# Research on Wind Speed Inversion Method for X-Band Networked SAR Satellite
However, since the observation azimuth of SAR is fixed, it is impossible to conduct multiple observations on the same sea surface unit within a short interval; therefore, SAR wind speed inversion cannot be carried out in the same way as the application of GMFs to scatterometers. If the wind direction is taken as initialization information, the incident angle and relative wind direction information of the GMF can be determined. Thus, the wind speed can be determined to complete the inversion of the sea surface wind speed [16]. At present, there are at least three ways to obtain the wind direction: (1) obtain the sea surface wind direction by using linear streak information contained in an SAR image [17; 18; 19]; (2) obtain the sea surface wind direction using a numerical prediction model [20; 21]; and (3) obtain the sea surface wind direction by spatiotemporally matching SAR data with wind direction information from a scatterometer [21]. A large number of early observations and studies have shown that the axial direction of the atmospheric boundary layer vortex is basically coincident with the direction of the sea surface wind vector, which appears as a wind streak with linear characteristics in an SAR image [22]. The wind direction of the sea surface wind field can be obtained by applying a two-dimensional Fourier transform to the image, but there is an uncertainty of 180\({}^{\circ}\) in the wind direction. According to numerous reports, SAR images with wind streak information account for only approximately 60% of all SAR images acquired over the sea surface [23]. Hence, this method suffers from substantial limitations in the inversion of the sea surface wind field, especially in operational applications. [PERSON] used wind direction information from the Navy Operational Global Atmospheric Prediction System (NOGAPS) and the SeaWinds scatterometer as the initial wind directions to conduct SAR wind speed inversions [21]. By comparison, the RMSE of the former wind speed inversion result, compared with the SeaWinds wind speed, is 1.78 m/s, while the latter is 1.36 m/s. When using wind direction data from numerical prediction models or scatterometers to invert wind speed, a resampler is needed to match wind direction data with SAR image pixels in time and space. Finally, wind speed is inverted according to a GMF. The problem with this approach is that wind direction data have a low spatial resolution; consequently, it is necessary to interpolate the wind direction to each pixel of the SAR image. However, many small-scale wind direction changes are ignored in this process, affecting the wind speed inversion accuracy. Moreover, the acquisition times of the wind direction data may be different from those of the SAR images; the greater the difference is, the worse the inversion result. In SAR wind speed inversion, wind direction data should be input when a GMF is directly used. As a result, the wind speed inversion accuracy is affected by the accuracy of the wind direction. Scatterometers make several effective observations of the same sea surface unit at different azimuth angles. Therefore, the maximum likelihood estimation technique can be used to estimate the wind speed and wind direction simultaneously. Employing this principle, [PERSON] et al. divided Environmental Satellite (ENVISAT) Advanced SAR (ASAR) images into \(25\times 25\) km units, and the difference in the incident angles between adjacent units was over 1\({}^{\circ}\)[24]. Under the condition that other relevant parameters are known, the cost function is established based on the differences in the incident angle between adjacent elements. The wind speed is inverted through a series of solution processes. However, this approach requires sea conditions with a uniform distribution of wind field and a relatively slow change, and external wind direction data are needed to determine the unique solution. When this method is applied to spaceborne SAR data, it is necessary to ensure that the incident angles of adjacent sub-images are different.