394 



SUBMARINE TARGET STRENGTHS 



TARGET STRENGTH IN DECIBELS 

 BOW 



BEAM 



BEAM 



STERN 



Figure 6. Aspect dependence (Mountain Lakes). 



from the deck of the submarine. The absolute values 

 in Figures 7 and 8, however, are not comparable be- 

 cause the theoretical calculations were carried out for 

 a projector very close to the submarine, while the 

 optical measurements applied to the ranges of several 

 hundred yards usually encountered in practical echo 

 ranging. 



Figure 9 is a smoothed curve showing the relative 

 target strength of the Italian submarine Vortice 

 plotted against aspect angle for altitude angles of 

 to 10, 10 to 20, 20 to 45, and 45 to 90 degrees, as 

 measured by Harvard observers; '^ for each curve, 

 the relative target strength at beam aspect was 

 arbitrarily set at 25 db. These data were obtained at 



a frequency of 26 kc, for submarine depths of 100 to 

 400 ft and ranges up to 1,000 yd. The aspect de- 

 pendence apparently becomes less marked and the 

 curve smoother as the altitude angle increases. It 

 might be pointed out, however, that for altitude 

 angles greater than 20 degrees, and a submarine 

 depth of about 400 ft, the sound beam does not com- 

 pletely cover the submarine at near-beam aspects. 

 Therefore the target strength would be expected to 

 show less aspect dependence. 



Figures 10 and 11 show target strength aspect 

 curves for different altitude angles, as measured in- 

 directly. Optical measurements on a submarine of the 

 S class are given in Figure 10 for altitude angles of 0, 



