# Phase Imbalance Analysis of GF-3 Along-Track InSAR Data for Ocean Current Measurement
## 3 Data Processing and Interpretation
### Analysis of Channel Phase Imbalance
#### 3.1.3 Phase Imbalance Caused by the Baseline Error and the Phase Mismatch of the Antenna Pattern
\begin{table}
\begin{tabular}{c c} \hline Radar Center Frequency & 5.4 GHz \\ Band Width & 100.0 MHz \\ Pulse Width & 25.0 us \\ Sample Rate & 133.3 MHz \\ PRF & 2529.6 Hz \\ Antenna Aperture & 15 m \\ Sat Velocity & 7567.3 m/s \\ Center Look Angle & 18.22\({}^{\circ}\) \\ Center Range & 799.822.9 m \\ Ground Width & 50 km \\ Ground Height & 50 km \\ Number of Azimuth Target & 1 \\ Number of Range Target & 101 \\ \hline \end{tabular}
\end{table} 
Table 6: Simulated SAR parameters. Figure 12: The relative change of phase imbalances in range caused by the attitude error. (**a**) The first set of GF-3 ATI data; (**b**) the second set of GF-3 ATI data. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \(\mathbf{\theta}_{pitch}\)(_ideal_) & \(\mathbf{\theta}_{yaw}\)(_ideal_) & \(\mathbf{\theta}_{pitch}\) & \(\mathbf{\theta}_{yaw}\) & \(\mathbf{\Delta\theta}_{pitch}\) & \(\mathbf{\Delta\theta}_{yaw}\) & \(\mathbf{\Delta x}\) & \(\mathbf{\Delta z}\) \\ \hline
0.0763\({}^{\circ}\) & \(-2.9730^{\circ}\) & 0.0770\({}^{\circ}\) & \(-2.9712^{\circ}\) & \(-0.0007^{\circ}\) & 0.0018\({}^{\circ}\) & \(-0.24\) mm & \(-0.09\) mm \\
0.0718\({}^{\circ}\) & \(-2.9936^{\circ}\) & 0.0715\({}^{\circ}\) & \(-2.9911^{\circ}\) & \(-0.0003^{\circ}\) & 0.0025\({}^{\circ}\) & \(-0.33\) mm & \(-0.04\) mm \\ \hline \end{tabular}
\end{table} 
Table 5: The attitude errors. the baseline error are coupled. Therefore, the phase imbalances caused by the antenna phase center position error and the phase mismatch of antenna pattern error cannot be distinguished. Therefore, it is necessary to calibrate the antenna pattern and antenna phase center position before the satellite is launched. This determines whether ATI-SAR can obtain accurate phase information and directly affects the ocean current inversion accuracy. In conclusion, The causes of the space-varying of the phase imbalance are shown in Table 7, where \(P_{A}(\theta)\) and \(P_{B}(\theta)\) represent the antenna phase pattern in the range of Channel A and Channel B, respectively. It is considered that the space-varying of the phase imbalance in range of GF-3 ATI mode may be caused by the squint imaging, the baseline error, and the phase mismatch of the antenna pattern in range, according to these two sets of GF-3 ATI data. Moreover, squint imaging is the main factor causing phase imbalance in range caused by the attitude error and antenna electronic miss-pointing, while the baseline error and the phase mismatch of antenna pattern in range are minor factors. Finally, the block processing method mentioned in this paper can be used to calibrate the phase imbalance. In addition, under the condition of accurate attitude measurement, the main reason for the space-varying of phase imbalance in GF-3 ATI experimental mode is the antenna electronic miss-pointing. Of course, it is necessary to carry out the subsequent GF-3 ATI experiments to further study the phase imbalances of the GF-3 dual channels.