TECHNIQUES OF NEW YORK MEASUREMENTS 



423 



reverberation studies. The average maximum ampli- 

 tude of five successive echoes, together with the 

 calibration constants of the equipment, was used to 

 compute the echo level at each aspect angle; an 

 auxiliary transducer measured the source level be- 

 fore and after each run. However, the transmission 

 loss was not measured directly. Transmission condi- 

 tions were fair, since the water was isothermal to a 

 depth of about 50 ft. Accordingly, inverse square 

 divergence and an attenuation coefficient of 5 db per 

 kyd were assumed in estimating the transmission 

 loss. Each target strength was computed from the 

 a\'erage echo level of five echoes; each range was the 

 average range over the five echoes, as measured on 

 the oscillograms. Aspect angles were estimated 

 trigonometrically. 



24.2 TECHNIQUES OF NEW YORK 

 MEASUREMENTS 



Two series of tests were made by BTL on ships in 

 Long Island Sound early in 1944, as part of a specific 

 development project to study the effects of short 

 pulse lengths and receiver bandwidth on echo rang- 

 ing,' and to measure echoes from surface vessels.^ 

 Because little time was allocated to this part of the 

 program, the work was discontinued as soon as 

 enough data were obtained to establish the range of 

 echo intensities to be expected. 



In the earlier measurements, made in Long Island 

 Sound near City Island, Hart's Island, and Execu- 

 tion Light, pulse lengths from 0.05 to 150 msec were 

 used at frequencies between 20 and 30 kc' Echo- 

 ranging gear including a transmitter and a receiving 

 system of adjustable characteristics was mounted 

 aboard a laboratory boat, the Elcobel, which was al- 

 ready equipped with a standard Navy projector 

 dome. Targets of these tests were various freighters 

 in the vicinity. No absolute echo levels or target 

 strengths were measured, since the experiment was 

 conducted largely to investigate the effects of pulse 

 •and receiver characteristics on reverberation, noise, 

 and echo character. Relative echo amplitudes were 

 found for diflEerent pulse lengths, however, and are 

 reported in Section 24.7. 



Later studies reported in more detail the reflect- 

 ing characteristics of a total of twenty surface ves- 

 sels.^ In these measurements, a crystal transducer 

 was mounted on the Elcobel, a 65-ft boat, in such a 

 way that it could be carried just below the keel while 

 under way, or lowered to a depth of 10 ft for echo 



ranging. In the lower position, the transducer could 

 be trained by means of a hand wheel on top of the 

 shaft; however, the speed of the Elcobel could not 

 exceed a few knots without interfering with the satis- 

 factory operation of the transducer. 



An oscillator aboard the Elcobel delivered pulses 

 approximately 3 msec long to the transducer, at a 

 frequency of 27 kc. The echoes received by the trans- 

 ducer were amplified, observed and photographed on 

 the screen of a cathode-ray oscilloscope, whose hori- 

 zontal sweep was proportional to the time — and 

 therefore to the range of the echo — and whose ver- 

 tical sweep was proportional to the amplitude of the 

 echo. 



In order to obtain target strengths, the average 

 range and the average peak amplitude of between 10 

 and 60 echoes were measured on the oscillograms. 

 The transmission loss was estimated on the assump- 

 tion of inverse square divergence and an attenuation 

 coefficient of 7 db per kyd, although such an assump- 

 tion probably was unrealistic since the water was 

 shallow during these measurements and the ocean 

 bottom was an effective reflector of sound. From the 

 average range, the average peak amplitude, and the 

 transmission loss, the diameter of the equivalent 

 sphere was computed — the sphere which would 

 theoretically return the same echo under the same 

 conditions. Then the target strength was readily 

 determined from the diameter of the equivalent 

 sphere by use of equation (10) in Chapter 19. 



24.2.1 Tests on Anchored Vessels 



In the first part of the second series of tests, the 

 targets were ships at anchor in the tideway of Long 

 Island Sound, near City Island, New York, where the 

 water was less than 100 ft deep. Echoes from five 

 freighters, a tanker, a Liberty ship, and a small 

 British carrier were measured. During these tests the 

 Elcobel was kept under way at a very slow speed, so 

 that both the range and the aspect angle of the tar- 

 get varied in almost all the tests. 



24.2.2 Tests on Moving Vessels 



The Elcobel also ranged on moving ships farther 

 out in Long Island Sound, in the vicinity of Lloyd's 

 Neck, Long Island. These tests were made, without 

 any advance arrangements, on passing ships whose 

 courses brought them close enough to the Elcobel to 

 make them satisfactory targets. Unfortunately, the 



