Radar Baok-Soatter from the Sea Surface 



2k' 



k, 



k°, k** 



Fig. 10, Upper panel . Interacting gravity -wavenumbers 



for a spectral -peak wavenumber small compared with the 

 Bragg wavenumber 2k'. Strong contributions to the 



second-order Dopplef~spectrum x ("cj) arise when k 

 Is close to the spectral peeik. Contour lines Indicate 

 curves of constant spectral energy. 

 Lower panel . The associated second-order Doppler 

 spectrum x^^^ consists of two Images of the wave fre- 



quency spectrum 

 Bragg line. 



En reflected on either side of the 



Bragg field by long surface waves of high amplitude. However, In 

 the present case the Doppler shift Is not determined by the frequency 

 shift (Of = - 2 k' u Induced by the long-wave orbital velocities, but 

 rather by the Intrinsic long- wave frequencies Wq. Each low- 

 frequency component splits the first-order Bragg line 

 <*)^'^ = Wj + o-|jWjj Into two lines co^^' = "s'^ ^ %• Since u « C,Wg, the 

 regions of validity for the wave -facet and wave -wave Interaction 

 models may also be expressed, respectively, as Wg « tOf and 



Wfl» 



Uf, 



383 



