# Phase Imbalance Analysis of GF-3 Along-Track InSAR Data for Ocean Current Measurement
## 1 Introduction  
The study of ocean currents is an important part of ocean dynamics and is of great significance to humans working on the sea and in coastal areas. Synthetic aperture radar (SAR) is a high-resolution imaging radar that can observe the Earth for all weather and all time. Since the first spaceborne SAR, SeaSat, was launched successfully in 1970, SAR has shown irreplaceable ocean observation advantages. According to previous studies, SAR can accurately measure ocean currents with high spatial resolution. Two methods of SAR current measurement are produced based on the characteristic that a target moving relative to the radar will make a Doppler frequency shift proportional to the speed. [PERSON] and [PERSON] [1, 2] proposed the concept and basic theory of ocean currents obtained by ATI-SAR in 1987 and demonstrated for the first time the ability of ATI-SAR to measure surface currents in San Francisco Bay, Mission Bay, and San Diego Bay in 1989. [PERSON] and [PERSON] [3] combined the in site acoustic Doppler current profiler (ADCP) measurement to explain how to extract the surface current from the interferometric phase and confirmed the analysis model of the directional propagation of Bragg resonance wavesin 1998. [PERSON] and [PERSON] [4] proposed an efficient model for the simulation of Doppler spectra and ATI signatures based on Bragg scattering theory in a composite surface model approach in 2000. In 2003, [PERSON] [5] reviewed the relationship between the phase difference measured by SAR and the average Doppler frequency and proposed a method to remove the Bragg wave phase velocity based on the C and L dual-band ATI-SAR technology, since the proportions of C-band and L-band Bragg wave components propagating in different directions are approximately equal. In 2005, [PERSON] et al. [6] used the data obtained during the Space Shuttle Radar Topographic Mapping Mission (SRTM) in February 2000 through additional along-track antenna separation of 7 m to first analyze the ocean current measurements by ATI-SAR. Comparing the currents in the Wadden Sea area of the Netherlands measured by ATI-SAR with the results of the numerical circulation model KUSTWAD, the correlation coefficient was as high as 0.6, and the root mean squared error was within 0.2 m/s. In 2010, [PERSON] et al. [7] showed preliminary results based on actual TerraSAR-X ATI data. The results obtained by using TerraSAR-X Aperture Switching (AS) mode in the Elbe River mouth area were consistent with the UnTRIMresults. It is believed that the typical current measurement accuracy of TerraSAR-X ATI under the effective spatial resolution of 1000 m can reach a performance of 0.1 m/s. [PERSON] [8] first proposed the Doppler centroid analysis (DCA) method in 2005. [PERSON] et al. [9] used the DCA method to monitor the Agulhas current in 2010 successfully. The current inverted by the DCA method can replace the altimeter measurement result. In 2010, [PERSON] et al. [10] verified that the sea surface velocity retrieved by the SAR DCA method has an accuracy of 0.05 m/s at a resolution of 10 km. In 2013, [PERSON] et al. [11] analyzed two TanDEM-X interferograms with effective baselines of 25 m and 40 m obtained in the Pentland Firth in Scotland and compared with the TerraSAR-X dual-receiving antenna (DRA) mode interferogram with an effective baseline of 1.15 m and the velocity field of single antenna data from the same scene obtained by DCA. Under the same measurement accuracy conditions, compared with DRA mode, the spatial resolution of the currents obtained from the TanDEM-X interferogram is higher than that of DRA mode. The DCA-based currents are less accurate than the ATI-based ones, but close to short-baseline ATI results in quality.