BIRDSALL: COHERENCE 



Does the consistency of propagation break down as signal frequen- 

 cies are separated? It is common to experience different fades at sig- 

 nals just a few Hertz apart. I would like to cite just one study to 

 indicate that the behavior in frequency is more coherent, and more 

 complicated, than a Markov process. Single-path loss measurements were 

 made at 61 frequencies spaced 5/6 Hz apart, covering the regime from 

 395 Hz to 44 5 Hz over a period of 7 hours. The transmission was over 

 43 miles across the Straits of Florida. The loss contours as a func- 

 tion of frequency and time show a lot of pattern; I hope enough to 

 encourage studies that go after the whole surface, and enough to dis- 

 courage attempts at determining a correlation bandwidth. The frequency 

 deviation plot for the same data shows major peaks of the order of 

 one millihertz wide. Low-magnitude broadband ripples and changes 

 slide across frequencies in time; however, the entire band has a rea- 

 sonable unity. Of course it is only about one-sixth of an octave, but 

 that is all many sonars (active) cover. Incidentally, these data were 

 taken with nine- foot seas overhead. 



Correlation time-constants for multipath propagation are another 

 popular concept. In some locations it may be a valid description of 

 multipath behavior. Again, I would like to cite one study to indi- 

 cate that multipath propagation may be more coherent than suspected, 

 but that much careful work will have to be done to discover the 

 coherent parts and to use them. The data spans one day, and used a 

 continuous transmission designed to yield the same time resolution as 

 a 20 millisecond pulse repeated every 1.2 seconds (but with 18 dB more 

 processing potential) . The data taken in November 1971 show a dominant 

 30-millisecond arrival alternately merging and contrasting with a 

 following weaker arrival. This routine structure shows a dramatic 

 change both in the duration of the arrival and in the phase pattern. 



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