## 3 Methods
### GPS Data Processing
The absolute positioning method uses a single ground GPS station to achieve high-precision positioning, and in turn, high-accuracy displacements and site velocities. Precise point positioning (PPP) is a typical absolute positioning method that has been widely applied in the geodesy community for plate tectonic studies and natural hazards (landslides, subsidence) monitoring. PPP techniques use un-differenced dual-frequency pseudorange and carrier-phase observations, precise satellite orbits and clock information, high-fidelity error modeling, and other information to determinethe position of a stand-alone GPS antenna [34]. Compared to the DD method, the PPP method has not been widely applied in civil engineering and land surveying communities. There are various reasons for this. One primary reason is that the majority of structural-monitoring projects focus on short-term monitoring during construction rather than long-term post-construction monitoring. With the realization of the importance of long-term SHM, PPP positioning will be increasingly applied in the field of civil and surveying engineering. The GAMIT/GLOBK software package (Version 10.7) is employed for DD processing [35], and the GIPSY-OASIS (V6.4) software package is employed for PPP processing [36]. The GIPSY-OASIS software package employs a single-receiver phase ambiguity-fixed PPP method for its static positioning. According to [PERSON] et al. [37], the GIPSY PPP daily solutions retain 2 to 4 mm horizontal RMS-accuracy (repeatability) and 7 to 9 mm vertical RMS-accuracy in North China. Since the baselines are extremely short (<2 km) for the riverbed monitoring project, we only use the L1 observations for DD processing. We found that for baselines within a couple of kilometers, using L1-only observations can result in a higher precision (repeatability) of relative positions than using the L1 and L2 combined observations [22]. Both DD and PPP methods can be employed in post-processing or near real-time processing. For near real-time DD processing, the information recorded by the base and rover GPS receivers is sufficient to solve the relative position of the rover. For PPP processing, many publicly available data sets are required, such as precise estimates of the GPS satellite orbit and clock offsets, earth orientation, transmitter antenna calibration, and wide lane and phase bias (WLPB) estimates obtained from a global network of ground GPS stations. For PPP processing employed in the GIPSY software package, those products are available to the public through the Jet Propulsion Laboratory (JPL) as three product lines: Ultra-rapid, rapid, and final according to the latency in availability ([[https://gipsy-oasis.jpl.nasa.gov](https://gipsy-oasis.jpl.nasa.gov)]([https://gipsy-oasis.jpl.nasa.gov](https://gipsy-oasis.jpl.nasa.gov))). All products span 30 h. The final products are available after ten days. The rapid products are available the next day at 16:00 Universal Time Coordinated (UTC), which is a delay of approximately 16 h after the end of the previous observation day. The ultra-rapid products are available in near real-time and are updated every hour. The accuracy of the products improves with longer latencies. All three products can be automatically fetched for automated processing. The accuracy (1 sigma) of the ultra-rapid, rapid, and final orbits are 5, 3.5, and 2.5 cm, respectively. According to our previous investigations on GPS datasets collected in Beijing [11], the RMS-accuracy of daily PPP solutions corresponding to the rapid orbit products is almost identical to the RMS-accuracy obtained from the final orbit products. The RMS-accuracy of the daily PPP solutions corresponding to the ultra-rapid products agrees with the accuracy corresponding to the final products within a one-millimeter level. The JPL rapid products are used in this study to simulate near real-time monitoring. Figure 4 depicts the comparisons of three-component displacement time series of GPS1 derived from daily DD and PPP solutions.