ECHOES FROM SURFACE VESSELS 



437 



25.3.2 



Echo Structure 



At off-beam aspects, echoes from submarines do 

 not reproduce the outgoing pulses because the entire 

 length of the submarine reflects sounds (see Section 

 23.8.2). The duration of the echo, measured on an 

 oscillogram, may be given by 



2L 



T = — cos e + T, 

 c 



(7) 



where T is the duration of the echo, L the length of 

 the submarine, c the velocity of sound, 6 the aspect 

 angle measured from the bow of the submarine, and 

 T the pulse length. On a sound range recorder, how- 

 ever, the echo length is about half that given in 

 equation (7), perhaps because only the stronger part 

 of the echo would be expected to show on a recorder 

 using chemically treated paper (see Figure 25 in 

 Chapter 23). 



25.3.3 



Fluctuation 



The fluctuation of echoes at off-beam aspects is 

 due not only to fluctuations in the transmission of 

 the sound each way (see Section 25.2.3), but also to 

 fluctuations resulting from interference phenomena. 

 The echo obtained from a long pulse will be the re- 

 sult of constructive and destructive interference be- 

 tween echoes from individual reflecting surfaces dis- 

 tributed over the length of the submaeing. Changes 

 in this interference pattern as the aspect or altitude 

 of the submarine changes slightly will increase the 

 observed fluctuation of echoes. 



25.4 ECHOES FROM SURFACE VESSELS 



Information on reflection from surface vessels is 

 even more fragmentary than on reflection from sub- 

 marines. The following conclusions are suggested by 

 the data but cannot all be regarded as confirmed. 



25.4.1 



Still Vessels 



Vessels at anchor seem to behave as targets in the 

 same way as submerged submarines. At aspects close 

 to the beam, target strengths may be very high, as 



much as 40 db, but at other aspects, for pulses 3 msec 

 long at a frequency of 27 kc, it is usually between 5 

 and 20 db (see Section 24.3.1). The strong echoes at 

 beam aspect are presumably the result of specular re- 

 flection from the hull of the ship. 



25.4.2 



Moving Vessels 



When a vessel is under way, beam echoes are about 

 the same as off-beam echoes (see Section 24.3.2). Ob- 

 served target strengths of moving destroyers and 

 merchant vessels lie between 10 and 25 db, for pulses 

 3 and 10 msec long at frequencies of 24 and 27 kc; a 

 systematic difference in the target strengths of differ- 

 ent ships is not evident (see Section 24.6). An in- 

 crease in speed from 10 to 20 knots apparently does 

 not affect the target strength appreciably (see Sec- 

 tion 24.5). A decrease in pulse length decreases the 

 resultant target strength, especially for pulse lengths 

 less than 10 msec, but the target strength does not 

 drop as rapidly as 10 log r, where t is the pulse length 

 (see Section 24.7). 



Echoes from moving vessels may arise from scat- 

 tering by bubbles of entrained air along the side of 

 the ship. This implies that the echo from a moving 

 ship may be treated as an echo from a short stretch 

 of wake (see Section 33.1.1). 



25.4.3 Dependence on Range 



Most of the- data on target strengths of moving 

 vessels show a marked increase in target strength as 

 the range increases from 200 to 600 yd, in one case 

 amoimting to more than 30 db (see Section 24.4). 

 Although some increase is expected from the geom- 

 etry of the ship (see Sections 24.4.2 and 25.2.1) and 

 from the failure of the sound beam to cover the en- 

 tire ship at short ranges (see Section 24.4.1), so 

 marked a change seems greater than can be explained 

 on any simple basis; it is quite possibly a statistical 

 accident. Beyond about 600 yd, it is reasonable to 

 assume that the target strength does not depend on 

 the range, and that its value lies within the spread 

 specified for surface vessel target strengths at off- 

 beam aspects in the preceding section. 



