###### Abstract
The dimmer brightness in the lower left corner of Figure 4a is due to the lower wind speed and the weaker echo energy due to the oil spill on the sea, which also results in errors in the Doppler velocity, as shown in the corresponding position in Figure 4b. The absolute values are small in the lower right part of the Doppler velocity map, and the overall velocity includes the influence of sea-surface wind and wave motion, which needs to be further removed. Figure 5 shows the SAR amplitude image of the GF-3 satellite and the Doppler velocity estimated from the single look complex (SLC) data. The dark part of the right corner of Figure 5a is obviously affected by the sea oil film, which reduces the signal-to-noise ratio of the image. The Doppler velocity of Figure 5b is estimated from the Doppler processing in reference [19]. It can be seen that the absolute value in the lower left corner of the image is  
Figure 2.4 Sentinel-1 SAR (**a**) amplitude image and (**b**) radial Doppler velocity. Figure 2.5 Spatial geographic location of the actual SAR data. small, in addition to the anomaly in the oil film position, and the overall velocity overlaps with the influence of the wind and wave motion. Although the overlap range of the above SAR data is small, it has clear velocity characteristics, which is conducive to the validation of simulation method. Therefore, a simulation experiment was performed in this sea area. The input system parameters are consistent with the real SAR data parameters, representing the ascending Sentinel-1 satellite and the descending GF-3 satellite, respectively, as shown in Figure 2.6 It should be noted that, at present, there are few ascending and descending data from a single satellite that can meet the requirements of irradiating the same sea area at a short time, and it is difficult to obtain them directly. In future applications, two or more satellites must cooperate to obtain the ascending and descending passes data of the same space-time sea area for current measurement. Therefore, the following dual satellite SAR data processing and system simulation are of practical significance. The hybrid coordinate ocean model (HYCOM) reanalysis current field data and the European centre for medium-range weather forecasts (ECMWF) wind field data are obtained for the simulation input, respectively, as shown in Figure 2.7 The HYCOM model reanalysis data, which combines satellite altimeter data with temperature and salt data obtained by a buoy, is widely used in ocean current research [20; 21; 22]. The date selected for the HYCOM data is the same as Sentinel-1 SAR data acquisition date with spatial resolution of 1/12 degree. Wind field data at the same time and place were obtained from the ECMWF website with spatial resolution of 1/4 degree, and interpolated to ensure the same number of data points as the input current field. From the input data, it is known that the velocity in the overlapping area is small, and the sea-surface wind speed is mainly in the range of 2-7 m/s, which belongs to the moderate wind speed. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Parameters** & **Sentinel-1 (Ascending)** & **GF-3 (Descending)** & **Units** \\ \hline Radar Frequency & 5.4 & 5.4 & GHz \\ Incident Angle & 26.3 & 35 & deg \\ Beam Direction & 15 & 165 & deg \\ Platform Speed & 7590 & 7568 & m/s \\ Orbit Altitude & 707 & 713 & km \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Simulation parameters. Figure 5: GF-3 SAR (**a**) amplitude image and (**b**) radial Doppler velocity. The given current and wind data are input into the SAR ocean imaging model to calculate the radial Doppler velocity \(V_{S}\) under the parameters of ascending Sentinel-1 satellite and the radial Doppler velocity \(V_{G}\) under the parameters of descending GF-3 satellite, as shown in Figure 7a,b, respectively.