-30fl (9 1m.)- 



BUOY 



Figure 4. — Test arrangements at Lake Murray. 



find the best method for removing weak target 

 information from the recorded data. Although 

 water tests had been made in the very small 

 tank at Straza Industries, it was not possible 

 to project sufficient radiated power into the 

 water for proper evaluation of the operating 

 performance of the equipment. A tank would 

 not have been suitable for this series of tests 

 because reverberations would influence the 

 receiver performance. Lake Murray, a reser- 

 voir within 7 miles (11,3 km.) of Straza In- 

 dustries, was made available for a test through 

 the cooperation of the San Diego County Fish 

 and Game Service. This enclosed body of water 

 was free of manmade noise on 2 days of the 

 week while closed to the public. Lake Murray 

 seemed to meet the evaluation requirements. 

 The equipment was transported to the site, and 

 recordings were made of various inanimate 

 objects. 



To evaluate the performance of the equipment 

 appropriately at Lake Murray required a target 

 of a known signal strength. Such a target, a 

 triplane 4 inches (10 cm.) on the side, was 

 built. Equipment was built to support the 

 triplane and also to permit it to rotate in a 

 4-foot (1.2 m.) circle (fig. 4). 



The first returns from the snnall targets 

 used in the Lake Murray tests indicated that 

 the receiver was not performing with the 

 required sensitivity. The receiver was there- 

 fore redesigned on the basis of the information 

 gained in these tests. Actually, the receiver 

 performed well but not to the required speci- 

 fications. The receiver did respond marginally 

 to small fish swimming near the transducers. 



A second test at Lake Murray indicated 

 that the receiver was performing with the 

 desired sensitivity and could respond to ex- 

 tremely small targets. This finding, however, 

 did not necessarily mean that the sonar 

 equipment would perform well on fish in small 

 contained bodies of water. After the second 

 test the next step was use of the equipment 

 on live fish of the desired dimensions and 

 species that were to be studied in the program. 



Swimming Pool Tests 



Before tests were started with live fish in 

 a large body of water, a series of tests were 

 made to evaluate various enclosed sites. One 

 test was carried out in a kidney-shaped 

 swinnming pool to evaluate the crosstalk or 

 reverberation produced by the walls of the 

 pool. The level of crosstalk was found literally 

 to overwhelm the receiver. The reverberation 

 level was so high that it nearly blotted out 

 large signal returns produced by an 18 -inch 

 (46 cm.^ square flat reflector of aluminiim. 

 It was impossible to detect the small triplanes 

 that had been used in the Lake Murray tests. 

 This first indication that the pool would 

 produce substantial reflections from its walls 

 was not totally unexpected, but the level of the 

 reverberation was surprisingly high. Before 

 a test site could be decided upon, it clearly 

 was necessary to transport the equipment to 

 the site and determine the reverberation level 

 of the tank. Since this test had been in a 

 kidney-shaped pool it was possible that a 

 pool with a different shape might produce 



