DYER: FLUCTUATIONS: AN OVERVIEW 



At the other extreme are very slow fades which have been 

 described as seasonal fluctuations. Perhaps the record time for 

 these is as long as 1 year and averaging times of 1 month or more 

 might be used. 



Between these obvious extremes are other kinds of fading, per- 

 haps three or more, worth distinguishing between because they appear 

 to be associated with separate mechanisms. For example, fast fading 

 fluctuations tend to have a fading range of 20 dB in amplitude, a 

 period of about 2 5 minutes, and a decorrelation time of about 5 

 minutes. The phase, on the other hand, typically ranges over 5 cycles 

 with a 60- to 80-minute period, and perhaps a 25-minute decorrelation 

 time. 



The intermediate fades yield rather different numbers. It should 

 be emphasized that this particular set of experiments is for one 

 frequency with fixed source and receiver locations. Hence, the only 

 motion that does occur is, in fact, the motion of the ocean. For the 

 intermediate fades, there is a definite period of 12.4 hours, with a 

 somewhat reduced fading range on amplitude and an increased fading 

 range on phase. 



Figure 3 illustrates a possible mechanism for the fast-fading 



case which leads to the general result that the fading period on 



amplitude is related to the fading period on phase by the simple 



ratio Tt/e 



rms 



The argument proceeds from a modal interference picture which 

 treats the mean square pressure as a sum of sinusoids. Whenever the 

 phase difference between pairs of modes approaches twice the average 

 phase, an amplitude beat is generated. Hence, that amplitude beat 

 is given whenever the average phase is of the order of ti . Typical 



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