the backscattered power at the frequency w would still 

 vanish. (The filter separating the modulated power from 

 the D.C. term must, of course, be sufficiently wide to 

 include the small doppler broadening due to the waves — 

 or the platform motion.) However, the mean square power at 

 oj is non-zero and is proportional to the wind-sea energy 

 spectrum at }S^ • (It may be noted that this corresponds 

 to a mean fourth product of the signal amplitudes which 

 could not be inferred from mean quadratic quantities — 

 essentially the mean power — and must therefore represent 

 a non-Gaussian effect. In accordance with the Central 

 Limit Theorem it can be shown that for large areas of 

 illumination the modulated term is small compared with the 

 D.C. term, which has Gaussian statistics.) 



By varying the difference frequency and azimuth angle, 

 the technique basically provides a measurement of the com- 

 plete two-dimensional wind-wave spectrum, with the exception 

 of a sign ambiguity in the direction of wave propagation. 

 However, the method is critically dependent on the determin- 

 ation of the coupling coefficients characterising the 

 modulation of the Bragg return by the spectral components 

 of the wind-sea. The electromagnetic contribution to these 

 interactions can be readily calculated, but the hydro- 

 dynamic terms are governed by the same long-short wave 

 interactions that arose in discussions of the energy 



25-42 



