# Phase Imbalance Analysis of GF-3 Along-Track InSAR Data for Ocean Current Measurement
## 3 Data Processing and Interpretation
### Ocean Current Measurements
#### 3.2.2 Verification of Inversion Accuracy  
In order to verify the accuracy of the ocean surface radial velocity extracted from GF-3 ATI data, mutual verification has to be carried out with the currents obtained by the DCA method using single-channel data. The Doppler centroid frequency calculated from the SAR signal of the ocean is caused by the motion of the stationary target relative to the satellite and the ocean surface motion [33]. The Doppler centroid frequency estimated from the SAR echo data can be expressed as:  
\[f_{dc}=f_{ocean}+f_{dp}, \tag{25}\]  
Figure 14: GF-3 ATI data procession flow diagram.  
Figure 15: The ocean current retrieved by the ATI method. (**a**) The radial velocities of the ocean surface; (**b**) the ocean current velocities.  
\[V_{r}=-\frac{\lambda}{2\sin\theta}f_{ocean}, \tag{26}\]  
where \(f_{ocean}\) is the Doppler shift caused by the motion of the ocean surface and \(f_{dp}\) is the Doppler frequency shift caused by the satellite orbit, attitude, antenna electronic miss-pointing, and the rotation of the Earth. Generally speaking, \(f_{dc}\) is zero for land scenes since GF-3 uses the 2D attitude steering approach for the zero Doppler centroid. Based on this feature, we can also use the land scene as a reference scene to eliminate the \(f_{dp}\) component in the Doppler centroid frequency of the ocean scene. The estimated Doppler centroid frequency can be calculated by the correlation Doppler estimator (CDE) method [34].  
For the DCA method, we also use the land scene as a reference scene to eliminate the \(f_{dp}\) component, which is caused by the satellite orbit, satellite attitude, antenna electronic pointing, and the rotation of the Earth in the Doppler centroid frequency of the ocean scene. The way to extract the velocities of the ocean current from the Doppler velocities of the ocean surface is exactly the same as the ATI method. The result is shown in Figure 16. The ocean current retrieved by the DCA method is about \(-140\)\(\sim\)\(-30\) cm/s.  
The mean difference between the current velocities obtained by the ATI method and the DCA method is \(-0.010\) m/s, and the root mean squared error (RMSE) is \(0.062\) m/s. The correlation coefficient reaches \(0.98\). As shown in Figure 17, the difference of the current velocities obtained by the two methods is mainly \(-0.05\) m/s \(\sim\)\(0.05\) m/s. Therefore, it can be proven that the accuracy of ocean current retrieval by ATI-SAR with a short baseline is basically the same as that of the DCA method based on single-channel SAR data.  
Finally, the accuracy of the inversion results is verified with the current velocities measured by high-frequency surface wave radar (HFSWR). First of all, the ocean currents measured by HFSWR are matched with those retrieved by GF-3 ATI data by bilinear interpolation. Comparing the current velocities measured by GF-3 ATI data and HFSWR, the RMSE is \(0.38\) m/s, the mean difference being \(-0.33\) m/s, and the correlation coefficient reaching \(0.62\), which preliminarily verify the correctness of the analysis of this paper. In addition, the ocean current inversion method based on GF-3 ATI data and its accuracy improvement still need to be further studied.  
Figure 16: The results retrieved by the Doppler centroid analysis (DCA) method. (**a**) The radial velocities of the ocean surface; (**b**) the ocean current velocities.