## 5 Discussion
Numerous investigations have demonstrated that a multi-GNSS approach (e.g., GPS + BeiDou) is able to improve the performance of real-time-kinematic (RTK) positioning, particularly in urban and mountainous areas, where the lack of visible GPS satellite numbers and multipath reflections limit the performance of RTK [55; 56; 57; 58; 59]. However, it has been reported that multi-GNSS observations do not noticeably improve the accuracy of static positioning compared to GPS-only observations; somehow, using multi-GNSS observations can even degrade the accuracy of static positioning compared to using GPS-only observations [60; 61; 62]. Accordingly, we processed GPS and BeiDou observations separately in this study. Figure 11 depicts the comparisons of three-component displacement time series of GPS1 derived from GPS-only and BeiDou-only observations, respectively. The displacements are the 24-h DD solutions with respect to the base station. Similar to the DD processing for GPS that only used L1 (1575.420 MHz) observations, the DD processing for BeiDou only used B1(1561.098 MHz) observations. The time series depicted in Figure 11 indicates that the BeiDou-derived displacements retain slightly larger scatters than the GPS solutions. The GPS-derived displacements retain 1-mm horizontal RMS-accuracy and 2-mm vertical RMS-accuracy; the BeiDou-derived displacements retain 2-mm horizontal RMS-accuracy and 5-mm vertical RMS-accuracy. However, the trends from the two datasets agree well in delineating the displacements of the antenna over time. The back and forth movement of the ground surface along the excavation direction and the settlement during the excavation were observed from both GPS and BeiDou observations. Most outliers occurring in the GPS-derived displacement time series did not occur on the same days in the BeiDou-derived displacement time series. Thus, the BeiDou-only measurements are beneficial to distinguish real positional-changes from outliers which appeared in GPS-only measurements. By parallel processing GPS and BeiDou observations in near real-time, an abrupt positional shift can be detected with a higher confidence, thus minimizing false alerts. There are various reasons to explain why the positional accuracy of BeiDou is below the accuracy of GPS. Figure 12 depicts two azimuth-elevation plots of phase residuals per satellite for station GPS1 on 8 May 2018. The scheme is often called \"skyplot\". The outer circle marks azimuths for the satellites and also indicates the horizon as seen from the station. The 90-degree elevation, directly above the station, is at the center of the circle. Thin red lines indicate the tracks of the GPS satellites during the 4-hour interval. The green traces along these lines are the time series of phase residuals (the mismatch between recorded and modeled satellite signal phases) for each satellite. Skyplots are an efficient tool to quickly assess the effects of multipath and water vapor. High residuals in the same place at different times most likely suggest multipath effects; high residuals appearing in a given place only at one time mostly suggest water vapor and other effects. The skyplots exhibited in Figure 12 indicate that the BeiDou signals were more seriously affected by multipath effects than GPS signals. The unique constellation design of BeiDou (GEO + IGSO + MEO) makes it more difficult to mitigate multipath effects compared to GPS (MEO only). The multipath issue of BeiDou signals has been intensively investigated recently [63; 64; 65; 66; 67; 68]. It is expected that the algorithms for modeling multipath effects on BeiDou observations will be improved in the near future. Figure 11: Comparisons of three-component displacement time series (GPS1) derived from GPS-only (L1) and BeiDou-only (B1) observations. The trend line is obtained by a moving average (unweighted) filter. The width of the sliding window is 11 points compromising the current point and five points before and after the current point.