## 2 Literature Review  
### InSAR Monitoring Technology  
SAR (Synthetic Aperture Radar) is a measurement and imaging technique that uses the phase of coherent radar signals to measure the surface of the terrain, and the variation in surface detail features over time [23]. In contrast to GPS technology, SAR technology can be applied to an area of about 1000 km\({}^{2}\), which is regularly scanned by single or multiple satellites. SAR technology does not need to arrange monitoring points on site, and the monitoring cost is low, while the efficiency is high. However, the temporal and geometric decorrelation of SAR becomes the limiting factor for applying SAR technology in settlement monitoring. In addition, the inhomogeneity of the atmosphere can also strongly affect the accuracy of the results [24]. Based on the SAR technique, InSAR (Interferometric Synthetic Aperture Radar) time-series technology, a robust measurement remote sensing method for estimating millimeter-scale surface deformation from space, has been developed to solve temporal and geometric decoherence [25]. Although the interferometric phase can be obtained by using the geometric relationship between the two SAR acquisition points, InSAR technology presents a problem with signal decorrelation. InSAR time-series analysis techniques mainly address signal decorrelation to estimate deformation and atmospheric signals [26]. These techniques can be divided into three types. The first type, named the D-InSAR technique, separates the phase contribution terms other than the surface deformation phase in the interferometric phase [27]. The second type is PS-InSAR permanent scatterer (PS) InSAR technology, which mainly analyzes time-coherent point scatters in urban areas [28]. The PSP-InSAR (persistent scatterer pairs InSAR) persistent scatterer method is a new persistent scatterer interferometry method [29]. D-Insar is a low-cost deformation monitoring tool for large area coverage, but the temporal decoherence and atmospheric inhomogeneity limit its use of D-Insar [30]. To address the limitations of D-InSAR, [PERSON] was the first to propose persistent scatter InSAR PSI. The PS-InSAR technology is a kind of InSAR technology, the main feature of which is to extract pixels with relative steady backward scattering characteristics from a range of interferograms, thus eliminating the effect of atmosphere interference on the beam signal. Validation activities have been performed in various studies since the first application of the PSI algorithm to confirm the accuracy of the results of the technique. The verification studies are mainly based on level measurements [30] and the Global Navigation Satellite System [31]. Subsidence monitoring in urban areas has received more and more attention from scholars such as those in Tokyo [8], Houston [32], Beijing [33], Shanghai [34], etc. Subspace is mainly divided into endogenous subsidence and exogenous [35]. Endogenous subsidence is related to internal geological activities such as earthquakes, faults, volcanism, etc. [36]. Subsidence caused by human activities is often regarded as exogenous subsidence, such as groundwater extraction [37], subway construction, etc. [11]. Urban subsidence is usually caused by different reasons, such as groundwater or gas and oil extraction [38; 39]. [PERSON] et al. [10] conducted a detailed study on Beijing based on time series using the data from 2012 to 2018 and found that groundwater is an important factor affecting subsidence. [PERSON] et al. [40] collected Sar data in Shenzhen from 2004 to 2017. According to the InSAR time-series analysis of Shenzhen, most of the uneven subsidence in Shenzhen often occurs in reclamation and coastal areas. [PERSON] et al. [40] found that the subway is also the main factor causing subsidence, especially in the reclamation area that the subway passes through. PSI technology is often used in the settlement monitoring of infrastructure, including subways [5], buildings [18], airports [20], highways [19], etc. [PERSON] et al.