INTRODUCTION TO SONAR 



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-" — UNDERWAY 

 OUTGOING PING K OhHz 



REVERBERATION- 13.9 KHZ 

 SUB ECHO- 14 KH! 



OOPPLER UP 



/ 



REVERBERATION- II 2 kHz 

 SUB ECHO-14 3kHz 



71.39 

 Figure 4-25. — Illustration of doppler. 



Figure 4-27. 



71.41 

 Doppler up. 



No Doppler 



Consider the echo from a submarine that 

 Is neither going away from nor toward the 

 sound beam. It either is stopped or is crossing 

 the soimd beam at a right angle. If the sub- 

 marine is in either of these situations, it re- 

 flects the same sound as the particles in the 

 water, and its echo has exactly the same pitch 

 as the reverberations, which means the submarine 

 echo has no doppler. Whenever the pitch of the 

 submarine echo is the same as the pitch of 

 the reverberations, therefore, you know that 

 the submarine is stopped or that you are echoing 

 off its beam, and you would report "No doppler." 

 Figure 4-26 illustrates a "no doppler" situa- 

 tion. 



SUB ECHO- 14 I KH2 

 REVERBERATION- 14 I kHz 



SUB ECHO- 13 9 kHz 

 REVERBERATION- 13 9kHz 



-* — UNDERWAY 

 OUTGOING PING l40kHz 



71.40 

 Figure 4-26. — No doppler. 



Doppler Up 



Suppose the submarine is coming toward 

 the echo ranging ship. In this situation, it is as 

 though the submarine were a train approaching 

 a car at a crossing. The sound transmission 

 reflected from the approaching submarine is 

 heard at a higher pitch than the reverberations. 

 When the echo from the oncoming submarine 

 is higher than the reverberations, report 

 "Doppler up." When making this report, you 

 are telling the conning officer ihat the sub- 

 marine is heading toward your ship and making 

 way through the water. This form of doppler 

 is illustrated in figure 4-27. Notice in the 

 illustration that the echo frequency of the sub- 

 marine on the ship's starboard quarter is the 

 sEime as the ship's sonar frequency, yet it has 

 up doppler. Remember, doppler is determined 

 by comparing the tone of the target echo and 

 the tone of the reverberations. 



Doppler Down 



Just as the tone of a train's whistle de- 

 creases in pitch as the train moves away from 

 you, so does the doppler of a submarine moving 

 away from your ship. No matter where the 

 submarine is located in relation to your ship, 

 if he is heading away from you, the tone of 

 his returning echo will be lower in pitch than 

 the tone of the reverberations. Figure 4-28 

 illustrates two situations when you would report 

 "Doppler down" to the conning officer. 



52 



