420 



SUBMARINE TARGET STRENGTHS 



to 





 BOW 



90 



BEAM 



ASPECT ANGLE IN DEGREES 



120 



ISO 



ISO 

 STERJI 



Figure 35. Echo elongation and projected length of submarine as a function of aspect angle. 



of the nearest echo, but the hypothesis that the en- 

 tire submarine reflected sound was not confirmed.^' 

 The elongation, plotted in Figure 35, amounted to 

 only about half the calculated exposed lengths of the 

 submarine, after the pulse lengths had been sub- 

 tracted from the echo lengths. Similar results have 

 also been obtained in this country using the same 

 technique. It may be pointed out, however, that a 

 sound range recorder, not an oscilloscope, was used 

 in these experiments, and that records from a sound 

 range recorder might be expected not to show the 

 weaker tail part of the echo. 



23.8.3 



Source of Echoes 



Beam echoes originate in large part at the hull of 

 the submarine, as described in Section 23.8.1, with 

 some additional contribution possible from the hull 

 and bilge keel. The echoes are nearly square-topped 

 and result from simple specular reflection from only 

 one or two surfaces on the submarine. 



Off-beam echoes, however, apparently come from 

 all parts of the submarine. Oscillograms of these 

 echoes are detailed and show a fine microstructure of 

 peaks and valleys, somewhat similar to reverbera- 

 tion, especially for short pulses. Since study of the 

 elongation phenomena suggests that echoes are re- 

 turned from most of the submarine, various peaks in 

 the detailed structure of an echo might be correlated 

 with discrete reflecting surfaces on the outside of the 

 submarine. Only short signals could be used, how- 



ever; otherwise the signals from individual reflectors 

 on the submarine might overlap. 



Accordingly a series of echo oscillograms from sig- 

 nals approximately 0.5 msec (0.4 yd) long were 

 studied at San Diego. ^' The target was a submarine 

 of the S class at quarter aspect, 135 degrees from the 

 bow. The echoes were recorded oscillographically, as 

 usual, but the film was run at a speed of about 13 in. 

 per sec, five times faster than normally. This high 

 speed lengthened each echo and permitted better 

 resolution of the echo structure. 



Each echo analyzed consisted of a number of sharp 

 spines, usually between twenty and fifty, which rose 

 clearly above a fuzzy background. The envelope of 

 these spines was roughly cigar-shaped while the en- 

 velopes of the less intense parts of the echo peaks 

 were similarly shaped but only about half the ampli- 

 tude of the spine structure. The distribution of these 

 spines appeared to be random, and no peaks or 

 groups of peaks could be definitely correlated with 

 individual reflecting surfaces, such as the conning 

 tower or ballast tanks. Thus the peaks may be more 

 the result of constructive interference of sound scat- 

 tered at random from the entire submarine, than of 

 strong reflections from discrete surfaces on the sub- 

 marine. 



Studies of other short-pulse echoes obtained at 

 other aspects from various submarines usually yield 

 somewhat similar results. The echo almost always 

 consists of a succession of peaks rising above the 

 background. These peaks, however, do not always 



