BIRDSALL: COHERENCE 



"signal detection theory" is a study of precisely how uncertainty 

 about signal waveform and uncertainty about the noise characteristics, 

 together with interfering power, affect signal-processing design. Per- 

 haps more important, it studies how uncertainty and noise combine to 

 limit ultimate performance in signal detection and information extrac- 

 tion. The theoretical results are complicated, but there are some guid- 

 ing general principles. 



One such general principle is that the effective signal-to-noise 

 ratio after processing will fall off sharply at low signal-to-noise 

 inputs. Just where this 'suppression effect' cuts in depends on the 

 degree of uncertainty about the signal and noise characteristics. When 

 the input is well above the 'knee' most reasonable processors work 

 about equally well, and signal uncertainty is not very important. The 

 other side of the coin is that processors can be designed to dig infor- 

 mation out of weak receptions only if there is substantial knowledge 

 of signal characteristics and this knowledge is used. Said again, if 

 detailed signal knowledge is available and is used, it may mean tens- 

 of-dBs of processing gain. 



BRIEF HISTORICAL REVIEW 



Twenty years ago many U.S. propagation people do\abted that there 

 was sufficient stability in signals propagated over long distances to 

 support detailed signal knowledge. There were notable exceptions. 

 Project Artemis of Hudson Labs was a courageous step forward in investi- 

 gating propagation stability as seen at a very large receiving array. 

 One must qualify all experimental results as being specific to the areas 

 and frequencies studied. For Artemis that means for the Atlantic area 

 south of Bermuda and in the neighborhood of 400 Hz. Artemis established 

 the predominance of RSR (refracted, surface-reflected) paths for long- 

 range propagation, and the importance of the bottom topography and 

 local internal waves in the neighborhood of a slope mounted array. 



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