# Error Source Analysis and Correction of GF-3 Polarimetric Data
## 3 Influence Analysis of Error Source
### Channel Imbalances of Transmit and Receive Channel  
Due to the non-ideality of the hardware, gains are different between H and V polarization channels. Choosing the H polarization as a basis, the ratio of gain between V and H polarization channels is defined as an imbalance, including amplitude and phase. In this study, complex values \(f_{T}\) and \(f_{R}\) stand for imbalance of transmit and receive channel, respectively.  
Like other space-borne PolSAR satellites [16; 17; 18], the GF-3 satellite implements an internal calibration system [2]. Internal calibration can track the performance of the radar system by transmitting calibration pulses and receiving them through a dedicated internal calibration loop [19]. Fully considering various requirements of calibration elements, the internal calibration system of GF-3 satellite is designed with multiple calibration loops to improve the calibration accuracy and meet the quantitative application. More details of GF-3 internal system can be found in [8]. Therefore, the imbalance of transmit and receive channel can be determined from the data obtained by the internal calibration system, namely as \"internal calibration data\" [2; 16]. This estimation process can be summarized as follows:  
1. Extract the four suitable echoes (HH, HV, VH, VV) of the internal calibration loop covering transmit and receive channel from the internal calibration data;
2. Do pulse compression and extract the peak amplitude and phase of the four compressed pulses, then calculate the imbalances between the polarization channels;
3. Extract the pulses of the reference internal calibration loop, and get the peak amplitude and phase of the compressed pulses, calculate the imbalances caused by the internal calibrator itself.
4. Eliminate the imbalances in (3) from the imbalances in (2), then the results of \(f_{R}\) and \(f_{T}\) can be calculated.  
Here, 30 internal calibration data groups of GF-3 Full Polarized Mode are processed to analyze the real situation of channel imbalances. To make the results more representative, these 30 data groups span over four months and cover various beams and bandwidths, the detailed information of which is shown in Table 2. Figure 2 shows the results of \(f_{R}\) and \(f_{T}\) extracted from these experimental data, which are numbered by using bandwidth as the primary key and time as the secondary key, and sorted in an ascending manner. Results in Figure 2 suggest that phase imbalances are obviously large and non-ignorable. Most amplitude imbalances of transmit channel are small, only 10% of the results are more than 0.1 dB. The receive channel has a larger amplitude imbalance, in which most values distribute around 0.4 dB and some results exceed the requirement of less than 0.5 dB. Thus, imbalances of transmit and receive channel need to be corrected. In addition, imbalance of the receive channel has a large fluctuation where the difference between minimum and maximum is over 4\({}^{\circ}\) in phase and close to 0.9 dB in amplitude, while the imbalance of the transmit channel is relatively stable. The difference between transmit and receive channel is mainly due to the fact that the amplifier of the transmit channel operates in the saturation region, while that of the receive channel works in the linear region. In summary, imbalances between the transmit and receive channels have a non-negligible effect on the GF-3 polarization observation and should be well corrected. Variability of them means that \(f_{R}\) and \(f_{T}\) must be calculated in a timely fashion from the internal calibration data for high accuracy.