# First Retrieval of Sea Surface Currents Using L-Band SAR in Satellite Formation
## 1 Introduction  
Ocean currents [1], which are the persistent and directional movements of seawater, are subject to various influences, including wind, temperature fluctuations, salinity gradients, and the Earth's rotation [2]. They are an essential parameter in the dynamic environment of oceans and are indispensable for navigation, climate regulation, marine disaster forecasting, and the distribution of marine life [3]. The accurate measurement of ocean currents poses a significant challenge for scientific investigation and practical applications. Spaceborne synthetic aperture radar (SAR) plays a vital role owing to its all-day, all-weather capability, wide field of view, and high spatial resolution. Ocean current inversion methods based on SAR can be broadly classified into two categories [4, 5]: Doppler centroid analysis (DCA) and along-track interferometry (ATI). The DCA method applies to single-antenna SAR data. It imposes lower system requirements but yields relatively lowspatial resolution. Moreover, it generates one-dimensional velocity fields along the range direction. Contrarily, ATI requires SAR data from at least two antennas to generate interferograms, which involves higher system complexity. However, this approach facilitates higher accuracy and spatial resolution for current velocity and is less sensitive to antenna pointing errors. ATI-SAR was initially proposed to detect ocean surface currents. The system sequentially monitors the same resolution cell on the ocean's surface by employing two antennas aligned along the flight direction. If the resolution cell contains motion along the radar line of sight, the distance between the second antenna and the resolution cell changes relative to the first antenna. This is manifested as a phase difference between the SAR complex images acquired by the two antennas, referred to as the interferometric phase [6]. Theoretically, the ocean current fields derived from ATI-SAR maintain the same high spatial resolution as SAR imagery [7]. This presents a significant advantage for studying sub-mesoscale oceanic phenomena like ship wakes. The Shuttle Radar Topography Mission (SRTM) was the first experiment to employ spaceborne ATI-SAR for sea surface current inversion [8; 9]. The mission utilised a deployable boom to establish a 7 m physical along-track baseline, which enabled an interferometric time delay of approximately 0.5 ms [10; 11]. At a spatial resolution of 1 km, [PERSON] et al. [12] successfully retrieved currents with an accuracy of approximately 0.2 m/s by processing the ATI data acquired by the X-SAR instrument over the Wadden Sea, the Netherlands. TerraSAR-X was the first spaceborne SAR system to implement ATI in divided antenna mode [6]. It supports two ATI configurations, a dual receive antenna (DRA) mode and an aperture switching (AS) mode, with a 1.15 m baseline length [13]. [PERSON] was the first to process AS-mode data, obtaining a current measurement accuracy of 0.1 m/s at a 1 km spatial resolution [14]. In the same vein, [PERSON] et al. [15; 16] employed RADARSAT-2's dual-channel ATI mode to acquire a current measurement accuracy of 0.1 m/s at a spatial resolution of nearly 800 m over the Florida Current (FC) region, with an effective along-track baseline of 3.75 m. The ATI experimental mode is supported by Gaofen-3, which establishes a 3.75 m effective along-track baseline. For the first time, [PERSON] et al. [17] utilized this experimental mode to process data from the Jiaozhou Bays (located near Qingdao in Shandong Province, China). They reported a root-mean-square error (RMSE) of less than 0.2 m/s in comparison to high-frequency surface wave radar (HFSWR) data, resulting in an effective resolution of 1 km. Furthermore, the TerraSAR-X/TanDEM-X constellation is the very first distributed interferometric SAR system to accommodate the ATI mode, with an effective along-track baseline of 25 m [18; 19].