## 6 Conclusions  
In view of the lack of large-scale subsidence research, insufficient analysis of subsidence mechanism and low timeliness of experimental data in Shanghai. In this paper, time-series InSAR technique was used to process the 36 Sentinel-1A SAR images from January 2018 to March 2020. Combined with the changes in precipitation, groundwater level, and urbanization process in Shanghai, the spatial-temporal characteristics of surface subsidence in Shanghai are analyzed in detail. The main conclusions are presented as follows:  
1. The internal coincidence accuracy of PSInSAR technique is analyzed. Most PS point standard deviations are less than 1 mm/year, which is relatively small. Therefore, the accuracy and stability of PSInSAR technique for surface subsidence monitoring in Shanghai area are relatively high. 2. The uneven subsidence in Shanghai is obvious. The surface subsidence in the north and east of Shanghai is stable, and the surface subsidence in some areas is uplifted. However, the subsidence in Songjiang District in the east, Jinshan District in the south and Fengxian District in Shanghai are more serious, with large area subsidence and outward expansion trend. The subsidence near Xinghuocum in Fengxian District is the most serious, with the maximum accumulated subsidence of \(-75\) mm, which is related to the exploitation of groundwater caused by industrial water in this area. 3. The study area is mainly a soft soil geological area, and the water content of soft soil is relatively high, which has a relatively large impact on engineering construction. This reason is the main cause that Shanghai area is prone to subsidence. In addition, seasonal changes in surface subsidence may be closely related to rainfall, and timely replenishment of groundwater by rainfall can reduce the subsidence effectively and slow down the occurrence of excessive subsidence. The surface subsidence in the study area is highly correlated with the change in groundwater level. That is, when the groundwater level rises, the surface will be uplifted or stable, and when the water level drops, the surface will obviously sink. 4. Three subsidence areas were identified along the Shanghai Metro Line 15, which are the area of Yongde Road Station, the area between Yuanjiang Road Station and  
Figure 18: Difference chart of night light data of Shanghai main urban area in 2020 and 2018. Shuangbai Road Station, and the area between Yaohong Road Station and Guilin Road Station. Among them, the subsidence of Yongde Road Station is the most serious with a maximum accumulated subsidence that reaches \(-40.2\) mm. The main reason for the subsidence is the natural consolidation subsidence of the surface after the completion of the subway construction. Additionally, the industrial water in large industrial zones in Shanghai area increases the amount of underground water extraction, which is the main reason for the subsidence of industrial areas. Urbanization and infrastructure construction are also highly correlated with surface subsidence to a certain extent. All authors contributed to the manuscript and discussed the results. [PERSON] and [PERSON] put forward the idea of this paper. [PERSON] processed and analyzed the sentinel data and contributed to the manuscript of the paper. [PERSON] made criticism and revised the manuscript. [PERSON] analyzed the relationship between rainfall and ground subsidence. [PERSON] analyzes the results of PSInSAR. [PERSON] analyzed the settlement of the metro line, and made critical comments on the manuscript. [PERSON] analyzed the settlement of the industrial areas, and made critical comments on the manuscript. [PERSON] processes and analyzes the NPP-VIIRS data. All authors have read and agreed to the published version of the manuscript.