# Measuring Ocean Surface Current in the Kuroshio Region Using Gaofen-3 SAR Data
## 3 Gaofen-3 Data Experiment and Results
### Results Validation  
Generally, the in-situ data derived by Argos drifter buoys, current meters, and so forth, is optimally used for the verification of ocean current results. In addition, satellite altimeter  
Figure 8: The retrieved ocean surface current field corresponding to the acquisitions in Figure 6. (**a–d**) correspond to the labels 1, 2, 3, and 4, respectively. Figure 7: The result of sub-aperture processing: (**a**) one of the sub-apertures intensity image, and the result of Doppler centroid estimation: (**b**) the corresponding Doppler image. data can also be used to verify geostrophic current. Unfortunately, there is no spatio-temporal synchronization in-situ data and altimeter data available for this experiment. In fact, since the ocean current retrieved by the altimeter is usually at a resolution of tens of kilometers, it is not suitable for comparison with the data with a smaller width and higher resolution in this article. Ocean circulation model data can also be used for ocean current verification. The hybrid coordinate ocean model (HYCOM) has been widely used in a variety of ocean current research projects [34; 35; 36]. Therefore, the HYCOM model data are used to verify the results in this article. In this section, HYCOM's global reanalysis dataset with a spatial resolution of \(1/12^{\circ}\) and a temporal resolution of 1 day is acquired for comparison. Figure 9 shows the HYCOM ocean current model data consistent with the SAR acquisition time and location. The black box represents the boundary of the experimental area. The length and color of the arrow represent the current velocity, and the direction of the arrow indicates the current direction. It is obvious that the ocean current state in the black box compares qualitatively well with the experimental results in Figure 8.  
In order to quantitatively analyze the relationship between experimental results and the model data, the results in Figure 8 were down-sampled and matched with the data in Figure 9, and the current velocity and direction scatter plots are shown in Figures 10 and 11, respectively. Figure 10 illustrates the statistical comparison of current velocity, where the minimum correlation coefficient is close to 0.9 and the maximum correlation coefficient is 0.98. Both root mean square error (RMSE) and bias are less than 0.1 m/s, which meets the requirements for accuracy of current velocity measurement in marine environment applications. Figure 9: The HYCOM model data corresponding to the ocean current results in Figure 8. (**a–d**) correspond to the labels 1, 2, 3, and 4, respectively. the model data are all greater than 0.95. Both RMSE and bias are less than 10\({}^{\circ}\). Although the HYCOM data represent the average state of the current field in the day, the Kuroshio current does not change much in one day. The results in Figures 10 and 11 suggest that, after using the proposed method to process Gaofen-3 data, the ocean surface current fields in the Kuroshio region are measured accurately. The current characteristics in this paper and HYCOM need to be further compared. The proposed method has the advantages of flexible data acquisition time and high spatial sampling rate. The Gaofen-3 system can image the sea area of interest and obtain the current information at specific times, with high spatial resolution of only a couple of kilometers. However, the disadvantage is that the acquisition of current field in the case of complex sea conditions and uneven sea surface scattering is not applicable, so it needs to be further studied in combination with other information. The advantage of the current field obtained from HYCOM data is that the model integrates a variety of sensor data and is applicable to a wide range of sea environment, but the disadvantage is that it only represents the average motion state of one day, and the spatial resolution is only tens of kilometers, and there is a lack of data in the nearshore sea area.