FISHERY BULLETIN: VOL. 69, NO. 4 



region. Therefore, although it would be advan- 

 tageous to avoid the interference region, it is 

 apparent that this is the region in which the 

 worl< must be done. 



METHODS OF TARGET STRENGTH 

 JVIEASUREMENT 



Analytical methods are of limited value in this 

 interference region and experimental methods 

 must be utilized to obtain any valid answers. 

 It is possible to conduct the needed experiments 

 either at sea or in the laboratoiy, with each type 

 of measurement having its limitations. Whether 

 the measurements are made at sea or in the lab- 

 oratory, target strength will be determined from 

 the sonar equation, meaning that the source level 

 of the transmitter, the sensitivity of the receiver, 

 and the propagation loss must be known. The 

 calibration of the transmitting and receiving 

 systems is a standard procedure, but propagation 

 loss is much more diflicult to determine if long 

 ranges are involved. One way to avoid the prop- 

 agation loss problem is to make measurements 

 at short ranges, and this is what is done in the 

 laboratory. This, of course, is impossible with 

 large targets. Another method is to use a ref- 

 erence target for which the target strength is 

 known. In this method neither the transmitting 

 nor receiving systems have to be calibrated and 

 the propagation loss does not have to be measured 

 because all echo levels are compared to the ref- 

 erence level. One of the best reference targets 

 is a thin-walled air-filled rubber sphere, although 

 for large targets buoyancy becomes a problem. 

 Another good reference target, which can be 

 used for large targets, is a tri-i)lane, three mu- 

 tually perpendicular planes, for which it can be 

 shown that any incident ray will be reflected in 

 a direction exactly opposite to the incident di- 

 rection. Hence a tri-i>lane acts as a single plane 

 ])erpcndicular to the incident rays and reflects 

 a large, calculable percentage of the incident 

 energy. It is possible to measure propagation 

 loss directly and this is fairly simi)le if a trans- 

 mitting and a receiving ship are utilized. Propa- 

 gation loss measurements can also be made by 

 placing a calibrated transponder in the vicinity 

 of the target. 



Along with the problem of accurately measur- 

 ing propagation loss, there are other jiroblems 

 associated with target strength measurements 

 at sea, the most critical of these being relative 

 motion between the sonar beam and the fish 

 target. Roll or pitch of the ship can be over- 

 come by using a stabilized sonar beam, hut drift 

 can cau.se the axis of the beam to move off target. 

 Care must also be taken so that the target sup- 

 port structure does not interfere with the mea- 

 surements. Other problems that can arise are 

 poor weather, high ambient noise levels, and ex- 

 traneous targets swimming into the beam. 



Of course, there are problems associated with 

 laboratory measurements also, the chief one 

 being the limitation on the size of the target. 

 The fish must be ]ilaced at a range great enough 

 to insure that the incident sound energy is ap- 

 proximately equal over the complete fish and to 

 insure that the fish is not in the near-field of the 

 transmitter nor the receiver in the near-field of 

 the fish. However, the range must not be so 

 great that reflections from the boundaries or fish 

 supiiort interfere with the measurements. 

 Nevertheless, by judicious choice of measure- 

 ment range and by using short pulse lengths, 

 unambiguous work can be done in a laljoratory 

 tank. In order to obtain a true value of target 

 strength the pulse lengths of the discrete fre- 

 quency pulses most often utilized by fisheries 

 sonars must be at least twice the length of the 

 target in the direction of projiagation, so that 

 an echo can be obtained from all parts of the 

 target simultaneously. 



A tyiiical block diagram of the electronics re- 

 quired for target strength measurements is 

 shown in Figure 1. The transmitting system 

 consists of a signal soui'ce of known frequency, 

 a means to generate puLses, amplifiers, a trans- 

 mitting transducer, a system to match the elec- 

 trical impedances of the amplifier and transduc- 

 er, and a means to measure the outgoing signal. 

 The receiving .system consists of a receiving 

 tiansducer, amplifiers, a means to gate out un- 

 wanted echoes, iiossibly a filter, and a means to 

 measure the received signal. The electronic 

 system is basically the same whethei' it is used 

 in the laboi'atory or at sea. 



If all fish were composed of the same homoge- 



706 



