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

 AND DOPPLER SPREADING 



I would like to go back now and look at one specular arrival and 

 the family of scattered arrivals that have the same number of 

 surface interactions. Figure 31 illustrates the SRBR structure plotted 

 in terms of the intensity versus arrival time for the same total 

 number of surface reflections (25) . The number next to each arrival 

 indicates the number of specular bounces it had made prior to emitting 

 the scattered ray which happens to hit the receiver. The one marked 

 "24" made 24 specular bounces before it emitted the ray. Note that 

 the rays which scatter at the ends (either near the source or near the 

 receiver) suffer the least loss. 



The same information can be expressed in terms of the grazing 

 angle that the arrivals make with the bottom after scattering (Figure 32) 

 All the arrivals that interact in the last half of the received pulse 

 have a significantly lower grazing angle, about 5 to 10 degrees, than 

 all those that are in the first half. Also the first half are all 

 upscattered, whereas the second half are all downs catte red. The 

 difference in the bottom loss with these different grazing angles is 

 enough to cause the consistent 3 to 6 dB sideband asymmetry. 



Figure 33 is the predicted Doppler spectrum (or more properly the 

 transfer function which must be multiplied by the surface-wave spectrum 

 to get the Doppler spectriom) . Note the 3 dB difference in the side 

 bands. The scattering event itself is symmetric. Because of the 

 differences caused by: (1) the angle at which the ray is emitted from 

 the surface and (2) the requirement that the ray hit the receiver, 

 the upscattered paths have significantly less loss than the down- 

 scattered paths. These results are quite consistent with what is 

 observed in measured data. 



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