a narrow but finite angular beam, so that all points within 

 the footprint reflect equal power back to the source) the 

 returned power I at a given delay time t , = -^-ii + 2t 

 is proportional to the area of the footprint. For small 

 footprint diameters relative to the source height h , 



the area Increases linearly with the penetration time t 

 (cf . Fig. 9) , so that 



{0 for t < 

 (3) 

 atp for tp < 



where the constant oi is determined by geometric factors 

 and the mean square wave slope, in accordance with the 

 specular reflexion model valid for normal incidence (cf. 

 table 1). The case of a square-shaped initial pulse 

 follows from the step-function solution (Including the 

 modifications considered below) by subtracting a second, 

 identical solution displaced in time. 



In the presence of waves , the sharp break at the onset 

 of the reflected pulse is smoothed through the early 

 arrivals of energy reflected from the crests of waves before 

 the pulse reaches the mean sea surface. The modification 

 of the reflected pulse shape contains useful sea state 

 information, but tends also to degrade measurements of the 

 mean sea surface elevation. Particularly Important for the 

 latter problem is the question whether the pulse distortion 



25-45 



