wave crests Is smaller than from the troughs, and the mean 

 backscattered power Is biased towards greater delay times. 

 Thus a linear extrapolation of the asymptotic pulse response 

 defines a virtual onset time corresponding to a calm- 

 surface elevation lower than the true mean surface. For 

 significant correlations between the wave height and the 

 wave slope squared the systematic error introduced in this 

 manner could become of the order of the r.m.s. wave height. 

 Although this will normally not exceed the achievable reso- 

 lution of currently planned space altimeters, it could 

 become serious should the sought-for dm resolution needed 

 for most oceanographic applications (tides, geostrophic 

 surface slopes, wind set up, etc.) become attainable. The 

 correction for these errors will require not only measure- 

 ments of the wave field but also an understanding of the 

 coupling between the long wind-sea components and the shorter 

 waves contributing to the mean square slope — which is 

 essentially the same two-scale interaction problem that has 

 been mentioned repeatedly above. 



10 . CONCLUSIONS 



Microwave measurements from space hold considerable 

 promise of yielding valuable synoptic data on sea state and 

 surface winds. However, the interaction between micro- 

 waves and the surface is highly complex and poses several 

 fundamental questions which require further extensive study 



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