Table 1. --Methods of identifying single fish by visual and acoustic observations. 



^ a, c, d, e, f, g, determine target strength and directivity. 

 ^ c, d, e, determine Doppler motion. 

 ^ a, determine Doppler motion. 



of the fish-detecting sonar, is discussed by 

 Volberg. 



Acoustic information that can be used to 

 classify fish targets is listed in the next two 

 columns. A subheading, called dynamic echo 

 return , is broken into two columns: one, motion 

 in the medium, and the other, body motion. 

 The motion in the nnedium column is best 

 determined by using a CTFM sonar (described 

 below) in the scanning mode that paints a 

 picture of the range and bearing of the specimen 

 at any instant. For normal sonar work the 

 Doppler effect produced by the velocity of the 

 fish is small enough to be neglected for any 

 ranges of interest. The normal CTFM sonar 

 cannot resolve the body motion column outlined 

 under the motion of the fish, with coordinates 

 referenced to the body of the fish. It is there- 

 fore necessary to use a Doppler technique 

 wherein the body of the fish produces a fre- 

 quency change proportional to the velocity of 

 portions of the body of the fish. Unfortunately, 

 the body motion of the fish cannot be removed 

 from the displacement motion in the medium. 

 That is, the velocity of the fish in the medium 

 produces an additional Doppler frequency term 

 which cannot be separated from the Doppler 

 effect produced by body flexure. The itenns 

 that determine Doppler motion can be found at 



H. W. Volberg. 1970. Acoustic target strength of 

 several species of fish. This publication, pp. 21-26. 



the bottom of table 1, The steps to be followed 

 in studying Doppler as a means of classifying 

 fish targets follow from the table: (1) deter- 

 mine the target strengths of the subject fish 

 species; (2) design a CTFM sonar system to 

 these standards; (3) obtain Doppler information 

 from fish body motions; and (4) examine the 

 Doppler data for characteristics that can be 

 used for target classification. 



OPERATING PRINCIPLES OF A CTFM 

 SONAR FOR FISHERY WORK 



The best known sonar, the pulsed sonar, gets 

 target range information from the time elapsing 

 between pulse transmission and echo recep- 

 tion. The CTFM sonar has the advantage of 

 transmitting and receiving continuously and 

 simultaneously and gets range information by 

 comparing the frequencies of the transmitted 

 and echo frequencies. Both get target bearing 

 information by using directional receiver hy- 

 drophones. 



In the simplified block diagram of a CTFM 

 sonar (fig. 1) the transmitted frequency is 

 modulated in a sawtooth manner between fj and 

 fi shown in figure 2 by the solid lines. For 

 illustrative purposes, fz can be 45 kHz and f^ 

 can be 59 kHz. An echo returning a time A, later 

 is received by the hydrophone. The frequency 

 Af of the beat note between the frequency ft, 

 being transmitted, and the frequency fp, of 



