DEFERRARI: FIXED-SYSTEM MEASUREMENTS OF TIME-VARYING MULTIPATH 

 AND DOPPLER SPREADING 



This paper consists of the presentation and. discussion of some re- 

 sults from fixed-system measurements. Basically three types of pheno- 

 mena are described: 



• Time-varying multipath for CW signals 



• Time-varying multipath for broadband signals 



• Doppler spreading 



Before presenting these data I would like to discuss a model used 

 for their interpretation. The basic model is a bilinear profile with 

 quasi-static fluctuations. A surface scattering model is included on 

 each path, and the paths are added coherently. 



Figure 1 shows typical range-averaged profile between Eleuthera 

 and Bermuda and a bilinear fit to it. We don't have any experimental 

 data of the sound speed fluctuations there, so for perturbations of this 

 profile we use a calculation made by Dr. Moore at the University of 

 Miami of the first-mode internal wave for an internal tide of wavelength 

 150 kilometers. Figure 2 shows the resulting perturbed profiles, and 

 Figure 3 shows the bilinear approximations to them. The perturbations 

 can then be described in terms of two parameters: the depth D of the 

 axis and the bilinear angle a. This model can be mode range-dependent, 

 as shown in Figure 4. The sound-speed profile becomes a function of 

 range by segmenting it and allowing the profile to change with range 

 and also with time. Figure 5 shows the bilinear fit to some actual 

 sound-speed measurements made about mid- range in the Florida Straits. 

 One profile was obtained every two hours for four days. If you look at 

 the sequence closely, you can see the effects of a tide. The gradients 

 change and the knee rises and falls by the tidal periodicity. 



The bottom sketch in Figure 6 is the surface scatter model we will 

 use. There is a specular reflection, unshifted in frequency, from the 

 surface waves, and Doppler- shifted sidebands separated in frequency 

 from the carrier by multiples of the surface-wave frequency. We have 



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