# Measuring Ocean Surface Current in the Kuroshio Region Using Gaofen-3 SAR Data
## 4 Discussion  
Considering that the marine environment is complex and full of many unknowns and uncertainties, many works about ocean surface current field measurement are mainly based on data-driven studies, especially in the application of using Doppler centroids to estimate ocean currents. As mentioned in the introduction, ENVISAT ASAR data has been used to study the sea surface current field in different regions including Gulf Stream and Agulhas Stream [5,6,8]. The SAR data acquired by different systems, such as ERS [37], TerraSAR-X [38] and Sentinel-1 [39], are also used in sea surface current measurement research, respectively, demonstrating the ability of currents retrieval. The work in this paper focuses on using the Gaofen-3 SAR data to carry out application research on the ocean surface current measurement in the Kuroshio region. In terms of this, the promising experimental results have proved the current measurement capability of the Gaofen-3 system. However, it should be noted that the results will also be affected by the system, environment, and processing methods. Since there is little knowledge about the actual sea surface state, it is hard to accurately and quantitatively analyze the dynamic process of the sea surface. Nevertheless, the main influencing factors and processes of the system and environment on the experimental results will be discussed below in this section, and the applicable scope of the proposed method will be given. The incidence angle will affect the SAR ocean surface imaging mechanism. At a medium incidence angle range from 20\({}^{\circ}\) to 60\({}^{\circ}\), which is consistent with the Gaofen-3 system design, the sea surface is dominated by Bragg scattering, and Bragg waves are the main scatterers on the sea surface, which is in line with the principles of the proposed method. At steep incidence angles, the specular scattering will increase imaging nonlinearity, which is not conductive to current measurement. Moreover, if the incidence angle is too large, the sea surface will be affected by wave breaking. In the experiment, the azimuth beam width determines the number of sub-apertures, which in turn also affects the current measurement results. When the azimuth beam width of the system is large, the number of sub-apertures will be more, and the number of azimuth angles will increase, which will lead to more equations used for least squares estimation to suppress the influence of random noise. However, in the case of the designed azimuth beam width, an increase in the number of sub-apertures will result in a decrease in sub-aperture time illuminating the ocean surface, which results in a decrease in the signal-to-noise ratio and current accuracy. Therefore, the number of sub-apertures is a compromise selection based on the data processing process. In this paper, six sub-apertures are used in the Gaofen-3 data experiment. Generally, in order to improve the accuracy of Doppler estimation, the size of the resolution cell for ocean current products exceeds 1 km. When the system resolution increases, the amount of data that can be used to estimate the Doppler center increases, which will also increase the accuracy of Doppler estimation. The resolution of the Gaofen-3 UFS mode data used in this paper is about 2 m, and the spatial resolution of the current field is 1 km. Usually, the wind field will cause the variation of sea surface wave motion, resulting in the deviation of current measurement results. Therefore, in traditional current measurement methods, such as DCA and ATI, it is necessary to first estimate the deviation value using the known wind field information, and then subtract it from the current measurement results. This process makes the current measurement process more complex, and the accuracy of wind field information and the difference of deviation estimation methods will affect the current measurement accuracy.