Assume the sequence of values recorded at Transducer No. 1 Is desired. Appro- 

 priate pulse signals are monitored on an oscilloscope (Fig. AT), and the time tnterval 

 between outgoing pulse and return echo is counted on the time Interval counter. 

 Time values are then printed out on a digital recorder. As a check on the quality of 

 the digital printout, an Esterline-Angus Recording mllliameter is used to record the 

 analog output from the digital recorder, which appears as a staircase function and Is 

 smoothed out by the slow response time of the Esterline-Angus Recording mllliameter. 



Normally, the counter displays time In milliseconds, and this Is proportional to 

 the sea height above the submarine. For convenience In computation, It Is desirable 

 to have the print-out In feet directly. This Is accomplished by introducing a 25 kc 

 signal to the external time base of the time Interval counter. The output of the 

 printer is then given directly In feet and tenths of feet . These values are coded sub- 

 sequently on punched cards for high speed computation of power spectra on the 

 Datatron . 



B . Effect of Submarine Motion on Surface Wave Measurements 



Sonic Surface Scanner wave measurements at a keel depth of 100 feet and made 

 under lower sea state conditions will not be affected seriously by the submarine 

 moving in response to the subsurface wave motion. However, as the sea surface wave 

 height grows under the action of the wind, or If low frequency swell propagates over 

 the hovering submarine, the low frequency (high period) components will Increase 

 the amplitudes of motion of the submarine. In effect, wave measurements will be made 

 with respect to a moving reference system. 



Now, If water depth Is great, the attenuation factor of a simple harmonic wave In 

 the neighborhood of the hovering submarine Is given by: 



K„ = e — for L = - 



^Z " L 2ir 



L = wave length 



T = wave period 



Z = depth to the submarine 



Depth in the operating area off Wilmington, North Carolina, was 250 fathoms, 

 and we will consider K for Z = 75 feet and 100 feet. These two depths represent 

 average depth to the Sonic Surface Scanner transducers and the average keel depth 

 for Sonic Scanner spectra data shown in Figures 10 and 11 . Expressing !<_, as a 

 function of frequency f for fixed depth Z: 



50 



