# First Assessment of GF3-02 SAR Ocean Wind Retrieval
## 4 Discussion  
In this paper, the accuracy of wind speed retrieval of GF3-02 SAR is evaluated for the first time. The SAR wind speed retrieved by different GMF models is compared with NDBC buoy wind speed, ERA5 wind speed, and HY-2B SCA wind speed, respectively. For VV polarization, the optimal RMSE is 1.40 m/s compared with NDBC buoy wind speed, 1.18 m/s compared with ERA5 wind speed, and 1.24 m/s compared with HY-2B SCA wind speed. For HH polarization, the optimal RMSE is 1.39 m/s compared with NDBC buoy wind speed, 1.19 m/s compared with ERA5 wind speed, and 1.52 m/s compared with HY-2B SCA wind speed. The contributions to the RMSE of the wind speed measured by  
Figure 7: Comparison of wind speed retrieval results from GF3-02 HH polarization data based on (**a**) CMOD5.N+PR_[PERSON], (**b**) CMOD5.N+PR_[PERSON], (**c**) CMOD5.N+PR_[PERSON], (**d**) CMOD5.N+PR_[PERSON], and (**e**) CMODH_HH and HY-2B SCA wind data.  
the GF3-02 SAR are expected in many aspects. Firstly, one of the sources of error is the accuracy of the GF3-02 SAR radiometric calibration. As the GF3-02 SAR external field radiation calibration has not been completed, this paper only uses the Amazon rainforest for preliminary radiation calibration. There may be certain errors in the calibration constants, which directly affect the accuracy of the sea surface NRCS. Secondly, the accuracy of wind speed retrieval largely depends on the GMF model used. After completing the external field radiation calibration, we can further evaluate and optimize the accuracy of the GMF model through a large amount of GF3-02 SAR data. Thirdly, the GF3-02 SAR wind speed retrieval accuracy is limited by the accuracy of the wind speed of the NDBC buoy, ERA5, and HY-2B SCA. The wind speed measured by the buoy can be regarded as the ground truth, but the number of scenes where the SAR data collected in this paper matches the buoy in space is only 10, and the result may be accidental. The collected SAR data are mainly concentrated in the waters near the Hawaiian Islands and the waters off the west coast of the United States, where there are also a large number of NDBC buoys. Therefore, we can verify the accuracy of the ERA5 data by comparing the ERA5 wind speed with the spatially matched NDBC buoy wind speed, thereby indirectly proving the accuracy of the GF3-02 retrieval wind speed results. Figure 8 shows the GF3-02 SAR data and selected NDBC buoys distributed in the waters near the Hawaiian Islands and the waters off the west coast of the United States. Figure 9 shows the comparison results of the ERA5 wind and the NDBC buoys wind at the same time as the SAR imaging moment. Compared with the NDBC buoys wind, the RMSE of the ERA5 wind speed is 1.03 m/s and the RMSE of the wind direction is 18.63\({}^{\circ}\). It is enough to show that the ERA5 wind has high accuracy to verify the retrieval results of the GF3-02 SAR data collected in this paper. In the same way, we matched the wind speed of the HY-2B SCA and the NDBC buoy shown in Figure 8 at the moment of SAR image imaging. The comparison results of the sea surface wind field are shown in Figure 10. Compared with the NDBC buoys wind at the moment of the GF3-02 SAR image, the RMSE of the HY-2B SCA wind speed is 1.64 m/s and the RMSE of the wind direction is 36.07\({}^{\circ}\), which may be affected by individual outliers. In addition, future work is devoted to collecting more GF3-02 SAR data matching the wind field in-site measured data, further analyzing the wind speed retrieval accuracy, and optimizing the wind speed retrieval model.  
Figure 8: The map of SAR data location (Blue boxes) and buoy location of NDBC (black dots) of (**a**) the waters off the Hawaiian Islands and (**b**) the waters off the west coast of the United States. The NDBC buoy stations are 51000, 51001, 51002, 51003, 51004, 51101, 46002, 46011, 46028, 46047 and 46059.