a = 0.04 yard , 6°, 2^ = 0.1 radian horizontal 

 by 0.2 radian vertical, A d = 20 yards and N = 

 l/yard-'. By use of the foregoing, the target 

 strength for the school appears to be about 19 

 db. This approach also considers the fact that 

 the vertical dimension of the beam is wider 

 than the depth of the school. Aq, thus the 

 vertical dimension is 20 yards. Also, an in- 

 cremental width, Sw, of 16 yards is taken to 

 agree with a typical CTFM sonar resolution. 

 Checking this result with the summation of fish 

 targets as determined from the target-strength 

 work at NTCF we obtain a target strength in 

 the order of 16 db. From this rough check 

 and the above solution to the equation atypical 

 school target strength of about 15 db appears 

 to be a very conservative value, for the para- 

 meters selected. 



To examine what the foregoing schooltarget 

 strength represents in terms of a CTFM sonar 

 set of the same beam pattern as selected in 

 the foregoing expressions, we can use the sonar 

 equations as follows: 



Next, for a 50 Hz analyzer filter of which 

 100 are used, the bandwidth contribu- 

 tion would be 



Bw 



10 log 50 = 17 db 



Bw = 17 db 



The sonar equation given below can now be used. 



2Nw 



S +T+ DI - N - Rd - Bv 



in which T = target strength 



N = noise level per Hertz bandwidth. 



By substituting tne foregoing values we obtain 

 T - N = 47 db 



If the assumed school target strength of 

 T = 15 db is used in N = T - 47, then N = 32 

 db/Hz. If the signal from the school is to be 

 seen, the foregoing requires certain limitations 

 on the noise such as maximum sea state of 

 about six or a ship noise of about -35 db at 

 the sonar operating frequency. This require- 

 ment represents a fairly quiet ship with a 

 velocity of some 3 knots. If the calculations 

 had been based on a range of 1,000 yards 

 then the permissible ship noise would be 

 about -20 db, which is about 5 to 8 knots for 

 a ship. For the same performance parameters. 



but with a pulse system of equivalent band- 

 width, the pulse system would suffer some 

 3 db disadvantage. In addition, the pulse system 

 would be totally inadequate owing to the much 

 slower rate of data caused by the roundtrip 

 time between pulses. 



FEASIBILITY STUDY 



Target classification from theory to practice 

 followed a series of phases. The first phase 

 was to determine some likely values for the 

 target strengths of fishes. This work was done 

 at the NTCF (Volberg, 1970). -^ From these 

 approximations it was possible to specify cer- 

 tain sonar design parameters necessary for 

 the construction of a sonar capable of detect- 

 ing single fish at ranges up to 100 m. 



The second phase was to construct this 

 sonar and use it in tank and field tests to 

 nnieasure sonar returns from fishand to record 

 fish Doppler signals to find out the best way 

 or processing them. If the results from this 

 second phase were encouraging, the next phase 

 would be to modify and package the sonar 

 for shipboard use and to design and construct 

 a suitable unit to process Doppler signals 

 for the sonar. 



For the second phase of the program, por- 

 tions of the sonar system were gathered 

 together and the system layout, system fre- 

 quencies, sweep rates, beam widths, and other 

 important parameters for the sonar were 

 established. Whenever possible, off-the-shelf 

 components were used. Early in this step we 

 saw that working at close range with such a 

 weak target would impose special problems 

 upon the sonar. Particular emphasis was 

 placed, therefore, on reducing the noise of the 

 sonar receiver to the minimum threshold level 

 dictated by thermal and ocean noise. 



The sonar system that evolved from the 

 effort had unique design and capability, Straza 

 Industries at this time possessed a small test 

 tank, 4 by 4 by 20 feet (1,2 by 1,2 by 6,1 m,), 

 which was used to determine some of the water 

 parameters of the sonar system. These tests 

 yielded sufficient information to indicate that 

 open-water, or tank tests, were necessary to 

 evaluate properly the performance of the 

 receiver. This work required a suitable test 

 site to whichthe instrumentation and equipment 

 could be transported for evaluation of the 

 sonar equipment. Several sites were selected 

 and used. 



Lake Murray Equipment Tests 



The first tests in a large body of water 

 were intended to determine the smallest signal 

 that the sonar receiver could resolve and to 



See footnote 2, 



