# Measuring Ocean Surface Current in the Kuroshio Region Using Gaofen-3 SAR Data
## 3 Gaofen-3 Data Experiment and Results  
### Experiment Description  
The experiment site is located in the Kuroshio Current region. The Kuroshio originates from the North Equator, enters the East China Sea via the Philippines, close to the eastern part of Taiwan, and then flows through the Ryukyu Islands, along the southern part of the Japanese archipelago, and ends in the waters near 142\({}^{\circ}\) E and 35\({}^{\circ}\) N. The Kuroshio has the characteristics of strong flow velocity, large flow, narrow flow range, deep extension, high temperature and high salt, and so forth.  
Gaofen-3 is a Chinese spacecraft carrying a C-band SAR, which was launched in August 2016, from Taiyuan (Shanxi Province, China). The Gaofen-3 satellite is capable of high resolution, large imaging width, high radiation accuracy, multiple imaging modes and long-term operation. It can monitor global marine and land information all day and all night, and expand the earth observation range and improve the rapid response capability through left-right attitude maneuver. The satellite has 12 imaging modes, which is the SAR satellite with the most imaging modes in the world. Gaofen-3 has outstanding flexibility, high attitude control accuracy and stability, and has the ability of continuous two-dimensional attitude guidance maneuver. In addition, the satellite adopts an independent health management mechanism to reduce the risk of whole satellite failure. Some Gaofen-3 SAR system parameters are shown in Table 1. For more specific parameters and technical details, please refer to [26].  
The four Gaofen-3 SAR datasets are single look complex (SLC) images acquired in ultra-fine strip-map (UFS) mode, of 3 m high resolution within 30 km swath owing to the dual-receive technique. The Gaofen-3 raw data is imaged by a special integrated SAR data processor, and a chirp scaling (CS) algorithm is used for UFS mode data. More details can be found in Reference [33]. The radar wavelength of the Gaofen-3 SAR system is 5.6 cm, the central incident angle is 22.5 degrees in ultra-fine strip-map mode. The GF-3 SAR system is right view, and implemented zero Doppler through attitude control. Further information about the data is shown in Table 2. Figure 5 depicts the geographical locations in the red marks.  
Figure 6 shows four SAR intensity images. It can be observed that all the images are relatively uniform in intensity, and are not affected by vortices, internal waves and other sea surface phenomena, which means that the weighted effect of the sea surface normalized radar cross section (NRCS) on Doppler centroid variation is minimal. Note that [PERSON] et al. [6] pointed out that the Doppler estimation method works best for the quasi-uniform radar cross section, which is consistent with the image features in this experiment.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Parameters** & **Values** & **Units** \\ \hline Radar Frequency & 5.4 & GHz \\ Incidence Angle & 20–60 & deg \\ Polarization & HH/HV/VH/VV & – \\ Spatial Resolution & 1–500 & m \\ Swath & 10–650 & km \\ Platform Speed & 7567 & m/s \\ Orbit Altitude & 755 & km \\ \hline \hline \end{tabular}
\end{table}
Table 1: Gaofen-3 system parameters.  
Figure 5: Geographic location of Gaofen-3 SAR data (marked in red).