366 



DIRECT MEASUREMENT TECHNIQUES 



21.2.4 



Harvard 



The target strength of the ItaUan submarine 

 Vortice was also measured by HUSL workers using a 

 special sonar first in the area of the Bahama Islands, 

 then off the coast of Florida near Port Everglades. 

 Sonar gear mounted aboard the USS Cythera (PY31) 

 echo ranged on the submarine at a frequency of 

 26 kc. 



The first series of tests was made near stern aspect 

 as the Cythera and Vortice followed parallel courses 

 at speeds from 2 to 6 knots.'" Cut-ons were obtained 

 by listening to the echoes. Very few data were col- 

 lected; only 114 echoes were obtained on the Vortice 

 during the two days of measurements so that the re- 

 sults cannot be considered conclusive. 



During the second and more complete series of 

 tests, the Cythera maneuvered around the Vortice in 

 order to determine the dependence of target strength 

 on aspect angle, altitude angle, and range." The 

 Vortice maintained a speed of 3 knots on a base course 

 at depths of 100, 300, and 400 ft. Echo intensities 

 were obtained for groups of approximately 10 echoes; 

 the source level was measured by training the pro- 

 jector at a monitor transducer, then feeding the 

 voltage across the monitor transducer into a cathode- 

 ray oscilloscope and finding the voltage that had to 

 be applied to the oscilloscope in order to balance it. 

 The speed of the Cythera was held close to that of 

 the Vortice to prevent bearings from changing too 

 rapidly; training the projector was accomplished by 

 cut-ons. A vertically directional beam from a QHF 

 transducer was used in addition to the original non- 

 directional beam. 



Aspect angles were estimated at intervals of 5 

 degrees; ranges correct to about 25 yd were read from 

 the sound-range recorder. Altitude angles were not 

 recorded; instead, they were computed from the 

 range, as read from the recorder, and from the depth 

 of the submarine, measured from the ocean surface 

 to the center of the control room about 12 ft above 

 the keel of the .submarine. 



21.2.5 



Fort Lauderdale 



Three runs were made off the coast of Florida by 

 observers from groups at Fort Lauderdale. In one 

 series of tests, the YP451 remained stationary and 

 echo-ranged on the USS Pintado (SS387) and the 

 USS Pipefish (SS388), two new fleet-type submarines 

 which ran past the YP451 at prearranged depths, 

 speeds, and ranges. The equipment aboard the YP451 



included a crystal transducer, driven at 60 kc, which 

 was suspended on a pendulous pipe so that it was 

 15 ft below the surface. The platform carrying the 

 transducer was stabilized by an automatic pilot gyro 

 control in one dimension, with its horizontal axis of 

 rotation normal to the axis of the sound beam. In 

 addition, the transducer was automatically trained in 

 elevation. The pendulum and gyro provided a plat- 

 form which was stabilized in the most critical direc- 

 tion, while the elevation control centered the sound 

 beam on the target vertically; the transducer was 

 trained manually on the target in the horizontal 

 place. The beam width was roughly 25 degrees hori- 

 zontally and 10 degrees vertically. A 6-string electro- 

 magnetic oscillograph recorded the echoes. 



Unfortunately, operations with the YP451 were 

 hampered by mechanical difficulties in the alternating 

 current generator and by failure of radio communica- 

 tion with the escort vessel which maintained sound 

 communication with the submarine. Although this 

 lack of communication resulted in unpredictable 

 maneuvers by the submarine, fairly satisfactory data 

 were obtained on echoes from the submarines. 



In the second and third runs, signals 30 msec long 

 were sent out at a frequency of 60 kc every 0.6 sec. In 

 the second run, a fleet-type submarine at periscope 

 depth followed a straight course at a speed of 6 

 knots. The echo-ranging transducer, mounted with 

 accessory equipment in a submerged unit, circled 

 about a fixed point, 230 yd from the course of 

 the submarine, in a radius of 125 ft and at a depth 

 of approximately 35 ft. Aspects were estimated 

 trigonometrically from observed ranges, which had 

 been corrected for the position of the echo-ranging 

 unit in its turning circle. 



During the third run, an R-boat was the target, at 

 a keel depth of 100 ft and a speed of 6 knots. The 

 range was decreased continuously; cut-ons were em- 

 ployed in training. Since the echo intensities varied, 

 depending on where the beam struck the submarine, 

 a series of echo maxima was obtained and was used 

 to calculate the target strength. These maxima are 

 illustrated in Figure 4, where the echo level — in 

 decibels below the source level — is plotted against 

 the range; each point represents an individual echo. 

 The target strength was computed from the received- 

 echo voltage, as measured on a film continuously ex- 

 posed to a cathode-ray oscilloscope, hydrophone 

 sensitivity, total power output into the water, 

 directivity index of the transducer, and the esti- 

 mated transmission loss. 



