FIXED-SYSTEM MEASUREMENTS OF TIME-VARYING 

 MULTIPATH AND DOPPLER SPREADING 



H. A. DeFerrari 

 Rosenstiel School of Marine and Atmospheric Science 



Signals transmitted through ocean channels will be spread in 

 time because of multipath and spread in frequency because of 

 scatter from the ocean wave surface. Fixed-system measure- 

 ments in the Florida Straits and between Eleuthera and Bermuda 

 make possible the observation of time-varying multipath inter- 

 ference and Doppler spectra. Results are summarized for 

 several short experiments using CW (420 Hz) and pulse CW trans- 

 missions. 



Fully coherent ray models are used to interpret experimental 

 results. These models predict the transmission loss and travel 

 time along all paths with sufficient accuracy to allow the co- 

 herent addition of arrivals at the receiver. Time-varying CW 

 multipath interference is simulated by introducing perturbations 

 to the sound-speed field and generating time series of phase 

 and transmission-loss fluctuations for comparisons with experi- 

 mental results. Model computations show that horizontally in- 

 variant internal waves produce sound-speed perturbations that 

 cannot cause both the phase and transmission loss fluctuations 

 which are consistent with experiment. When horizontal fluctua- 

 tions are introduced to the sound-speed perturbations, statis- 

 tics of CW transmission fluctuations match experimental results. 



Pulsed CW transmission can also be simulated by coherent addi- 

 tion of received pulses. Broadband characteristics of received 

 signals exhibit selective fading. The frequency of the fade 

 is sensitive to small perturbations of sound speed while the 

 fade bandwidth depends on average characteristics of the pro- 

 pagation channel and is relatively insensitive to the typically 

 observed fluctuations of sound speed. 



Doppler spectra and scattering functions are presented and dis- 

 cussed. Combined propagation and scattering models show that 

 unsymmetric surface-scatter sidebands can result from bottom 

 interactions . 



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