SONAR AND RADAR 



of the FM bats, during each individual pulse of sound. 

 When the echoes of such pulses were received, the 

 frequency change was audible in the beat note. In 

 one typical setting of the apparatus the transmitted fre- 

 quency, and of course the echo, was varied from 20,800 

 to 19,200 c.p.s. If the local frequency was set at 19,000 

 c.p.s., the beat note would vary from 1800 to 200 

 c.p.s. and this would produce an extreme chirp or 

 "Wheeoough" sound. One advantage of this type of op- 

 eration was that at any particular instant of time the 

 many reverberations or multiple echoes from the ship's 

 hull and the water surface had traveled different dis- 

 tances and hence had different frequencies as they ar- 

 rived at the receiving hydrophone. This tended to create 

 an audible difference between the important chirping 

 echoes from a submarine and the noise level of rever- 

 beration from which they had to be discriminated. The 

 important echo was a clear chirp, the competing rever- 

 berations an irregular and shifting mixture of frequen- 

 cies. Very likely bats obtain a similar advantage from 

 their frequency-modulated pulses. 



In another type of operation the sonar system used a 

 constant frequency in the emitted pulse, and the opera- 

 tor listened for slight differences in the pitch of the 

 audible beat note. Slight differences between the echo 

 frequency and the local frequency can produce large 

 changes in the audible ping. These differences can be 

 used to determine the relative motion of the target by 

 means of what is called the Doppler effect. This change 

 in frequency resulting from the motion of the source 

 causes the rising pitch of a train whistle as the train ap- 

 proaches you. To understand the Doppler effect, let us 

 consider a concrete example. Suppose that the sonar 

 ship is moving east at 10 meters per second while emit- 

 ting a 0.1 second pulse of 20,000 c.p.s. sound, that is, 



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