DYER: FLUCTUATIONS: AN OVERVIEW 



The experimental geometry consisted of a fixed string of receivers at 

 the depths indicated and a source closing from a range of 7 miles to 

 a closest-point-of-approach of 2 miles and continuing on out to about 

 7 miles. The source frequency is 130 Hertz and the source speed is 

 6 knots. 



Notice that for the shallow (300-meter) receivers, periodicity 

 is about 10 minutes, whereas for the deeper receivers, between 2,000 

 and 3,000 meters, the periodicity is of the order of one or two 

 minutes. These time scales are consistent with the spatial scales 

 that exist in the ocean, as sampled by the various source-receiver 

 geometries. The temporal scales associated with internal-wave motion 

 in this geometry would lead to periods of 40 to 50 minutes for this 

 frequency. This is a good example of an experiment which yields 

 time scales that result from the structure of the acoustic field in 

 the ocean and not from the ocean dynamics. 



The conclusions these various results suggest are that the ocean 

 can move and hence give some structure to received-signal fluctuations 

 and, also, the platforms can move resulting in additional fluctuation 

 structure. Both of these possibilities must be considered. In fact, 

 in many practical circumstances there are sources moving near 6 knots 

 and a technique is needed to combine situations where fluctuations 

 due to platform motion and ocean dynamics are comparable. No theory 

 adequately takes both into account. In fact, no theories adequately 

 treat either of the two separate mechanisms. 



Figure 9 addresses a few more facets of the fluctuation problem. 

 In a data record which addresses fast fading but is also long enough 

 to include, for example, intermediate fading, variations appear in 

 the mean, p , of the individual fast-fade processes. 



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