## 4 Gaofen-3 Interferometry Analysis and Refinement
### The Refinement of the Orbit-Related Artifacts  
Figure 6 shows that when compared to the orbit track of the Sentinel-1 satellite, the orbit track of the Gaofen-3 satellite was unstable while observing the study area on different image acquisition dates. This instability led to a significant orbit error in range and azimuth direction. Due to the inaccurate orbit information of Gaofen-3, the accuracy of the perpendicular baselines calculated by orbital parameters could not reflect some changes in the azimuth.  
The spatial baseline errors usually increase the phase density and decreases the accuracy of phase unwrapping. The accuracy of baseline refinement will influence the quality of the interferograms. The conventional method uses orbit parameters to calculate the initial baseline, which assumes that the spatial baseline does not vary with time, which is suit  
Figure 5: Spatial and temporal baseline distributions of the interferograms generated from the Gaofen-3 (**a**) and Sentinel-1 (**b**) SAR datasets from August 2020 to April 2021.  
Figure 6: The track of SAR satellites based on the orbit state vector in image acquired date over the study area: (**a**) the track of the Gaofen-3 satellite observing the study area; (**b**) the track of the Sentinel-1 satellite observing the study area.  
able for parallel baseline estimation. Figure 7 shows that the high-density interferometric fringes in most of the wrapped interferograms resulting from initial baseline calculations are inaccurate by the errors in state vectors of orbit. In contrast, the interferogram itself could be used to improve the accuracy of perpendicular baseline estimation based on the interferogram fringe frequency. Therefore, we estimate the perpendicular baseline using the frequency information of the interferogram. Since the main frequency of the interferogram is not at the zero frequency, FFT (Fast Fourier Transformation) techniques were applied to shift it to zero frequency without assistance from the GCPs (Ground Control Points) and accuracy DEM.  
Due to the method of combined orbit information and interferogram fringes frequency without considering the detailed characteristics of the interferograms, the residual phase from the baseline errors still remained in wrapped interferograms. Furthermore, the baseline-induced error is often combined with atmospheric artifacts. Subsequently, the 2-D quadratic polynomial method is applied to model the residual phase of the baseline. After removing the phase-related interferometric baseline of the topographic, the height dependence of the atmospheric phase was modeled by a linear regression method. Then the residual orbit error and atmospheric are removed from the wrapped interferogram. As shown in Figure 8, the number of fringes was reduced greatly after removing the artifacts that related to inaccurate orbit information from the wrapped interferograms. However, the orbital phase is still obvious in some interferograms since we could not obtain the precise orbital information of the Gaofen-3 satellite during the observation period, and the orbital phase could not be estimated precisely.  
There were no obvious baseline artifacts in the 10 interferograms that were processed using the StaMPS-InSAR technique. Therefore, the quality of the interferograms generated by the Gaofen-3 images is influenced by the artifacts with inaccurate orbit information. As for the artifacts related to the orbit in Gaofen-3, the interferograms serve more than those in other SAR satellites. Refining and removing the artifacts with inaccurate orbit information could assist the Gaofen-3 SAR images in achieving high-accuracy subsidence over the study area.  
Figure 7: The wrapped interferograms with inaccurate orbit information of Gaofen-3 SAR images.