436 



SUMiVlARY 



reveal a definite structure. For observations on one 

 S-boat, the main echo consists of two cornponents 

 separated by a distance of about 4 yd; the first com- 

 ponent may come from the broadside of the sub- 

 marine, while the second component may be an echo 

 from the bilge keel or conning tower. After this main 

 echo comes a much weaker secondary echo, pre- 

 sumably resulting from sound reflected from the 

 submarine straight up to the surface, back down to 

 the submarine, and then back to the projector (see 

 Figure 9 in Chapter 21, and Figure 29 in Chapter 23). 

 The presence of this echo structure will be expected 

 to modify slightly the conclusions in the preceding 

 section, since for long pulses the different components 

 will combine. Such a combination will increase the 

 average target strength 3 db at most above its value 

 for very short pulses. 



25.2.3 



Fluctuation 



The fluctuation of beam echoes may be primarily 

 attributed to the fluctuation in the transmission of 

 the outgoing and incoming sound (see Section 21.6). 

 Much of this fluctuation is apparently due to the 

 presence of surface-reflected sound (see Section 

 21.5.4). Estimates of the fluctuation of transmitted 

 sound are given in Chapters 7 and 10. In addition, 

 for pulses more than a few milliseconds long, inter- 

 ference between the different components of the echo 

 will somewhat inci'ease the fluctuation. 



25.3 OFF-BEAM ECHOES FROM SUB- 

 MERGED SUBMARINES 



At aspect angles more than about 20 degrees off 

 the beam, echoes from submarines originate along 

 the entire length of the vessel and probably result 

 from both specular and nonspecular reflection (see 

 Section 23.8.2); they ai'e 10 to 15 db weaker than 

 echoes at beam aspect. The echo does not reproduce 

 the outgoing pulse (see Figures 25 and 28 in Chapter 

 23). 



25.3.1 Off -Beam Target Strengths 



Observed submarine target strengths at oil-beam 

 aspects and at long ranges lie mostly between 5 and 

 20 db for pulses 100 or more msec long, and usually 

 between 10 and 15 db (see Section 23.1.1). The 

 spread of values is apparently real to some extent, 

 since at different aspect angles echo characteristics 

 are markedly different. At certain off-beam aspects 



and altitudes, strong specular reflections from nearly 

 flat surfaces, such as the conning tower, may give 

 target strengths greater than 20 db (see Section 

 23.2.2); these reflections depend critically on the 

 particular .submarine measured. 



Variation with Submarine Class 



No variation in the oil-beam target strengths of 

 different submarmes has been observed in either the 

 direct or indirect measurements to be greater than 

 the estimated experimental error in the direct meas- 

 urements (see Section 25.2.1). 



Variation with Submarine Speed 



No important variation of target strength with 

 submarine speed has been observed at off-beam as- 

 pects (see Section 25.2.1). 



Variation with Range 



At off-beam aspects, submarine target strengths 

 decrease with decreasing range (see Section 25.2.1). 

 At ranges less than the length of the submarine, 

 off-beam target strengths are roughly equal to beam 

 target strengths. Under such conditions, a submarine 

 may be approximated by a cylinder at off-beam as- 

 pects except bow and stern, and equation (6) may 

 be used. 



Variation with Pulse Length 



Since at off-beam aspects the echo does not usually 

 reproduce the pulse and the echo length considerably 

 exceeds the pulse length, for pulses 100 or more msec 

 long, some variation of target strength with pulse 

 length may be expected (see Section 23.5.1). Ob- 

 served target strengths decrease with pulse length 

 for signals shorter than 100 msec (see Section 23.5.2). 

 The decrease is most marked for pulses shorter than 

 10 msec, but even for such short pulses the target 

 strength does not decrease as rapidly as the pulse 

 length, or rather, as rapidly as 10 log t, where t is the 

 pulse length in milliseconds. 



Variation with Frequency 



No variation of target strength with frequency is 

 expected at off-beam aspects (see Section 23.6.1). 

 This conclusion is contradicted by some target" 

 strength measurements at a frequency of 60 kc, which 

 give much higher results than similar measurements 

 at 24 kc (see Section 25.2.1). However, the differences 

 between beam and off-beam target strengths are 

 about the same at 60 kc as at 24 kc, so that if the ob- 

 served frequency effect is real, it is the same at all 

 aspects/ 



