# Error Source Analysis and Correction of GF-3 Polarimetric Data
## 4 Polarization Correction and Verification
### Quality Assessment of Corrected Data  
To evaluate the performance of this method and also to monitor the quality of corrected polarimetric data, data quality assessment is necessary. The desired amplitude and phase of channel imbalances are less than 0.5 dB and 10\({}^{\circ}\), and the isolation is desired to be higher than 35 dB. To analyze the quality assessment of corrected data, the point-target method and the common distributed-targets method are used to extract the residual channel imbalance and isolation in imagery. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Number** & **1** & **2** & **3** & **4** & **5** \\ \hline Latitude (deg) & 4.38 & 24.45 & 31.89 & 44.05 & 51.7 \\ \(\Omega\) (deg) & 0.02 & 0.07 & 0.04 & 0.02 & 0.05 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Estimation FR of GF-3 PolSAR data. Point-Target Assessment  
The point-target method is often used to evaluate calibration accuracy after polarization calibration [32]. This method utilizes corner reflectors which are artificially placed in the observation scene to estimate the isolation \(\delta_{triangle}\), amplitude imbalance \(f_{triangle}\) and phase imbalance \(P_{triangle}\) in the image domain. As a commonly used trihedral corner reflector, these measures can be calculated by the Equation (10). \[\left\{ \begin{array}{c}\delta_{triangle}=-20\log 10(\max(abs(M_{HV}/M_{ HH}),abs(M_{VH}/M_{HH})))\\ f_{triangle}=20\log 10(abs(M_{VV}/M_{HH}))\\ P_{triangle}=phase(M_{VV}/M_{HH})\end{array} \right. \tag{10}\]  
where \(M_{ij}\) is the measured scattering component of the trihedral corner reflector. Using the point-target method, polarization accuracy of five groups of GF-3 polarization datasets containing the trihedral corner reflector are analyzed. The results are shown in Table 7 where \"Mode\" means different beam. These results reveal that isolation of these data is better than 37 dB and the imbalance of VV channel relative to HH channel is within 0.2 dB in amplitude and less than 5.3\({}^{\circ}\) in phase. The quality of five group data has satisfied the requirements of polarization applications; therefore, the correction method proposed in this paper is effective. (2) Common Distributed-Targets Assessment  
The point-target analysis of corrected data reveals the preliminary conclusion that the proposed polarization correction method effectively corrects the polarization errors. To fully confirm this conclusion, more GF-3 PolSAR data need to be assessed. However, the point-target method relies on corner reflectors and cannot be widely used for polarimetric data assessment of the GF-3 entire polarization system. Hence, the method based on common distributed targets [23], is applied to assess the polarization quality of GF-3 using more data. This method does not depend on calibrators and particular distributed targets, which can meet the need of normalized quality assessment, and can give quantitative results of channel imbalance and isolation. Though the accuracy of the common distributed-target method is lower than the point-target method, the method is with high feasibility to verify the validity of the proposed polarization correction method for the GF-3 polarimetric system. Here, the quality of 43 calibrated PolSAR images is analyzed by using the method based on common distributed targets. These data are from 24 beams with the view angle ranging from 18.9\({}^{\circ}\) to 41.2\({}^{\circ}\), include multiple bandwidths and pulse durations, and span time of six months. Assessment results of these data are shown in Figure 8, where these data are numbered by using view angle as the primary key and time as the secondary key, and sorted in ascending way. In addition, the imbalance in this result figure denotes the difference between VV and HH channel. Despite the view angle or bandwidth of data being different, channel imbalances of all data observed at different times are basically maintained within 0.5 dB in amplitude and do not exceed 10\({}^{\circ}\) in phase.