DEFERRARI: FIXED-SYSTEM MEASUREMENTS OF THE TIME-VARYING MULTIPATH 

 AND DOPPLER SPREADING 



Figure 23 illustrates a modeled case for a typical sound-speed 

 profile measured in the Florida Straits with a small perturbation added 

 to it. On the left is an amplitude-frequency-time plot and on the 

 right is a phase- frequency- time plot. Note the two fades at early 

 times. As time goes on they slide across the band. Traveling with 

 the deep fade is a 180-degree fade shift. The other fade isn't quite 

 as deep and its phase shift is somewhat less than 180 degrees. 



While no one would claim to be able to predict when these fades 

 will occur, certain features are predictable, notably the bandwidth. 

 The bandwidth depends on average characteristics, not on the detailed 

 fluctuations in the sound-speed profile. The precise time of the fade 

 is determined by extremely small changes in the profile and hence is 

 not predictable. 



The models not only predict the frequency response but can also 

 simulate spatial processing; for example, a coherent summation at several 

 points. The modeled fade cells as shown in Figure 24 are small and 

 isolated at 100 Hz. One of the few advantages of ray theory is once 

 you make this computation you can change the frequency and easily 

 consider several frequencies. Figure 25 is the same kind of plot for 

 200 Hz. (Note: there's a scaling of a hundred to one from range to 

 depth so these contours are actually very elongated. The contour in- 

 terval is 5 dB.) Figure 25 is the same thing at 420 Hz. 



I would now like to present some Doppler-spread data. Figure 27 

 is a typical Doppler spectrum I measured in the Florida Straits. The 

 carrier line at 420 Hz has been suppressed to emphasize the sidebands. 

 The sidebands are characteristically asymmetric and differ by 3 to 6 dB 

 almost always. The spectrvun appears to be a replica of the surface- 

 wave spectrum. 



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