DEPENDENCE ON PULSE LENGTH 



407 



TARGET STRENGTH IN DECIBELS 

 BOW 



BEAM 



BEAM 



30 MILLISEC 



10 MILLISEC 



1 MILLISEC 



STERN 

 Figure 21. Dependence of target strength on signal length for an S-boat at 60 kc. 



cosine of the aspect angle. The mean curve connect- 

 ing the mean points is also indicated. However, this 

 dependence on pulse length is relatively small and 

 not very reliable in view of the large scatter. 



Measurements were made at a frequency of 60 kc 

 on another S-boat at creeping speed and a keel depth 

 of 100 ft, with signal lengths of 1, 10, and 30 msec.-' 

 A significant variation in relative target strength 

 with signal length was observed and plotted in Fig- 

 ure 19 for bow, beam, and stern aspects where each 

 point at beam aspect represents about 40 echoes, at 

 stern aspect about 20 echoes, and at bow aspect only 

 a few. Surprisingly, a large variation with pulse 



length is found at beam aspect; here theory would 

 predict a minimum dependence, since the echoes for 

 the most part accurately reproduce the pulses. An 

 attenuation coefficient of 20 db per kyd was assumed 

 in calculating the transmission loss; ranges averaged 

 about 300 yd. 



Figures 20 and 21 show target strength as a func- 

 tion of aspect angle for three different signal lengths, 

 1, 10, and 30 msec, for a submarine of the S class at 

 24 and 60 kc. A definite dependence on signal length 

 is evident from an examination of the three curves in 

 each figure, although the actual target strength val- 

 ues are rather low. The dependence on aspect angle 



