BIRDSALL: COHERENCE 



The arrival frequency shifted 0.5 millihertz and 12 hours later changed 

 again by 0.8 millihertz. Even the weak trailing arrivals show re- 

 peatable phase often lasting for hours. There is strong evidence of 

 consistence and pattern, but it is complicated, and one time-constant 

 does not describe this type of data. 



HALF-TIME SUMMARY 



Coherence is a complicated subject, but worth pursuing because of 

 the potential gain in apt signal processing at low signal-to-noise 

 ratios. Coherent propagation may lead to complex receptions which must 

 be sorted out, and some physical measurements will appear to be much 

 more coherent than others. The lack of regularity in one class of 

 measurement does not imply incoherent propagation, and simple measure- 

 ments of correlation may be deceptively uninformative . 



In the second half of this paper a model of propagation from a 

 submerged moving source will be presented to show how a complicated 

 and apparently incoherent signal may be received even though the pro- 

 pagation itself is totally coherent, totally deterministic. The model 

 agrees well with measurements in many respects, but I beg your in- 

 dulgence for leaving that to another paper. The pujrpose of this pre- 

 sentation is to emphasize my theme: Much of the randomness in under- 

 water acoustics is not inherent randomness, but is rather a manifesta- 

 tion of complicated deterministic phenomena. 



AN EXAMPLE MODEL 



Picture a deep ocean with a single classical sound speed profile 

 that applies everywhere, and a fixed source at 150 meters depth and 

 600 km from a deep receiver. The numbers are purely for example 

 sake. This source emits a steady pure tone, let us say at 250 Hz. 



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