## 2 Methodology  
Aiming at the problems of interference incoherence and atmospheric delay error that exists in DInSAR technique, PSInSAR technique can reduce the influence of interference incoherence and atmospheric delay error on the calculation results effectively. Unlike DInSAR technique, PSInSAR focuses on the use of SAR image sequences acquired from the same area during a certain period. First, according to the scattering characteristics of ground objects and statistical analysis method, the permanent scatterers with high correlation in time series in the study area are detected. Second, based on the phase time series of these specific targets, modeling and analysis are carried out to obtain high-precision deformation measurement results. The main steps are detailed as follows [32; 33; 34]:  
(1) Assume the N+1 SAR images covering the same area. Arrange the SAR images according to the time series. Calculate the temporal baseline, spatial baseline, and doppler centroid frequency difference between the images. Establish the relevant model according to the three aforementioned factors to select the reference image. \[\rho^{m}=\frac{1}{K}\sum\
olimits_{k-1}^{k}\left[\left(1-\frac{B_{\perp}^{k,m }}{B_{c}}\right)\times\left(1-\frac{T^{k,m}}{T_{c}}\right)\times\left(1-\frac {B_{DC}^{k,m}}{f_{c}}\right)\right] \tag{1}\]  
where \(\rho^{m}\) is the comprehensive correlation coefficient of an interference pair; \(B_{\perp}^{k,m}\), \(T^{k,m}\), and \(B_{DC}^{k,m}\) are the perpendicular baseline, temporal baseline, and Doppler centroid frequency difference between interference images \(k\) and \(m\), respectively. The critical conditions of the aforementioned parameters are \(B_{c}\), \(T_{c}\), and \(f_{c}\), respectively. The image with the maximum value is the reference image to select the optimal reference image. (2) After calculation, the image on 29 June 2019 was selected as the reference image in this paper. All the secondary images were registered to the reference image, and N interference pairs were generated after differential interference processing. The spatial-temporal baseline distribution is shown in Figure 1, and the specific spatial-temporal baseline distribution parameters of Sentinel-1A data are listed in Table 1.  
(3) The amplitude deviation index method was used to select the high-amplitude value as the candidate PS point according to the strong reflection characteristics of PS points. The algorithm of amplitude deviation index is shown in (2). The interference phase information contained in the candidate PS points can be expressed as Equation (3) [35; 36]:  
\[D=\frac{\sigma_{i}}{m_{i}} \tag{2}\]  
\[\phi=\phi_{ref}+\phi_{def}+\phi_{topo}+\phi_{atm}+\phi_{noi} \tag{3}\]  
where \(D\) is the amplitude deviation index, \(\sigma_{i}\) is the amplitude standard deviation of the images with time series, and \(m_{i}\) is the mean amplitude of the time series image. In Equation (3), \(\phi\) is the interference phase; \(\phi_{ref}\) represents the flat-earth phase; \(\phi_{def}\) refers to the surface deformation phase; \(\phi_{topo}\) corresponds to the phase caused by topographic relief; \(\phi_{atm}\) denotes atmospheric phase error; and \(\phi_{noi}\) depicts the rando/m noise phases (e.g., orbital errors, thermal noise, and spatial decorrelation). Among them, \(\phi_{ref}\) can be eliminated by precise orbit data, \(\phi_{topo}\) can be eliminated by external DEM. (4) After separating the linear deformation and elevation error from \(\phi\) using the correlation characteristics of atmospheric phase with low correlation in time and high correlation in space, the high-pass filtering in time and low-pass filtering in space are used to obtain the estimated values of atmospheric phase and nonlinear deformation in each scene of the differential interferogram.