DYER: FLUCTUATIONS: AN OVERVIEW 



trapped in the duct. The constant of proportionality involves param- 

 eters such as the depth of the receiver, z , and the depth of the 

 duct, D. 



Using this scale, or whatever scale is appropriate to the problem 

 of interest, the decorrelation time is determined by the scale length 

 and the range rate. When making measurements with a moving platform, 

 the impact of this time on the measurements must be addressed. 



Notice that the decorrelation time for platform motion and ocean 

 dynamics is proportional to the wavelength, as shown in Figure 6. 

 Hence, there should be a particular value of range rate which makes 

 the two decorrelation times equal. This speed depends on the path 

 geometry but for this case appears to be on the order of 3 to 5 knots. 

 That is, if the ocean is scanned at speeds substantially in excess of 

 3 to 5 knots, the fluctuation time scale will be governed by the 

 structure that exists in the ocean as if the ocean were standing 

 still and didn't have, say, internal waves. On the other hand, if 

 the ocean were scanned at speeds significantly less than a few knots 

 (for example, the one- third of a knot in drift used by Spindel et al. 

 (1974) , the time scales may well be those associated with internal 

 waves or other ocean dynamics. 



Some evidence for this is indicated by the NRL experiments where 

 range rates were 7 knots and horizontal scale lengths of 65 kilometers 

 were measured corresponding to the convergence zone spacings. A 

 closer examination of their spectral decomposition in wave number 

 (really interference scales) shows at 14 Hertz about a 9 kilometers 

 interference length which is roughly consistent with the results in 

 Figure 7. 



The transmission-loss data shown in Figure 8 were supplied by 

 Earl Hays and are a good example of the effects of platform motion. 



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