on the sonar's effective range, its ability to determine the 

 depth and movement of fish, and the likelihood that the 

 sonar targets can be specifically identified. 



The sonar emits a beam of inaudible high-frequency sound 

 in a specified direction. A target such as a fish reflects a 

 portion of that sound to transducers aboard the ship. The 

 signal is translated in two forms: as an audible sound and 

 as a moving point of light on the face of the cathode-ray 

 screen (fig. 28) . The sonar operates in two modes, a "search" 

 mode of 52 to 32 kilocycles per second for distant targets 

 (about a mile away) and a narrower and more intense 

 "classify" mode of 290 to 260 kilocycles per second for closer 

 examination of nearby targets. 



Throughout the summer and fall of 1966, the Toicnsend 

 Cromwell coursed the waters of the Hawaiian Islands to 

 test the equipment and train operators. During much of the 

 period the sonar was operating 24 hours a day. It will take 

 several months and a thorough reflective scrutiny of the 

 material before the scientists will be able to say with 

 confidence all they have discovered. 



One positive gain of the cruises, says Heeny S. H. Yuen, 

 who supervised the sonar studies, has been confidence in the 

 equipment and its potentialities. For years the Townsend 

 Cromirell and Charles H. Gilbert have been equipped with 

 underwater viewing ports. Ma.ximum visibility has been 

 20 yards and not always that. With the sonar, a single 

 skipjack tuna 20 inches long, hung beneath a buoy, could 

 be observed from 258 yards. Schools of skipjack tuna could 

 be observed as far as 440 yards — and on one occasion, 715 



FIGURE 27. The new CTFM sonor oboord the TOWNSEND CROM- 

 WELL ollows scientists to frock individual fish and fish schools 

 beneath the surface of the woter. hiigh-frequency sound emitted by 

 the transducers is reflected by the fish back to the ship Eoch return 

 appears os a point of light on a cothode-ray screen Icenteri. The 

 sonar is kept on torget by meons of o joy stick, which con be seen on 

 the console beneoth the cathode-ray screen. 



FIGURE 2%. This sector of the cothode-ray screen shows sonar returns 

 mode by fishes to the port of the ship (note bearings). The lorge 

 circle is o cursor of light which the technician uses to isolate a target. 



41 



