## 3 Results
### Effect of Wind Wave Spectra on NRCS Simulation  
The wind wave spectrum describes the quasi-periodic nature of ocean surface oscillations. It plays an indispensable role in the study of microwave electromagnetic scattering from the sea surface [39; 40; 41; 42]. Therefore, the performance of different wind wave spectra in scattering prediction should be analyzed by comparing the scattering simulation obtained using the conventional TSM based on four spectra with the reference data at the wind speeds of 3-16 m/s under moderate incidence angles for VV and HH polarizations. We simulated the scattering from a Gaussian sea surface based on the D, A, E, and H18 wave spectra for a sea surface size of 2 km \(\times\) 2 km and a facet size of 10 m \(\times\) 10 m. Furthermore, the Sentinel-1 data, ASCAT measurements, and NRCSs calculated by the empirical model were used as the reference data. The ASCAT NRCS was averaged within \(\pm\)0.5 m/s wind speed, \(\pm\)6\({}^{\circ}\) wind direction, and \(\pm\)0.5\({}^{\circ}\) incidence angle bins, and the Sentinel-1 NRCS was averaged within \(\pm\)0.5 m/s wind speed, \(\pm\)6\({}^{\circ}\) wind direction, and \(\pm\)0.5\({}^{\circ}\) incidence angle bins. The NRCS predictions of the conventional TSM from a Gaussian sea surface based on different wind wave spectra and the reference data as a function of incidence angle in upwind direction under wind speeds of 5, 10, and 15 m/s are illustrated in Figure 4. This figure shows that the NRCSs computed based on the A spectrum are the highest in level, whereas those calculated using the D spectrum are the smallest. This phenomenon can be explained by the behaviors of these spectra in the Bragg resonance wavenumber region shown in Figure 1d-f, in which the density of the A spectrum is always the largest and that of the D spectrum is the smallest in general. Figure 4 also shows that the NRCSs based on the D spectrum always underestimate backscatter under various wind speeds at the incidence angles of 30\({}^{\circ}\)-60\({}^{\circ}\) compared with the reference data. At a low wind speed (5 m/s) and under VV polarization, the A spectrum overestimates the scattering. As the incidence angle increases, the error becomes larger and even reaches 3 dB. The E spectrum underestimates the NRCS at small incidence angles (\(>\)35\({}^{\circ}\)) and overestimates the scattering values at large incidence angles (\(<\)35\({}^{\circ}\)). The variation trend of the reference data with the incidence angle is consistent with that of the H18 spectrum, but the H18 spectrum underestimates the scattering by approximately 2 dB at the incidence angles of 30\({}^{\circ}\)-60\({}^{\circ}\). Under HH polarization, all four spectra underestimate the NRCS compared with the Sentinel-1 data. This phenomenon might be caused by wave breaking, which greatly affects the scattering under HH polarization and cannot be described by the TSM. In this paper, we paid more attention to the Bragg scattering at moderate incidence angles, and the non-Bragg scattering caused by wave breaking was not taken into consideration. Moreover, the A spectrum performs better overall than other spectra in NRCS simulations under HH polarization because the overestimation of the A spectrum offsets part of the underestimation of TSM. Furthermore, all four spectra initially overestimate and then underestimate the backscatter in comparison with the empirical model, CMOD5.n + PR. At moderate wind speed (10 m/s) under VV polarization, the reference data are consistent with the predictions based on the A spectrum at small angles (\(<\)36\({}^{\circ}\)) but close to the predictions of the E spectrum at incidence angles of \(>\)44\({}^{\circ}\). For incidence angles of 36\({}^{\circ}\)-44\({}^{\circ}\), the reference data are between the results of the A spectrum and the E spectrum. The H18 spectrum is more consistent with the reference data as a function of the incidence angle. Under HH polarization, the performance of four spectra in predicting scattering is similar to that at a low wind speed (5 m/s).