## 5 Conclusions  
Most previous studies of facet scattering were carried out based on a Gaussian sea surface constructed using the E spectrum or JONSWAP spectrum [6; 7; 8; 9; 10; 11; 12; 13; 14]. The effects of different wind wave spectra and the non-Gaussianity of the sea surface on the average scattering and scattering distribution need to be further analyzed under different incidence angles, wind speeds, and wind directions. Meanwhile, most studies focused on the influence of swell on the mean NRCS, and there are few studies that analyze the impact of swell on facet scattering. Therefore, in this paper, we analyzed the effects of the wind wave spectrum, non-Gaussianity of the sea surface, and swell on the prediction of facet scattering based on the facet TSM by comparing the simulated results with the Sentinel-1 data, the ASCAT data, and the GMF at the wind speed range of 3-16 m/s, the wind direction range of 0\({}^{\circ}\)-360\({}^{\circ}\), and the incidence angle range of 30\({}^{\circ}\)-50\({}^{\circ}\) under VV and HH polarizations. First, we compared the performances of the TSM based on the four wind wave spectra of D, A, E, and H18 and the two composite spectra of AE and AEH18 in predicting mean scattering. The results show that the prediction based on the A spectrum is more consistent with the Sentinel-1 data and GMF under HH polarization. Under VV polarization, the A spectrum has an overall satisfactory performance under different incidence angles compared to the Sentinel-1 data, the ASCAT data, and the GMF, except for high incidence angles (e.g., > 35\({}^{\circ}\)), where the combined spectra performed slightly better. Therefore, the  
Figure 27: Comparisons of the scattering images based on different facet sizes in the upwind direction at the incidence angle of 40\({}^{\circ}\) and wind speed of 10 m/s under VV polarization. The surface size is 2 km \(\times\) 2 km. **(a)** The facet size is 1 m \(\times\) 1 m; **(b)** the facet size is 5 m \(\times\) 5 m; **(c)** the facet size is 10 m \(\times\) 10 m.  
Figure 26: Comparisons of profiles of the sea surface height and profiles of scattering intensity under VV polarization based on different facet sizes in the upwind **(left)** and crosswind **(right)** directions at the incidence angle of 40\({}^{\circ}\) and wind speed of 10 m/s. The surface size is 2 km \(\times\) 2 km. Blue solid curves, 1 m \(\times\) 1 m; red solid curves, 5 m \(\times\) 5 m; green solid curves, 10 m \(\times\) 10 m. **(a)** Profiles of sea surface heights in the upwind direction; **(b)** profiles of sea surface heights in the crosswind direction; **(c)** profiles of NRCSs in the upwind direction; **(d)** profiles of NRCSs in the crosswind direction. A spectrum was found to have a better applicability generally. This result is different from the conclusion in [39] that a composite wind wave spectrum has a better applicability than single wave spectra. This might be because in [39], the influences of the spectra on the NRCS prediction were analyzed based on the advanced integral equation model. In addition, we compared the distribution of the simulated scattering using TSM based on different spectra with the Sentinel-1 data in the IW mode, and we found that the spread of the Sentinel-1 scattering distribution was wider than that of the simulation, which may be because the Sentinel-1 data contain noise. The A spectrum achieved a more consistent scattering distribution with the Sentinel-1 data at low incidence angles, while the composite spectrum performed better at high incidence angles under VV polarization. Second, we analyzed the influence of the non-Gaussianity of the sea surface on the mean NRCS simulated by TSM based on the A spectrum. The results show that the upwind-downwind asymmetry of backscattering can be predicted well by the modified TSM which is constructed by incorporating bispectrum correction into the conventional TSM. This is consistent with the result reported in [45].