## 3 Results
### The Influence of Directional Wave Spectra on NRCS Simulation at Different Azimuth Angles
However, at a moderate wind speed, the reference data are consistent with the predictions based on the A spectrum when the incidence angle is less than 35\({}^{\circ}\), in good agreement with the E spectrum for the incidence angles of 35\({}^{\circ}\)-39\({}^{\circ}\), and consistent with the H18 and D spectra for incidence angles larger than 39\({}^{\circ}\). At a high wind speed, the A spectrum performs better than the other spectra generally, except that the H18 spectrum is in good agreement with the ASCAT data when the incidence angle is larger than 38\({}^{\circ}\). The above analysis reveals that the satisfactory sea surface spectrum for NRCS simulation at different wind speeds and incidence angles varies, and we conjecture that a combination of these spectra might have wider applicability. Therefore, we chose the A, H18, and E spectra, which have better performance, to obtain composite spectra. Two composite wave spectra were obtained through averaging: the average of the A spectrum  
Figure 8: Simulated NRCSs based on D + Cosine, A + Cosine, E + Cosine, H18 + Cosine, AH18 + Cosine, and AEH18 + Cosine, and their comparison with the ASCAT data and CMOD5.n as functions of incidence angle at wind speeds of 3 m/s, 9 m/s, and 16 m/s along the (**a–c**) upwind direction, (**d–f**) downwind direction, and (**g–i**) crosswind direction. and the H18 spectrum (hereafter, \"AH18\"), and the average of the A spectrum, E spectrum, and H18 spectrum (hereafter, \"AEH18\"). It can be seen from Figure 8 that in upwind direction, the AH18 spectrum always performs well at a low wind speed compared with the reference data. The NRCS simulated with the two combined spectra are almost the same at a moderate wind speed, and the differences between the predictions of the two combined spectra and the reference data are within 1.5 dB. At a high wind speed, both the AH18 and the AEH18 spectra underestimate the mean NRCS in comparison with that of ASCAT and CMOD5.n, and the AH18 spectrum works relatively better than the AEH18 spectrum. In the downwind and crosswind directions, the performances of the two combined spectra at different wind speeds and incident angles are similar to the performance in the upwind direction, except that in the crosswind direction at a moderate wind speed, the prediction with AH18 has a better consistency with the reference data when the incidence angle is larger than 40\({}^{\circ}\). In order to show the performances of different wind-wave spectra more clearly and intuitively, Table 1 shows the offset and standard deviation of the simulated scattering shown in Figure 8 relative to CMOD5.n. Table 2 shows the offset and standard deviation of the simulated scattering relative to the ASCAT measurements. The incidence angle is divided into 30\({}^{\circ}\)-40\({}^{\circ}\) and 40\({}^{\circ}\)-50\({}^{\circ}\). The mean biases and standard deviations of the simulated scattering relative to the reference data under the three wind speeds are shown in each incidence angle cell. After comparing the offsets and standard deviations of the six wave spectra, we obtained the best-  
In general, the above comprehensive evaluation of different directional wave spectra in the NRCS simulation reveals that the directional spreading function has little influence on the prediction of the incidence angle and wind speed dependences of scattering at different azimuth angles, while the wave spectrum has significant influence. The AH18 spectrum has better performance at low and moderate wind speeds, and the A spectrum works better at a high wind speed. Then, the effects of wave spectra and directional spreading functions on the estimation of azimuthal fluctuation of ocean backscatter are analyzed. Figure 9 illustrates the predictions of the modified TSM with different wave directional spectra input, and their comparison with CMOD5.n and the ASCAT measurements as functions of azimuthal wind direction at incidence angles of 30\({}^{\circ}\) and 50\({}^{\circ}\) under wind speeds of 3 m/s, 9 m/s, and 16 m/s.