Apparently they are the result of scattering from the 

 entire length of the submarine, instead of reflection 

 from only one or two major surfaces such as the 

 conning tower or screws. They suggest that in addi- 

 tion to specular reflection from the hull, nonspecular 

 reflection or diffuse scattering also occurs, especially 

 at aspects away from the beam. The exact mechanism 

 by which sound is reflected from the entire length of 

 the submarine is unknown. 



Similar elongation phenomena were analyzed by 

 British observers in an effort to determine the origin 



