# Phase Imbalance Analysis of GF-3 Along-Track InSAR Data for Ocean Current Measurement
## 3 Data Processing and Interpretation
### Analysis of Channel Phase Imbalance
#### 3.1.1 Sampling Time Delay  
The dual-channel sampling time delay can be obtained by analyzing the inner calibration data. Firstly, the pulse compression is carried out for the dual-channel inner calibration signals, and then, the sinc function interpolation algorithm is adopted to re-sample the signals after pulse compression. Finally, the time difference between the peak amplitude points of dual-channel signals is compared to obtain the sampling time delay error. A shown in Figure 8, it is obvious that there is a certain time delay between the Channel A signal and the Channel B signal of the first set of data and the second set of data. Furthermore, the range position of the image of Channel A relative to the image of Channel B will become inconsistent, caused by the sampling time delay, antenna phase center position error, and attitude error. The position offset of the dual-channel image in the range can be obtained by the correlation algorithm for image registration [31,32], which is represented by \(\Delta P\). After calculating, the position offsets in range caused by each error are shown in Table 4. The range position offsets of the first set of GF-3 ATI data  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Data** & **Antenna Beam** & **Look Direction** & **Direction** & **Look Angle** & **Center** & **Imaging Time** \\ \hline
1 & 162 & R & ASC & 18.22\({}^{\circ}\) & 106.0\({}^{\circ}\)W, 40.30\({}^{\circ}\)N & 27 April 2018 00:49 \\
2 & 162 & R & ASC & 18.22\({}^{\circ}\) & 106.0\({}^{\circ}\)W, 40.03\({}^{\circ}\)N & 10 November 2020 00:49 \\
3 & 163 & R & ASC & 20.23\({}^{\circ}\) & 120.0\({}^{\circ}\)E, 35.66\({}^{\circ}\)N & 29 October 2019 09:50 \\ \hline \hline \end{tabular}
\end{table}
Table 3: GF-3 radar parameters.  
Figure 7: The space-varying of phase imbalances in range. (**a**) The first set of GF-3 ATI data; (**b**) the second set of GF-3 ATI data.  
and the second set of GF-3 ATI data are \(-0.0364\) pixels and \(-0.0326\) pixels, respectively. The range sampling time delays of the first set of GF-3 ATI data and the second set of GF-3 ATI data are \(-0.266\) ns and \(-0.249\) ns, respectively, so the corresponding image range position offset is \(-0.0355\) pixels and \(-0.0332\) pixels. Therefore, the range image position offsets caused by antenna phase center position error and attitude error are only \(-0.0009\) pixels and \(0.0006\) pixels.  
Therefore, the position offset of the GF-3 ATI dual-channel image in range is mainly caused by the sampling time delay. At the same time, it also shows that the GF-3 ATI data are indeed the baseline errors that may be caused by antenna phase center position errors  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Data** & \(\Delta\)**\(P\)** & **Sampling Time Delay** & **Baseline Error** \\ \hline the 1 st & \(-0.0364\) pixels & \(-0.0355\) pixels & \(-0.0009\) pixels \\ the 2 nd & \(-0.0326\) pixels & \(-0.0332\) pixels & \(0.0006\) pixels \\ \hline \hline \end{tabular}
\end{table}
Table 4: The position offset of dual-channel images in range caused by each error.  
Figure 8: After the dual-channel signal pulse compression of the internal calibration data, the peak position is partially enlarged. (**a**) The head calibration result of the first set of data; (**b**) the tail calibration result of the first set of data; (**c**) the head calibration result of the second of set data; (**d**) the tail calibration result of the second of set data.  
and attitude errors. Moreover, the range error between the dual channels caused by the baseline error is only on the order of millimeters.