# First Retrieval of Sea Surface Currents Using L-Band SAR in Satellite Formation
## 1 Introduction
[PERSON] [20] conducted a comparison of the current measurements from the TerraSAR-X single-satellite DRA mode and the TerraSAR-X/TanDEM-X dual-satellite formation over the same oceanic region. The single-satellite DRA mode necessitated a spatial resolution of 1 km to achieve an accuracy of 0.1 m/s, whereas the dual-satellite formation with a long baseline enabled measurements at a resolution of 33 m. This enhancement is consistent with theoretical predictions that the optimal effective baseline length for X-band spaceborne ATI systems is between 20 and 30 m. Table 1 summarizes the preceding information. One of the key challenges in achieving the operational implementation of spaceborne ATI-SAR is optimizing the baseline length, which directly impacts the accuracy of surface current measurements. Additionally, it is essential to maximize temporal coverage, defined as the proportion of time within a satellite's orbit during which the required measurement accuracy is maintained. ATI-SAR has strict requirements for baseline length. If the baseline is extremely short, the time delay between the two channels is also short, resulting in a phase that is easily overwhelmed by noise [20]. Conversely, the delay time between the two interfering channels will exceed the sea surface's coherence time, which occurs when the surface scattering is stable. The coherence coefficient will be minimal, and consequently, it is impossible to obtain an effective interference phase. The length of the optimal baseline has an approximately inverse relationship with the SAR frequency. There are two approaches for realizing optimal baseline operation for spaceborne ATI systems. The first is to use a solitary satellite. Given the hardware platform constraints, it is necessary to increase the SAR frequency band. For example, the Wavenill [21] and SEASTAR [22] satellite mission concepts, successively proposed by the European Space Agency (ESA), both consist of a satellite carrying a single Ku-band SAR payload. This instrument is based on the ATI solution with two squared-look directions to invert the ocean surface current vectors. The optimal baseline can be reduced by approximately 10 m owing to the higher band, which simplifies the satellite technology. Both studies conducted airborne proof-of-concept experiments and obtained encouraging results [23; 24]. However, there are currently no thoroughly evaluated spaceborne platforms. Another approach is via satellite constellations. [PERSON] [25] demonstrated that for the TanDEM-X/TerraSAR-X constellations, the optimal X-band along-track effective baseline is 20-30 m. However, this baseline length is shorter than the minimum safe distance between satellites. Thus, the constellation can only operate within this optimal baseline for short periods, limiting temporal coverage. Contrarily, the optimal effective along-track baseline length for an L-band spaceborne ATI-SAR system is approximately 10 times that of a C-band system [26], reaching around 200-300 m. This significantly enhances the feasibility of using satellite constellations to achieve an optimal baseline. The actual temporal coverage depends on the specific satellite formation design, which is beyond the scope of this study. The LuTan-1 constellation's A and B satellites (LT-1A/1B) were successfully launched from the Jiuquan Satellite Launch Center in China on 26 January and 27 February 2022, respectively [27]. LuTan-1 represents the world's first L-band distributed SAR dual-satellite system. This development also marked the first opportunity to invert ocean surface currents using the L-band spaceborne interferometric synthetic aperture radar (InSAR) technique. Compared to single-satellite systems, satellite formation systems employing longer baselines are prone to introducing larger errors. Owing to the limited precision of navigation systems and the irregular baseline variations induced by helical flight paths, baseline errors typically exhibit temporal variability.