# 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  
After removing the phase imbalance caused by squint imaging, the main reasons for the remaining phase imbalances are the phase mismatch of the antenna pattern in range and the baseline errors caused by the antenna phase center position error and the attitude error. The remaining phase imbalances of the first set of data and the second set of data are shown in Figure 11, the accuracy of which depends on the accuracy of the estimated Doppler centroid frequency, satellite velocity, and effective baseline between channels. It can be found that the change of phase imbalance along the distance is less than 1\({}^{\circ}\), which basically meets the requirements of ocean current velocity measurement, and there is no consistent trend of the remaining phase imbalance between different scenes of the same set of data. In addition, only the attitude changes in the same imaging process of GF-3, and it can be concluded that GF-3 does have the baseline error caused by attitude error. The baseline error caused by attitude error can be analyzed according to the GF-3 attitude information in echo data. We select the central scenes of two sets of data to analyze the attitude error. \(\theta_{pitch(ideal)}\) and \(\theta_{yaw(ideal)}\) represent the ideal pitch angle and yaw angle of satellite attitude, which are obtained by calculating the satellite orbit information using the 2D attitude steering approach for the zero Doppler centroid. \(\theta_{pitch}\) and \(\theta_{yaw}\) are the actual pitch and yaw angle measurements. The position errors of the dual-channel receiving position caused by the attitude error are shown in Table 5. For the first set of data, the pitch error is \(-0.0007^{\circ}\), and the yaw error is \(0.0018^{\circ}\), corresponding to \(\Delta x=-0.24\) mm and \(\Delta z=-0.09\) mm. Similarly, for the second set of data, the pitch error is \(-0.0003^{\circ}\), and the yaw error is \(0.0025^{\circ}\), corresponding to \(\Delta x=-0.33\) mm and \(\Delta z=-0.04\) mm. Moreover, the magnitude of baseline error can correspond to the magni  
Figure 11: The relative change of phase imbalances in range after removing the phase imbalance caused by squint imaging. (**a**) The first set of GF-3 ATI data; (**b**) the second set of GF-3 ATI data. tude of the position offset in the distance direction. On the premise of the accurate attitude measurement value, and the phase imbalance caused by the baseline error due to the attitude error can be expressed by Equation (19). We carried out simulation experiments using the real radar parameters of the first set of GF-3 ATI data to analyze the phase imbalance caused by the deviation of the antenna phase center position causing the attitude error. The radar parameters used in the simulation are shown in Table 6. As shown in Figure 12, the phase imbalance between the two channels caused by the attitude error also has a near-linear change in the range, which is not a pure linear change. The phase imbalances caused by the attitude error of these two sets of data only changed by 0.0976\({}^{\circ}\) and 0.1242\({}^{\circ}\) from near range to far range, which is far less than the actual error. Therefore, on the premise that the attitude measurement is accurate enough, the baseline error caused by the satellite attitude error has little effect on the phase imbalance. Moreover, there is no approximately linear change of the phase imbalance on both sides of the image, and the changing trend of the two sets of GF-3 ATI data with the same antenna beam is consistent. Therefore, it may be caused by the phase mismatch of the antenna pattern in range. However, the phase mismatch of the antenna pattern and