## 3 Results
### The Spatial Distribution of Differences Between BDS and GPS Positioning Results  
We present the position residual time series for station YNSM (south China), station AHBB (east China), station QHGC (north China) and station XJBY (west China) from three sets of positioning results in Figure 4 and Figure S4, respectively. There are always several abnormal jumps in BDS residual time series for stations in every territory of China, which is unseen for GPS results. The maximum horizontal difference can reach 30 mm, and the maximum vertical difference can reach 80 mm. The BDS/GPS station position time series are coincidence with GPS results everywhere. As shown in Figure S1, 13 BDS satellites are used for positioning, four GEO of them are almost fixed in space, and only include four MEO satellites which is a similar trajectory pattern and positioning contribution with that of GPS satellites. However, 32 GPS satellites are involved in positioning, and the spatial geometric distribution of GPS satellites is more reasonable than that of BDS satellites. Therefore, we think BDS observation is a limited supplement for combined BDS/GPS positioning under the current BDS MEO satellite distribution, and this status maybe results in the similar results between GPS and BDS/GPS observations. To elaborately specify the characteristic of the spatial distribution for differential time series between BDS and GPS results, we present the mean value of differential time series from 44 CMONOC stations on horizontal and up components, respectively, in Figure 5. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Satellite** & **North (mm)** & **East (mm)** & **Up (mm)** \\ \hline BDS & 6.1 & 7.1 & 22.1 \\ GPS & 2.8 & 2.9 & 5.8 \\ BDS/GPS & 2.4 & 2.2 & 5.0 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The average RMS of residuals position time series for BDS, GPS, and GPS/BDS combined solutions, respectively. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Title** & **North (mm)** & **East (mm)** & **Up (mm)** \\ \hline mean & \(-0.8\) & \(-1.5\) & \(-5.5\) \\ RMS & 6.8 & 7.3 & 21.7 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: The mean and RMS of differences between BDS and GPS residual position time series We try to investigate the spatial pattern of the direction of the mean horizontal differences. The directions of mean horizontal differences for station XJBY and station XJKE are both eastward, while they are northwestward for station XJBL, which is on their southwestern area, and station QHLH, which is on their southeastern area. The mean horizontal differences are nearly nil for station DLHA, station XZYD and station QHTT, while significant value points to northeast direction for station QHGC, which is not far away from station DLHA. These two examples indicate that it is random without any disciplinary pattern for the spatial distribution of the direction of mean horizontal differences. In southwest China, south China, north China and northeast China areas, the directions are all randomly distributed, thus confirming the aforementioned conclusion. We intend to identify the spatial pattern of the quantity of the mean horizontal differences. It is \(\sim\)3 mm for the quantity of the mean horizontal differences in the district southern of N30\({}^{\circ}\), \(\sim\)3-8 mm in the district between N30\({}^{\circ}\) and N43\({}^{\circ}\), and \(\sim\)10-15 mm in the district northern of N43\({}^{\circ}\). The different quantities of the mean horizontal differences in China indicate their enlargement pattern and the decline in the accuracy of BDS positioning results with the increasing on the latitude of the station. Reference [35] concluded that the accuracy of GEO positioning shows a declining pattern with the increasing latitude of the station, which is origin from the fixation of five GEO satellites in the southern sky-area of China. Analogously, the visibilities of five IGSO satellites have similar declining patterns with the increasing latitude of the station.