Radar Baok-Soattev from the Sea Surface 



20 



10 



b- 



X 1228 MHz, CROSSWIND 15 KNOTS 

 O 8910MHz, CROSSWIND 15 KNOTS 

 — THEORETICAL DEPENDENCE 

 FOR RMS TILT ANGLE AT 7.5° 



o 8 



eg 



10° 20" 



DEPRESSION 



J I ■ I.I.I I I I I 



30° 40" 

 ANGLE 



60° 90° 



Fig. 3. Theoretical and observed ratios of Bragg backscatter cross 

 sections for vertical and horizontal polarisation at wave 

 lengths 24 cm (1228 MHz) and 3.4 cm (8910 MHz) (from 

 Wright [ 24] ) 



weak Bragg return for horizontal polarisation. Not predicted by 

 first-order Bragg theory is the observed cross-polarised return, 

 which is generally only slightly smaller than or comparable with the 

 backscatter for horizontal polarisation; this can be explained by the 

 wave-facet interaction model [ 23] . 



Although the observed cross sections cr^^ and or^^ are in 

 good agreement with theory, the Doppler spectra for these polar- 

 isations point to limitations of the first- order model. In the cm- 

 dm bands, the Bragg lines are found to be broadened into Gaussian 

 shaped distributions with bandwidths of the same order as the Bragg 

 frequency (cf. Fig. 4. , frora Valenzuela and Laing [ 20]). Earlier 

 measurements by Hicks et al. [ 13] indicate that the mean frequencies 

 of the distributions -- which were not measured by Valenzuela and 

 Laing -- may also be considerably higher, by factors of the order 

 2 to 4, than the theoretical Bragg frequency. 



369 



