## 3 Methods
### Monitoring the Stability of the Base Station  
In general, DD processing with short baseline is able to achieve higher displacement-accuracy than PPP processing. However, DD processing requires one extra station (reference) to be continuously operated in the field during the entire monitoring period. The stability and data quality of the reference station will therefore directly affect the accuracy of the displacement measurements at rover stations. It can be a challenge to keep a reference station strictly stable for several months to years in the field, particularly in urban environments. Instant movements at a few millimeters or gradual movements at a couple of millimeters per week at a reference site could be unnoticed. Thus, the unnoticed movement at the reference site could propagate to the positions of rovers and lead to a wrong interpretation of the stability of the monitoring objects. For real-time monitoring, it could even lead to false alerts. So, the stability of the base station needs to be continuously monitored. The PPP method solves the position of an antenna with one stand-alone GPS station without directly using observations from other ground stations. The positions are referred to a global reference frame that defines the orbit of satellites. Currently, the International GNSS Service (IGS) Reference Frame 2014 (IGS14) is utilized to define the satellite orbits by IGS [38]. In general, a global reference frame is realized by minimizing the overall secular horizontal velocities of hundreds of selected stations distributed worldwide. As a result, linear velocities at GPS sites referred to a global reference frame are often dominated by long-term tectonic drifts. Minor structural deformation could be obscured by the underlying plate motions with respect to the global reference frame [32]. A stable regional or local reference frame is needed to exclude the effects of the plate motions. In this study,  
Figure 4: Three-component displacement time series at GPS1 derived from daily Precise Point Positioning (PPP) and Double Difference (DD) solutions. The PPP solutions are aligned with a stable regional reference frame (NChina16); the DD solutions are referred to the base station (BASE). Trend lines are obtained by applying a moving-average (unweighted) with a sliding window on the GPS-derived displacements. The width of the sliding window is 11 points comprised of the current point and five points before and after the current point. we transform the IGS14 coordinates to the Stable North China Reference Frame 2016 (NChina16), which was established from over six years of observations from 15 continuous operating reference stations (CORS) fixed on the North China Craton [37]. The frame stability of NChina16 is within 1 mm/year. Figure 5 depicts the daily PPP solutions of the base station with respect to NChina16. The antenna monument of the base station was reinforced on 17 April 2018 (day of the year: 107) by welding three legs (Figure 3a). The field reinforcement was handled in a cautious way to avoid disturbing the base antenna. However, an abrupt horizontal displacement indeed happened. The daily displacement time series clearly indicate that the reinforcement caused a 0.8-cm movement toward the south (Figure 5a) and a 1.3-cm movement toward the west (Figure 5b). The disturbance in the vertical direction is not so obvious partially because of large scatter of the vertical displacement time series (Figure 5c). The unexpected positional shift will be unnoticed without continuous monitoring using the absolute positioning method. An unnoticed positional change at the base station will lead to an abrupt step in the positional time series of rover antennas. Figure 5: Plots depicting the stability of the base GPS (BASE) using Precise Point Positioning (PPP) processing, a stable regional reference frame, and a seasonal model (vertical) of ground deformation. The displacement time series of BJUT are illustrated for comparisons.