8. THE WAVE-FACET INTERACTION MODEL 



Two straightforward generalisations of the lowest order 

 scattering models have been Investigated. In the first 

 case , the perturbation expansion in terms of surface wave 

 height, which yields Bragg scattering to first order, is 

 extended to quadratic and higher powers. The second order 

 wave-wave interaction theory yields a useful description of 

 backscatter at decameter wavelengths or longer([23] [25] [8]), 

 but is of only limited value in the microwave band, since 

 for short wavelength radiation the principal wind-sea com- 

 ponents violate the basic interaction condition (surface 

 wave height )/(electromagnetic wavelength) << 1. 



This difficulty Is avoided In the composite-wave or 

 wave-facet interaction model, which is based on an alterna- 

 tive two-scale expansion method (cf [9] [10] [49] [57])- 

 In this case the Bragg scattering waves are assumed to be 

 superimposed on a random ensemble of longer carrier waves 

 (the wind sea), which are represented locally by plane 

 facets of dimension small compared with the wind-sea wave- 

 length but large compared with the wavelength of the Bragg 

 waves. It is then assumed that Bragg theory can be applied 

 as before in the local reference system of the moving, 

 inclined facet. Modifications of the Bragg return arise 

 both through changes in the angle of incidence and the 

 orientation of the polarisation planes relative to the local 



25-38 



