# First Retrieval of Sea Surface Currents Using L-Band SAR in Satellite Formation
## 6 Conclusions  
Sea surface currents play a crucial role in the dynamic ocean environment. ATI-SAR technology applied to ocean current inversion yields precise measurements, a large mapping area, frequent observations, and high resolution. However, the operational implementation of interferometric SAR for ocean current inversion is challenging. One critical issue is maintaining an optimal baseline configuration for an extended duration within a satellite's orbital period. The optimal baseline length of an ATI system is approximately inversely proportional to the operating frequency. A potential approach to address this challenge is using lower frequency bands, such as the L-band, which allows for the implementation of satellite formations to achieve the optimal operational state for the ATI system. LuTan-1 is the first L-band spaceborne distributed interferometric SAR system. Although not initially designed to measure ocean currents, this instrument offers valuable guidance regarding the operational implementation of ATI-SAR on spacecraft. On 18 July 2022, sea surface current observational data were obtained for the YanTai Sea region in China using the LuTan-1 satellite. The LuTan-1 InSAR system operates in a double-helix formation, comprising both along-track and cross-track configurations. It  
Figure 14: The sea surface velocity retrieved by ATI. The color map represents the radial current velocity. Figure 15: Enlarged views of selected subregions from Figure 14: (**a**) Region 1, (**b**) Region 2. The color map represents the radial current velocity. operates within the optimal effective along-track baseline range of 200 to 300 m at this latitude. This study represents the first validation of ocean current measurements in L-band spaceborne ATI systems using LuTan-1. Owing to the limitations in navigation system accuracy, distributed satellite formations introduce more significant baseline errors compared to single-satellite systems. These errors make it challenging to eliminate the cross-track baseline phase component using only satellite-provided navigation parameters. To address this issue, an easily implementable two-step strategy was proposed. The critical aspect of this approach is the use of a parameterized model for baseline error estimation, which facilitates an unambiguous physical interpretation compared to the commonly used polynomial fitting methods. To assess the precision of the inversion results, they were compared with CMEMS data. Although LuTan-1 was not specifically designed for measuring ocean currents, the quantitative comparison results were relatively consistent. The mean differences between the currents obtained from CMEMS and LuTan-1 were 0.032 m/s and 0.041 m/s, respectively, with a correlation coefficient of 0.610. This study demonstrates the viability of measuring sea surface currents using a spaceborne L-band SAR system by processing LuTan-1 interference data. It also offers insights into the potential for the future operational deployment of a spaceborne ATI-SAR system for ocean current velocity measurements. However, several limitations need to be addressed. One notable limitation is the lack of local real-time data on wind speed, sea surface velocity, and sea surface height. Herein, the local sea surface current velocity and height data were sourced from CMEMS, whereas the sea surface wind speed data were obtained from the ECMWF. These data sources represent the average measurement over a long duration and large area. LuTan-1 acquired data within a few seconds of the synthetic aperture. They are not compatible, and this mismatch leads to errors during result verification and processing. Conceptualization, B.P. and T.L. ([PERSON]); Data Curation, B.P., X.Y. and T.L. ([PERSON]); Formal Analysis, B.P. and C.X. ; Funding Acquisition, X.W. and H.H. ; Investigation, B.P. and T.L. ([PERSON]); Methodology, B.P., T.L. ([PERSON]), X.W. and H.H. ; Project Administration, H.H. ; Resources, B.P., X.Y. and X.W. ; Software, B.P. ; Supervision, H.H. ; Validation, B.P. ; Writing--Original Draft, B.P.