# Research on Wind Speed Inversion Method for X-Band Networked SAR Satellite
## 4 Results
### Accuracy of Wind Speed Inversion
#### 4.1.1 Experimental Results of SAR Data with Different Incident Angles  
The first set of experiments determined the relative wind direction angle to be 240\({}^{\circ}\). When simulating SAR data, a fixed value of noise is added to the backscatter coefficient of a single pixel. The purpose of adding noise is to understand the effect of noise on wind speed accuracy. Under the conditions of noise of 0 dB, 0.5 dB, and 1 dB, respectively, multiple sets of wind speed inversion experiments were conducted by changing the incidence angles of the three networked SARs. The experimental results are shown in Table 3.  
In the table, RMSE1 is the root-mean-square error of the comparison between the wind speed extracted without introducing wind direction information and the actual wind speed. RMSE2 is the root-mean-square error of the comparison between the wind speed extracted with the wind direction information and the actual wind speed. The definition of root mean square error is shown in (7).  
\[\text{RMSE}=\sqrt{\frac{\sum\ \left(Y_{i}-X_{i}\right)^{2}}{n}} \tag{7}\]  
It can be seen from the table that as the noise increases, the wind speed accuracy begins to deteriorate. When the incident angle is 30-50\({}^{\circ}\), the accuracy of wind speed inversion is relatively high. As the angle of incidence increases, the absolute value of the backscatter coefficient in dB units begins to increase. The proportion of noise in the backscatter coefficient is reduced, so the effect of noise on wind speed accuracy is reduced. Under different noise conditions, the comparison between the inversion wind speed and the actual wind speed is shown in Figure 2.  
It can be seen from the figure that as the noise increases, the accuracy of the wind speed results screened by the two methods will decrease. When the noise is small, the introduction of wind direction information can further improve the accuracy of the extracted wind speed. With the increase of noise, the wind speed extraction accuracy of the second method is gradually smaller than that of the first method. When the noise is greater than 1 dB, the introduction of wind direction information will reduce the accuracy of wind speed extraction.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Noise** & **Incidence** & **Actual Wind Speed** & **RMSE1** & **RMSE2** \\ \hline
0 dB & \(23^{\circ};26^{\circ};29^{\circ}\) & 1–30 m/s & 0.86 m/s & 0.00 m/s \\ \hline
0 dB & \(33^{\circ};36^{\circ};39^{\circ}\) & 1–30 m/s & 0.20 m/s & 0.00 m/s \\ \hline
0 dB & \(43^{\circ};46^{\circ};49^{\circ}\) & 1–30 m/s & 0.32 m/s & 0.00 m/s \\ \hline
0.5 dB & \(23^{\circ};26^{\circ};29^{\circ}\) & 1–30 m/s & 2.56 m/s & 2.61 m/s \\ \hline
0.5 dB & \(33^{\circ};36^{\circ};39^{\circ}\) & 1–30 m/s & 0.80 m/s & 0.82 m/s \\ \hline
0.5 dB & \(43^{\circ};46^{\circ};49^{\circ}\) & 1–30 m/s & 1.38 m/s & 0.98 m/s \\ \hline
1 dB & \(23^{\circ};26^{\circ};29^{\circ}\) & 1–30 m/s & 3.24 m/s & 3.29 m/s \\ \hline
1 dB & \(33^{\circ};36^{\circ};39^{\circ}\) & 1–30 m/s & 1.52 m/s & 2.01 m/s \\ \hline
1 dB & \(43^{\circ};46^{\circ};49^{\circ}\) & 1–30 m/s & 1.67 m/s & 1.39 m/s \\ \hline \hline \end{tabular}
\end{table}
Table 3: Experimental results (constant relative wind direction).