sonar system, if the sound is reflected from a 

 moving object and the receiver is stationary. 

 In table 1, column 4, items c, d, and e deter- 

 mine the character of a Doppler signal 

 returned from a fish moving with respect to 

 the medium. These parameters are concerned 

 with the fish's position and velocity components 

 with respect to the hydrophone and projector. 

 Superimposed on the total or average velocity 

 of the body of the fish would be the minute Dop- 

 pler changes produced by body flexure. These 

 body flexures are necessary for propulsion of 

 the fish or are a result of the propulsion or 

 respiratory mechanisms. Item a in column 5 of 

 the table lists this characteristic. The relation 

 of tail beat to velocity is one of the factors that 

 influence Doppler effects. 



Figure 3 shows the block diagrann for the 

 Doppler system constructed as a part of this 

 progrann. Many of the parameters for this 

 Doppler system were adjusted to be compatible 

 with the hydrophone projector and receiver of 

 the fishery sonar. The sensitivity of such a 

 system is the sanne as the sensitivity of the 

 sonar itself. One difference is that range de- 

 termination can no longer be made. Since the 

 system is only a velocity-measuring device it 

 is valuable only in determining whether or not 

 motion with sufficient target strength is gen- 

 erated. Because only a portion of the fish 

 produces Doppler effects due to the swimming 



motion, the information content is weaker than 

 that in a normal sonar return. The theoretical 

 voltages present in the Doppler receiver are 

 noted in figure 3. 



Target Strengths of Single Fish 



From the work done at the NTCF, Straza 

 determined that the target strength on a beam 

 aspect for individual fish was about -25 db, on 

 the average. Some of the specimens tested in- 

 dicated target strengths as low as -34 db. Since 

 the target strength for a fish without a gas 

 bladder apparently is a function of the projected 

 area of the fish, it is important that the fish 

 be of reasonable size for the evaluation of the 

 signal returns. In all the experiments the re- 

 turns from the fish were large enough for the 

 sonar to detect and give information on its 

 range. The Doppler unit is concerned only 

 with that fraction of the fish that is moving at 

 the velocity that can best identify th^ motion of 

 the species. The signal level for the Doppler 

 unit is therefore less than the signal for the 

 sonar. Enough infornnation was gained in these 

 tests to indicate that with larger fish the 

 Doppler signal would be great enough to permit 

 acquisition of Doppler data. Furthermore, an 

 increased number of fishes, such as a school, 

 would increase the signal-to-noise ratio of the 

 system and pernnit rapid evaluationof the data. 



Figure 3. — Doppler sonar block diagram. 

 4 



