## 4 Gaofen-3 Interferometry Analysis and Refinement  
### Gaofen-3 Interferometric Analysis  
The InSAR coherence is a key factor in estimating the quality of interferograms which is related to the amount of phase error in interferograms. To understand the sources of decorrelation in the Gaofen-3 interferograms over Beijing, the interferograms from 27 January 2021 to 25 February 2021 of Gaofen-3 images (Figure 3a) were selected to analyze their interferometric performance. We also included the interferograms between 23 January 2021 to 28 February 2021 of Sentinel-1(Figure 3b) to assist in the analysis of Gaofen-3 SAR images (the details in Table 3). It can be seen from Figure 3 that the coherence performance and value of Sentinel-1 interferogram are better than those of Gaofen-3, especially in main urban distracts, and Figure 4 shows the histograms of coherence value for Gaofen-3 and Sentinel-1 interferograms represented by marigold color and green colors, respectively. The majority of coherence values for the Gaofen-3 interferogram fall within the range from 0.2 to 0.7, while the values of Sentinel-1 interferograms range from 0.3 to 0.9. A total coherence model is introduced to analyze the phase noise in the Gaofen-3 interferograms. According to the total coherence model (Equation (1)), the coherence of thermal noise was estimated by the value of the Signal Noise Ratio (SNR), where the Noise Estimation Sigma Zero (NESZ) as \(SNR^{-1}\) is used to estimate the coherence of thermal \(\gamma_{thermal}\). The maximum NESZ is \(-21.3\) dB and \(-22\) dB for Gaofen-3 and Sentinel-1 satellites, respectively, so the \(SNR^{-1}\) is \(10^{-21.3}\) and \(10^{-22}\), respectively [15; 51]. The coherence of thermal \(\gamma_{thermal}\) is close to 1 for both Gaofen-3 and Sentinel-1 based on Equation (2). Thus, the thermal noise is not the dominate inducement of the noise in Figure 3. Due to the similar time span of the two interferograms, the noise induced by temporal decorrelation \(\gamma_{temporal}\) is also excluded. We also observed the difference in Doppler centroid frequency based on the parameters of the Gaofen-3 SAR data, where the difference in Doppler centroid frequency in this interferogram reached 19 Hz, and the azimuth bandwidth was 1482.5 Hz. Therefore, the decorrelation caused by the difference in the Doppler centroid is minimal and only takes up nearly 1% of the \(\gamma_{DC}\) decorrelation based on Equation (6). The decorrelation of the baseline is the result of the difference in the incidence angle, the perpendicular, critical baseline (\(B_{c}\)), and the topographic slope of the earth's surface according to Equation (4) [37]. We derived the critical baseline (\(B_{c}\)) from the parameters of two satellites, which are approximately 10 km and 6.6 km for Gaofen-3 and Sentinel-1 based on Equation (5) (assuming there is no topographic slope). The larger the perpendicular baseline, the more serious the phase decorrelation to SAR interferograms over the study area. Figure 5 shows that the perpendicular baselines of Gaofen-3 images range from 134 m to 1926 m, and most of them are more than 500 m, while the perpendicular baselines of Sentinel-1 images range between 2 m and 196 m. The maximum perpendicular baselines of Gaofen-3 are nearly ten times the perpendicular of the Sentinel-1 SAR data during the observation period. The perpendicular baseline is 1156 m for the interferogram of the Gaofen-3 images, which leads to the correlation of \(\gamma_{baseline}\) decreasing to around 0.1156. While the perpendicular baseline is 45 m for the interferogram of the Sentinel-1 SAR images, the perpendicular baseline error to the coherence of the Sentinel-1 interferogram is negligible. Therefore, our results indicate that the Gaofen-3 SAR images own the capability of interferometric over Beijing city to a certain degree. However, the coherence is seriously contaminated by long perpendicular baselines, which limit the application of the Gaofen-3 images in ground surface monitoring.