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

 AND DOPPLER SPREADING 



on the shore and dissipating while propagating seaward. The effect was 

 the same as taking the perturbations and distributing them evenly over 

 the whole range. Basically it provides enough phase shift but not 

 enough interference. 



However, using the tidal frequencies and adding a little bit more 

 perturbation to the system we can get the same amount of interference 

 that we see in the experiments. Figure 18 illustrates the procedure 

 where a 4-meter random internal-wave component is added to the decay- 

 ing tide. It adds no significant contribution to the phase (see middle 

 graph) other than the same jitter. However, the jitter introduces 

 more fades (bottom graph) . Each ray basically interacts with fluctua- 

 tion components of comparable cycle distances. Hence they select the 

 appropriate component from the internal-wave spectrum. 



There are a lot of other modes that could be added to the pertur- 

 bation but all I have put in are tidal-like frequencies. I have broken 

 it up spatially, and it seems to be enough. 



The fades that we see are not strictly continuous wave — that 

 is they have some bandwidth associated with them. One way to measure 

 it is to transmit a broadband signal, a pulse and look at the received 

 time series. Figure 19 illustrates the result of transmitting a 20 

 millisecond pulse. The signal that arrived was about 100 milliseconds 

 wide representing the superposition of many pulses with slightly 

 different travel times corresponding to different RBR rays. Also a 

 lower level group is seen which appears from the model studies to be 

 an SKBR arrival. 



Figure 20 illustrates the behavior of such pulses during the time 

 that a CW signal is fading. The top figure shows the CW amplitude 



487 



