## 3 Methods  
### Small Baselines Subset InSAR Analysis  
With the development of the InSAR technique and the increasing of synthetic aperture radar (SAR) images, [PERSON] et al. [21] proposed the method called Persistent Scatterer InSAR (PSInSAR) based on the differential InSAR technique. The main idea of the PSInSAR is to use multiple SAR images covering the same area to analyze the stability of the amplitude and phase and, then, to identify the pixels that are less-affected by the spatiotemporal decorrelation. To obtain accurate deformation information, the components of the phase need to be jointly analyzed and modeled to remove errors. In the PSInSAR method, a unique master image is selected from all images to generate interferograms. The strategy will result in the presence of long-space baseline interference pairs. It produces a low  
Figure 4: Deformations on the Muyubao landslide: (**a**) road damage on the west boundary, (**b**) deformation of buildings, (**c**) a local collapse on the west side, (**d**) road damage on the east side, (**e**) a collapse in the middle of the west side and (**f**) a tensile trench on the east side. density of the target reflectors in case of nonurban areas. Hence, in rural areas with few artificial buildings, the permanent scattering pixels may be very sparse [22]. The small baseline subset method was initially proposed to overcome the problem of decorrelation by making full use of the interferograms with both small temporal baselines and short perpendicular baselines [23; 24]. In rural areas, plenty of pixels have no dominated scattering characteristics in the SAR image. The pixels showing a slow decorrelating filtered phase (SDFP) are widely distributed in natural terrain and maintained good coherence during a short time interval. The Stanford Method for Persistent Scatterer (StaMPS) was proposed in 2004 [25]. In the StaMPS SABS method, these kinds of pixels are identified and analyzed to monitor surface displacement. The SDFP pixels are selected through their phase characteristics. In order to reduce the calculation burden, an initial subset of pixels containing almost all SDFP pixels are firstly selected through amplitude analysis. The amplitude dispersion value is calculated as the indicator of phase stability. The wrapped phase is composed of spatially correlated phase and spatially uncorrelated look angle error. The spatially correlated phase, including ground deformation, elevation error and orbit error, is estimated by the bandpass filtering in the frequency domain. The spatially uncorrelated look angle error mainly consists of the spatially uncorrelated elevation error. It is estimated through its correlation with the perpendicular baseline. The residual, obtained by removing these two terms from the wrapped phase, gives an estimation of the decorrelation noise \(\gamma x\) (Equation (1)), which indicates the stability of the pixel phase:  
\[\gamma_{x}=\frac{1}{N}\left|\sum_{1}^{N}\exp\left\{\sqrt{-1}(\psi_{x,i}- \widetilde{\psi}_{x,i}-\Delta\hat{\phi}_{\theta,x,i}^{u})\right\}\right| \tag{1}\]  
where \(N\) is the number of interferograms, \(\psi_{x,i}\) is the wrapped phase, \(\hat{\phi}_{\theta,x,i}^{u}\) is the spatially uncorrelated look angle error and \(\widetilde{\psi}_{x,i}\) is the spatially correlated term. The final SDFP pixels are selected through the threshold analysis of \(\gamma x\). Then, these SDFP pixels can be decomposed with the three-dimensional phase unwrapping method [26]. The unwrapped phase on a given pixel can be expressed as:  
\[\psi_{x,i}=\mathrm{W}\left[\phi_{D,x,i}+\phi_{A,x,i}+\phi_{O,x,i}+\phi_{T,x,i} +\phi_{N,x,i}\right] \tag{2}\]  
where \(\psi_{x,i}\) is the unwrapped phase, and \(\phi_{D,x,i}\), \(\phi_{T,x,i}\), \(\phi_{A,x,i}\), \(\phi_{O,x,i}\) and \(\phi_{N,x,i}\) are the phase components due, respectively, to ground deformation, topographic error, atmospheric disturbance, inaccurate orbit information and noise. In the StaMPS SBAS analysis, a theoretical framework for three-dimensional phase unwrapping was proposed.