# Phase Imbalance Analysis of GF-3 Along-Track InSAR Data for Ocean Current Measurement
## 5 Conclusions  
GF-3 ATI mode performs imaging through dual-receive fine stripmap mode, which has an effective along-track antenna separation of 3.75 m. Using the ATI method to invert the current requires the high precision of the interferometric phase. Simply subtracting the constant phase imbalance between the two channels is not sufficient for measurement accuracy. The space-time variability of the phase imbalance between channels, caused by antenna phase center position error, attitude error, antenna electronic miss-pointing, antenna pattern mismatch, and other reasons, cannot be ignored. This paper introduces a phase imbalance calibration method based on SAR data. Using the characteristic that the phase difference of two channels changes linearly with the Doppler frequency in the Doppler frequency domain, the signal delay and phase imbalance between channels are calculated by the least squares method. The advantage of this method is that it can carry out dual channel image registration and target radial velocity measurement when the along-track interferometric baseline and satellite velocity are unknown, and the influence of the along-track interferometric baseline error and the satellite velocity measurement error can be ignored. Finally, the space-time change of the phase imbalance is solved by image block processing.  
Limited by the number of GF-3 ATI data, we mainly analyzed the phase imbalance in range of two sets of GF-3 ATI data with the same antenna beam combined with the simulation experiment using actual GF-3 parameters. It was concluded that the phase imbalance in the range of GF-3 ATI mode is mainly caused by squint imaging, the baseline error, and the phase mismatch of the antenna pattern in range. Furthermore, the space-varying of the phase imbalance caused by the baseline error and the phase mismatch of the antenna pattern in range is far less than that caused by squint imaging, which is mainly caused by satellite attitude error and antenna electronic miss-pointing. What is more, on the premise of accurate attitude measurement results, the influence of the GF-3 attitude control error on the phase imbalance can be ignored, and the main reason for the space-varying of phase imbalance in GF-3 ATI experimental mode is the antenna electronic miss-pointing.  
After the phase imbalances' calibration, we carried out the ocean current by the ATI method and the DCA method. The results obtained by the two methods are relatively consistent with the correlation coefficient of 0.98. The mean difference is \(-0.010\) m/s, and the RMSE is 0.062 m/s. Compared with the current measured by HFSWR, the RMSE is around 0.38 m/s. Therefore, it illustrates the feasibility of using GF-3 data to measure ocean currents, whether dual-channel or single-channel data. It can be proven that the accuracy of ocean current inversion through the short-baseline ATI-SAR is basically the same as that of the DCA method based on single-channel SAR data.  
Conceptualization, J.Y. and X.Y.; Data curation, J.Y., X.Y. and L.Z.; Formal analysis, J.Y.; Funding acquisition, B.H.; Investigation, J.Y. and J.S.; Methodology, J.Y., X.Y. and B.H.; Project administration, B.H.; Resources, L.Z.; Software, J.Y.; Supervision, C.D.; Validation, J.Y., J.S. and M.S.; Visualization, J.Y.; Writing--original draft, J.Y.; Writing--review & editing, J.Y., X.Y., B.H., M.S. and X.W. All authors have read and agreed to the published version of the manuscript. This research was funded by the National Natural Science Foundation of China under Grant Number 41976169.  
The authors declare no conflict of interest.