Figure 12. — Photograph of 3-dlmenslonal model used to establish correlation between motion pictures and 



recorded data. 



Three models were constructed to demonstrate 

 the Doppler correlation characteristics be- 

 tween data and the motion pictures. 



CONCLUSIONS FROM FEASIBILITY STUDY 



The results of the work accomplished clearly 

 indicate the suitability of CTFM sonar for 

 ranging on fish specimens. This fact was 

 determined early in the program. 



The remainder of the effort and time was 

 given to generation of techniques for presen- 

 tation of the weak Doppler return from indi- 

 vidual fish targets. The results obtained were 

 not decisive but indicated that with a greater 

 number of specimens the signal-to-noise ratio 

 improved and the data extraction techniques 

 relaxed. The application of a low-frequency 

 DELTIC to the analysis of the Doppler returns 

 produced a simple presentation. The techniques 

 and circuitry that were developed overcame 

 many difficult problems. 



In view of the higher target strengths of 

 schools of fish, it is clear that reasonable 

 ranges of detection can be achieved with this 

 type of sonar (e.g., 1,000 yards--914,4 m.). 

 The coherence of the dynamics of the school 

 and increased signal strength provide addi- 

 tional significant analysis information. 



SHIPBOARD TARGET CLASSIFICATION 

 EXPERIMENTS 



The initial attempts to detect Doppler effects 

 caused by fish showed that the technical 

 problems could be solved. How well theory 

 could be reduced to practice was to be tested 

 in the next, the at-sea, phase. The equipment 

 used in Phase II was modified and packaged 

 for shipboard use. We decided that "realtime" 

 signal analysis was necessary and incorporated 



a DELTIC analyzer in the system; however, 

 we also provided an instrumentation recorder 

 capable of storing target echo signals for 

 laboratory analysis should the need arise. 



Equipment 



Our target-classification sonar system was 

 a modification of a Straza Electronics 500 

 series CTFM sonar (table 3). CTFM sonar 

 has several desirable features for use on 

 pelagic fishes. A wide-beam projector con- 

 tinuously fills a large volume of water with 

 sound, the frequency of which is repeatedly 

 varied from high to low in a saw-tooth manner. 

 A narrow-beam hydrophone rapidly scans the 

 insonified volume (25°/second in our sonar). 

 All targets in the projector beam return 

 echoes at a frequency that differs with time 

 (or range) from the projected frequency. 

 Mixing of the transmitted and received fre- 

 quencies produces a difference frequency cor- 

 responding to a range. This continuous proc- 

 ess gives a much faster information rate 

 than is possible with a conventional pulsed 

 sonar systenn. This high information rate 

 makes it possible to maintain contact with 

 fast-moving schools. Each target is a source 

 of continuously reflected sound. The hydro- 

 phone scans each target for a time equivalent 

 to several pulse lengths of a pulsed sonar; as 

 a result the target returns are averaged. Such 

 signal-averaging of short-term fluctuations in 

 echo strength increases the likelihood of de- 

 tection of any particular target. 



Because range information was obtained 

 from a difference frequency, CTFM sonar 

 already has some of the circuit design that 

 was required for resolution of the Doppler 

 effect. An addition of a second mode of opera- 

 tion CTD (Continuous Transmission Doppler), 

 that used most of the CTFM circuits plus a 



14 



