BIRDSALL: COHERENCE 



The measures of space coherence were primarily (1) pulse time of arrival, 

 and (2) the linear correlation coefficient estimated from clipped pro- 

 cessing. That correlation would drop sharply to one-half, and then fall 

 off slowly as the spacing increased. Tracking and prediction of non- 

 plane wave fronts was limited by the speed and size of the available 

 computers. 



In the same time frame the NEL studies with pseudo-noise trans- 

 missions, controlled transmissions that covered over one-third of an 

 octave, showed substantial waveform repeatability. 



Almost all experiments over long ranges involve at least one moving 

 platform. Great care has been taken to reduce the fluctuation of the 

 platform, through using submarines and drifting ships. This care is 

 influenced by the experimenter's opinion of the stability of the medium. 

 There is little to be gained by reducing the platform instability effects 

 far below the effects that will be caused by the mediiim. As instrumen- 

 tation improves we often repeat the old experiments and get different 

 results. In a drifting-ship to bottomed- receiver experiment in 1963, 

 using CW (a 420 Hz tone) , across the Straits of Florida, a frequency 

 stability of 4 millihertz was observed. That stability was comparable 

 to the frequency source stability and to the ship station keeping. 

 Subsequent fixed-site experiments with improved sources confirmed this 

 stability in the Straits of Florida and over the old Artemis range in 

 the Atlantic. 



This millihertz frequency stability is a nice example of the com- 

 plexity of 'coherence'. It does not mean that the received signal looks 

 like a pure tone. The signal shows substantial amplitude fluctuations 

 and some phase fluctuations, which are now recognized as the effect of 

 forward scattered surface reverberation. Mother Nature thoughtfully 

 arranged for this reverb to lie in frequency sidebands some 50 to 500 



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