DYER: FLUCTUATIONS: AN OVERVIEW 



These results indicate that it is impossible to state on a 

 single-number basis what the fading parameters are. Nonetheless, it 

 is possible to discuss trends in these data and to try to understand 

 why changes such as these occur. 



Figure 6 shows results taken under more or less comparable con- 

 ditions, where the decorrelation time for amplitude, xA, is plotted 

 versus the carrier frequency. There were three experiments: one by 

 Nichols and Young (1968) at about 270 Hertz, DeFerrari ' s (1974) 

 (which was included in Figure 2) at about 400 Hertz, and Webb and 

 Tucker (1970) at about 800 Hertz. The decorrelation time seems to be 

 reasonably described by something that is intuitively appealing — 

 namely, that the frequency times time is a constant approximated by 

 1800 (when the frequency is in Hertz and the decorrelation time is in 

 minutes). This suggests, for example, that at 100 Hertz the decorre- 

 lation time may be as long as 18 minutes. 



The previous results have been attributed to effects of ocean 

 dynamics. Figure 7 addresses the question: What about making 

 measurements with moving platforms? For moving platforms, the notion 

 of a spatial scan is introduced. There are spatial fluctuations in 

 the acoustic field if the ocean is considered completely stationary 

 or "frozen." These fluctuations are described in terms of a correla- 

 tion coefficient relating changes in range and changes in depth. A 

 rangewise scale, i , and a depthwise scale, i , are then defined in 

 terms of the 1/e points in the correlation coefficient. 



Preston Smith proposed a theory which may have a direct bear- 

 ing on these measures. In the second case of an isogradient duct, 

 the radial or the rangewise scale was found to be proportional to 

 the wavelength divided by the square of an angle, 9 . This angle 

 is, in fact, the angle that encloses all the refracted rays that are 



374 



