# Measuring Ocean Surface Current in the Kuroshio Region Using Gaofen-3 SAR Data
## 1 Introduction  
Current is ubiquitous in the global ocean, the information about which is valuable in many applications. The circulation current in different regions of the deep ocean will exchange material and energy, which is of great importance for the global climate process. Moreover, monitoring currents in the coastal waters is conducive to navigation, offshore oil and gas field development, fishery resources management, and so forth. According to the data requirements survey report on oceanic variables issued by EuroGOOS, the ocean surface current velocity and direction were the two most requested [1]. Therefore, it is necessary to investigate various methods for measuring the velocity and direction values of the ocean current field. Compared with the in-situ current measurement tools, such as drift buoy and current meter, which are time-consuming and limited in spatial measurement range, the spaceborne synthetic aperture radar (SAR) has become a vital tool for ocean remote sensing mainly due to its day/night and quasi-all-weather capability and its high-spatial resolution [2]. There are two common techniques used for current measurement with SAR data, that is, one is Doppler centroid analysis (DCA) and the other is along-track interferometry (ATI) [3, 4]. DCA is suitable for single-antenna SAR data processing and has low requirements for the system. [PERSON] [5] first proposed the DCA method in 2005 and processed the ENVISAT ASAR data to retrieve the radial velocity of the sea surface in different areasincluding the current field near the Gulf Stream. In the following years, the DCA method was used many times to measure the ocean surface current field in the Agulhas Current regime [6; 7; 8]. These studies show that DCA has good applications in these regions with strong boundary currents. The Kuroshio Current is also one of the strongest western boundary currents, but there have not yet been any relevant SAR experimental results applied to the Kuroshio region. In addition, the DCA method using Doppler centroid shift can only derive the line-of sight (LOS) velocity component of ocean surface current, not including the direction values. The ATI method uses interferometric phase to achieve high-resolution ocean surface current measurements. Moreover, ATI systems can be transformed into ones with squint beams to measure the ocean current vectors [9; 10; 11]. However, ATI needs to operate the SAR data of at least two antennas; the related system is complicated, the experimental data are small, and it is far from scale application than DCA using single SAR data. Like the Gulf Stream in the North Atlantic, the Kuroshio is a powerful western boundary current and forms the western limb of the North Pacific Subtropical Gyre. Owing to the influence of the geostrophic Coriolis force, the remote sensing measurement of Kuroshio current speed can be performed with the spaceborne altimeters [12; 13], but the spatial resolution is relatively rough, reaching tens of kilometers, and the data around the land are easily missed. Gaofen-3 is China's first C-band full-polarization synthetic aperture radar imaging satellite, which provides reliable high-resolution SAR image data for ocean remote sensing [14]. So far, Gaofen-3 SAR data have been used to study ocean surface waves [15], ocean winds [16] and internal waves [17]. Gaofen-3 provides data support for the ocean current retrieval in the Kuroshio region, which will promote relevant application research. This paper uses four available Gaofen-3 SAR datasets to measure the ocean surface current in the Kuroshio region. In order to retrieve current velocity and direction information at the same time, a novel method combining sub-aperture processing technology and least squares technology is proposed and applied. The following second part introduces the principle and process of the data processing method in detail. The third part is to carry out the ocean current measurement experiment of the Gaofen-3 SAR data and verify the experimental results.