## 4 Discussion  
_Effect of Ocean Parameters on the NRCS Simulation_  
The parameters of the sea surface have a substantial impact on the efficiency of scattering calculation. Therefore, we analyze the influence of ocean parameters on both the scattering prediction and the computational efficiency based on the conventional TSM + Gaussian sea surface using the A spectrum and cosine spreading function in this section. The effects of the sea surface size on the prediction of backscattering are explored in detail in this section. We found that the size of the ocean surface area has little effect on the scattering distribution and the average NRCS at different wind speeds under various incidence angles. However, the computation time increases dramatically as the surface size increases. An example based on a Gaussian sea surface with areas of 2 km \(\times\) 2 km and 10 km \(\times\) 10 km, facet size of 5 m \(\times\) 5 m, wind speed of 10 m/s, and incidence angle of 40\({}^{\circ}\) is taken to demonstrate the influence of the sea surface size on the NRCS simulation. The pdfs for height, slope, and scattering in this example are shown in Figure 23, and the scattering images associated with this example are shown in Figure 24. The range and azimuth slope were calculated in the upwind and crosswind directions, respectively. Figure 23 shows almost no difference in the pdfs for the sea surface elevation and slope under various ocean sizes. Moreover, the surface size has no impact on the scattering distribution. From Figure 24, the patterns of the scattering images with different sea surface sizes can be determined to be almost identical at the same scale. A comparison of the average NRCS based on different sea surface sizes revealed that the mean scattering from the sea surface with an area of 2 km \(\times\) 2 km is almost the same as that from the sea surface with an area of 10 km \(\times\) 10 km for a given wind speed, incidence angle, and facet size. For example, the NRCS from the sea surface with an area of 2 km \(\times\) 2 km and that from the sea surface with an area of 10 km \(\times\) 10 km under VV polarization is \(-\)15.24 dB when \begin{table}
\begin{tabular}{c c c} \hline \hline
**Ocean Surface** & \
\begin{tabular}{c} **Wavelength of** \\ **the Dominant Wave (m)** \\ \end{tabular} &
\begin{tabular}{c} **Direction of the** \\ **Dominant Wave** \\ **(degree)** \\ \end{tabular} \\ \hline Sentinel-1 data & 200.0 & 180.0 \\ Tayfun sea surface without & 131.3 & 203.2 \\ swell & 200.0 & 180.0 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: The wavelength and direction of dominant wave obtained from the image spectra in Figure 22.  
Figure 22: Similar to Figure 18 but corresponding to ID 3.  
the wind direction is upwind, the wind speed is 10 m/s, and the incidence angle is 40\({}^{\circ}\). However, the calculation time increases considerably when the sea surface size is increased. The experiments outlined above were coded in MATLAB and run on a PC with an Intel(R) Xeon(R) Gold 6130 CPU. The code required approximately 30 min to obtain the NRCS for a single wind speed and incidence angle for a sea surface with an area of 2 km \(\times\) 2 km; in comparison, a period of 4320 min was required to calculate the backscatter for a sea surface with an area of 10 km \(\times\) 10 km under the same conditions. Figure 23: Comparison of the distributions of sea surface with different ocean surface sizes at the incidence angle of 40\({}^{\circ}\) and wind speed of 10 m/s in the upwind (**left**) and crosswind (**right**) directions. The facet resolution is 5 m \(\times\) 5 m. (**a,b**) Pdfs for elevation; (**c,d**) pdfs for slope; (**e,f**) pdfs for backscatter under VV polarization. Solid curves, the size of the sea surface is 2 km \(\times\) 2 km; dashed curves, the size of the sea surface is 10 km \(\times\) 10 km. The impact of the facet size on the NRCS was analyzed for a specific sea surface area.