BIRDSALL: COHERENCE 



phase tracker's average slope yields frequency estimates which have a 

 'capture' behavior, locking onto the frequency of the momentarily 

 largest line pair. 



This example was based on only 2 path pairs, and the 2.7 km range 

 change was ignored in the calculations to emphasize the simple beat. 

 Add a few more path pairs, take into account the opening range and 

 the attendant path and angle changes, and you will obtain a complica- 

 ted fade pattern, and a frequency estimate that hops around. Is that 

 bad? No, that's good! It's a basic part of propagation, and it occurs 

 in every test and every operational situation with a moving source at 

 long range and not on the surface. The fluctuations were coherent. 

 There was no randomness in the example. 



That concludes coherence lesson number three : not everything 

 that varies is a random variable, nor do fluctuating receptions imply 

 incoherent propagation. 



APOLOGY 



My brief history omitted reference to almost everyone, and I owe 

 an apology both to the researchers and to this audience if they were 

 expecting a scholarly review. 



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