INTRODUCTION TO SONAR 



You learned in chapter 3 how to determine 

 aspect from doppler effect. When you can also 

 observe an echo indication at the same time 

 you hear it, you can learn more about the target 

 than you can from audio responses alone. A more 

 reliable method of determining target aspect 

 than that based on range rate alone is to compare 

 the recorder traces with the doppler effect. 

 Naturally, this method takes soma practice and 

 requires a knowledge of how the traces appear 

 for different target aspects. 



Doppler and Recorder Traces 



Doppler and recorder traces work together 

 in serving as a check on each other. Tliat is 

 why it is essential to use your eyes and ears 

 simultaneously. When operating the recorder, 

 look at the traces and listen to the doppler. If 

 you hear no doppler, you have either a beam 

 trace or one from a motionless target. (Doppler 

 is a function of target motion. If a submarine 

 is dead in the water, there is no doppler indica- 

 tion, but the traces will record the target in its 

 proper aspect. Thus, traces for a stern-on 

 target, which is dead in the water, are recorded 

 as stern-on, but instead of having doppler down, 

 you will have no doppler.) 



For purposes of this text, assume that the 

 target is in motion. This assumption allows 

 simple comparison of doppler and recorder 



traces. Thus, doppler up means a bow or direct- 

 bow target; doppler down means a quai'ter or 

 stern-on target. 



Individual Echoes 



Suppose a pulse of sound hits a submarine 

 that is beam-on to the sound signal. All parts 

 of the outgoing pulse strike the submarine at 

 approximately the same time. All parts of the 

 pulse are thus reflected and started on their 

 return journey simultaneously. In figure 7-19 

 you can see that parts 1, 2, 3, and 4 start back 

 together. These echoes hit the transducer simul- 

 taneously, and when the returning echo reaches 

 the receiver, it is transformed into a short, dark 

 line on the recorder paper. A beam trace is the 

 same length as a recorded pulse length. 



The beam trace has no doppler, but it has a 

 definite "smack" as you hear it. It sounds sliort 

 and sharp, and it appears on the paper very much 

 as it sounds. Both edges of the trace are sharp 

 and distinct, and the entire trace is solid and well 

 defined. Because it is easy to recognize, it is the 

 simplest of the recorder traces. Study figure 7-19 

 until you can recognize the trace easily. 



Consider the quarter aspect target in figure 

 7-20. The stern of the submarine is nearer than 

 the bow, so that portion of the transmitted pulse 

 marked 1 hits the submarine first and starts back 

 first. It is followed by the portion marked 2, and 

 so on. 



SIGNAL 



SIGNAL 



ECHO 



BEAM 







QUARTER 





^: i-*; — T*-^ s — ^ 



w .■ ■ — - 









51.29(71) 

 Figure 7-19. — Beam aspect traces. 



51.30(71) 



Figure 7-20. — Quarter aspect traces. 



128 



