vations in their regions of theoretical validity (cf [9] [10] 

 [^9] [57]), but yield only limited information on the wind- 

 sea spectrum in terms of the cross sections and doppler 

 spectra, cf. table 1 (the application of the specular 

 reflexion model to altimeter data is discussed in §9). The 

 Bragg model determines only the wave spectrum at the two 

 (very high) Bragg wavenumbers. The specular reflexion model 

 yields the mean square wave slope and vertical orbital velo- 

 city^ of which only the latter moment (determining the doppler 

 bandwidth) is significantly dependent on the principal wind- 

 sea components. However, it should be noted that the theore- 

 tical doppler spectra apply to the idealized case of a 

 parallel incident beam. In operation from a moving satellite, 

 the finite beam angle of a real scatterometer would lead to 

 variable doppler shifts due to the platform motion which 

 would normally mask the wave-induced doppler spectrum. 



Two courses may be pursued to overcome the limitations 

 of the lowest-order models. Firstly, interrelationships 

 may be established between the accessible high wavenumber 

 range of the spectrum, the wind-sea spectrum and the local 

 surface wind. In this respect it is encouraging that the 

 high-frequency range of the spectrum does not appear to 

 represent a universal equilibrium governed solely by white- 

 capping, but contains a free energy factor governed, among 

 other processes, by the coupling to the principal wind-sea 



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