# Research on Wind Speed Inversion Method for X-Band Networked SAR Satellite  
[PERSON]  
1 College of Oceanography and Space Informatics, China University of Petroleum, Qingdao 266580, China; [EMAIL_ADDRESS] (Y.W. ); [EMAIL_ADDRESS] (L.L. ); [EMAIL_ADDRESS] (X.Q. ); [EMAIL_ADDRESS] (X.Z.) [PERSON]  
2 College of Control Science and Engineering, China University of Petroleum, Qingdao 266580, China 2*  
SAR is an active microwave imaging radar. SAR transmits electromagnetic wave signals to the sea surface and receives echo signals. The echo signal is processed to an SAR image. When the electromagnetic wave irradiates the target feature, the object will spread the electromagnetic wave energy in all directions in a spherical shape. This phenomenon is called scattering. Objects that scatter electromagnetic waves are scatterers. Scattering opposite to the incident direction of electromagnetic waves is called backscatter. The intensity of the backscattered energy is described by the radar cross section (RCS), usually expressed by \(\sigma\). SAR improves the range direction resolution by compressing radio pulses, and uses the Doppler shift of the transmitted and received scattered signals to increase the azimuth direction resolution, thereby achieving high spatial resolution imaging. In addition, the working band of SAR is in the microwave band. Signals in this band have higher wavelengths than visible and infrared light, and can penetrate clouds. Therefore, the synthetic aperture radar imaging has the characteristic of not being affected by light conditions, cloud, and rain. SAR's all-weather, high spatial resolution ocean observation advantages are particularly suitable for large-area target information imaging. Spaceborne SAR can realize a wide range of observations on the Earth's surface, and the observation range is affected by the satellite's orbit and SAR working mode; but the time resolution of spaceborne SAR is low. Affected by the satellite flight speed and the rotation of the Earth, at present, the time resolution of spaceborne SAR can be less than 12 h.  
Three main methods are available for extracting wind information from synthetic aperture radar images. The first method estimates the wind speed from the azimuth cut-off degree in the SAR spectrum according to the relationship among the wind speed, azimuth cutoff, and wave height [4]. The dependency of the azimuth cut-off on the square root of the significant wave height is approximately linear, while the dependency on the wind speed is approximately linear only if the sea state is fully developed [5]. [PERSON] used the azimuth cutoff (\(\lambda\)c)-to-wind speed linear model to invert wind speeds [4]. The root-mean-square error (RMSE) of the extracted wind speed is 1.8-2 m/s. The second approach uses an optimal inversion method to estimate vector winds, combining the SAR data and background numerical weather prediction model data [6]. The third method is based on the empirical relationship between the wind vector and SAR information, also known as geophysical model function (GMF), such as the C-band model (CMOD) [7] and the X-band model (XMOD) [8]. SAR wind speed inversion based on GMFs is a mature and widely used method. Among them, XMOD1 and XMOD2 are applicable to co-polarized X-band SAR data [8; 9; 10; 11]. CMOD4, CMOD-IFR2, CMOD5, and CMOD5.N are suitable for C-band co-polarized SAR data [7; 12; 13; 14]. The GMF was first applied to scatterometers [7]. The scatterometer uses different azimuth angles to make multiple observations of the same sea surface. According to the maximum likelihood estimation technique, a GMF-based objective function was established to obtain all the possible combinations of the wind speed and wind direction, and the possible results were filtered to find the optimal solution [15].