DYER: FLUCTUATIONS: AN OVERVIEW 



amplitude periods are then on the order of 1.5 divided by the number 



of fade cycles. Referring back to Figure 2, for fast fading, T /AG = 



9 



16 minutes, which is approximately two- thirds of the observed T 

 of 25 minutes. The agreement for the decorrelation times is not as 

 good but still in the right ballpark. This argument does not seem 

 to work for the other fading types which suggests that for this partic- 

 ular frequency range and these particular choices of averaging times 

 and record lengths the underlying mechanisms may be different. The 

 intermediate fading rates may, in fact, be related closely to the 

 modal interference that is caused by internal-wave motion, and the 

 slow rates may be caused by planetary waves that have a different 

 kind of behavior with respect to the fading process. 



The results of Figure 2 should not imply that a simple reduction 

 of experiments to a single number table is, in fact, possible. 

 Figure 4 displays results obtained by Stanford (1974) , where two 

 amplitude time series are spatially separated by only 40 meters 

 vertically and 80 meters horizontally. The periods of fade, T , 

 differ by a factor of 2, although the amplitude fade range is about 

 the same. 



Figure 5 illustrates results obtained by Spindel et al. (1974), 

 and the experiment differs in two respects from that reported pre- 

 viously: the range is somewhat different; and, perhaps more sig- 

 nificantly, there is a drift velocity of about a third of a knot, 

 rather than a zero range rate. (Nonetheless, as will be shown sub- 

 sequently, this drift rate may be not too significant.) More impor- 

 tantly, these results show a tremendous depth dependence to the 

 fading. The fading range on phase is of the order of 26 cycles with 

 a period of 140 minutes for the deep receiver. For the shallow 

 receiver above the main sound channel, the fading range is 10 cycles 

 with a fading period of 64 minutes. 



371 



