# Error Source Analysis and Correction of GF-3 Polarimetric Data
## 5 Discussion  
As product quality is of crucial importance, the polarimetric error must be calibrated and corrected for the GF-3 satellite. In the above sections, we presented the calibration of polarimetric error for GF-3  
Figure 8: Quality assessment results of corrected data estimated by distributed-target method: (a) amplitude imbalance; (b) phase imbalance; and (c) isolation.  
PolSAR system. The designed polarimetric performances of GF-3 satellite were 0.5 dB and 10\({}^{\circ}\) relative accuracy between channels, and cross-pol leakage below \(-35\) dB. The comprehensive assessment results by the common distributed target showed that the quality of corrected PolSAR data had reached this expected goal. Besides, for the more accurate five measures in Table 7, the performance was assessed to be around 0.2 dB, 5\({}^{\circ}\) and \(-37\) dB.  
Until now, various calibration methods focused on the determination of polarimetric error including channel imbalances, crosstalks, and FR using the external reference objects--particular distributed target and standard corner reflector [6; 31; 32; 33]. Simultaneously, existing PolSAR satellites also used external calibration methods to solve distortion parameters and usually depended on the Amazon Rainforest and triangle corner reflector [11; 14]. As to the polarimetric calibration task of GF-3, there are many kinds of data and the aim is to have completed the initial calibration of all data within 5 months. Hence, detailed analysis about the system should be done so as to perform a more efficient and affordable external calibration. Moreover, the GF-3 antenna is highly isolated, which causes fewer parameters to be calibrated [22; 23]. And, the GF-3 has the capacity of internal polarimetric calibration and precisely models the antenna before satellite launching [21], which helps us to get more information about the error of SAR system and antenna.  
Based on previous studies on the errors of antenna, attitude, and ionosphere [5; 6; 7], we considered more details by combining the actual conditions of GF-3 to give the full-link error model for the first time and the calibration way of GF-3 polarization error. Especially, we pointed out the specific sources of antenna phase imbalance and found that no coincidence of H and V antenna center at range caused this phase imbalance larger with the view angle increasing. Though the error model and these analyses are for GF-3, these works can provide references for the calibration of follow-up PolSAR satellites. It should be noted that the external calibration was still used to extract the absolute phase imbalance in Section 3.3 [24], but only for three typical beams. In addition, the accuracy of antenna model greatly affects the quality of corrected data. With time passage and the device aging, this model now used may be inadequate. However, this problem could be solved by the internal calibration such as single T/R channel calibration and phase encoding calibration. These calibration modes can help us to understand the working status of single T/R channel. So, we will monitor system changes by internal calibration and data quality in real time.