## 3 Results  
### GNSS Measurement  
Two dual-frequency GNSS stations were installed for analysis to verify the accuracy of the SBAS analysis results, as shown in Figure 1 from April 2017. A reference GNSS station located at a stable place 1.1 km away from this point is adopted for a node to node displacement calculation. Assuming the reference GNSS station is stable without displacement. The selection of GNSS station sites was based on preliminarily DInSAR  
Figure 4: Extracted candidate points produced with SBAS (March 2017 to June 2021). analysis. The differential GNSS calculation is based on open source code RTKLIB. The road could be blocked due to heavy rainfall. Thus, the recorded data was sent through the internet to the lab and calculated on another server every hour. The return period of Sentinel-1 data is 12 days. The hourly solution is too much for DInSAR verification. Thus, a 24 h solution is adopted as well. As shown in Figures 5 and 6, there are two significant events from April 2017 to June 2021. The first event hit this area in June 2017, just two months after the GNSS stations were installed. The rainfalls hit in June 2017. The first event (1 st June to 4 th June) has 902.5 mm rainfall and 640 mm rainfall after several days (11 th June to 18 th June). This event induced about 120 mm in the east direction, 38 mm in the south direction, and 78 mm in the vertical direction at GNSS1. There is another significant rainfall event in 2019. The hourly rainfall reached a peak of 52 mm, but accumulated rainfall was less than the event in 2017. However, displacements were measured in this event. The south direction displacement at GNSS2 is more than east since its slope direction is heading south. The horizontal displacement at GNSS2 is almost the same as vertical displacement. But the horizontal displacement at GNSS1 is much more than vertical displacement. The displacement result means that the location of GNSS2 is at a single sliding surface and GNSS1 is at multiple sliding surfaces, which means there is a deeper sliding surface underneath. The field identification and numerical simulation are executed to verify this observation. The vertical displacement is adopted for verification. Vertical displacements at GNSS1 and GNSS2 are extracted for SBAS SAR comparison, as shown in Figures 7 and 8. GNSS1 vertical displacement is very consistent with ascending SBAS within centimeter accuracy. The descending SBAS is quite different from actual displacement behavior owing to topography. This result shows that the aspect of the mountain is controlling the selection of tracks to analyze landslide behavior. The GNSS2 is installed inside Lushan elementary school with a slope facing south. The comparison results show that the descending track is more consistent than the ascending track. However, the ascending track has more accurate displacement after four years of waiting. Thus, the following result discussion will focus on the ascending track in this study area. Moreover, it is interesting that both GNSS and SBAS measured uplift after the main events in 2017. The mechanism will be discussed in the next section. The slope (steepness) and aspect (azimuth) of GNSS stations are shown in Table 1. For the accuracy of DInSAR results, the GNSS measurements are adopted as an exact reference. Only the data on the same day of the DInSAR result and GNSS solution is used. As shown in Table 1 of GNSS1 data, the ascending orbit result shows a 13 mm error with a 10 mm standard deviation. Moreover, the correlation coefficient is 0.95, which means highly correlated. For the descending orbit at GNSS1, the error is 84 mm. The correlation coefficient is \(-0.69\), which implies that descending data cannot be used in this location. For the site of GNSS2, the error and standard deviation of ascending data are more significant  
Figure 8: Measured GNSS displacement and SAR displacement at GNSS2. Figure 7: Measured GNSS displacement and SAR displacement at GNSS1. than GNSS1, but the correlation coefficient is only 0.51.