## 5 Results  
### Measurement Quality  
The post-fit residual is a key indicator of accuracy or precision of observations and their modeling. RMS of the post-fit residuals of ionosphere-free range (PC) and phase (LC) observations are listed in Table 4 and illustrated in Figure 6. For each station, RMS of the three satellite types are shown for comparison.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline  & \multicolumn{3}{c}{**LC (mm)**} & \multicolumn{3}{c}{**PC (m)**} \\ \cline{2-7}  & GEOs & IGSOs & MEOs & GEOs & IGSOs & MEOs \\ \hline
**CENT** & 6.8 & 9.0 & 10.3 & 1.4 & 0.9 & 0.9 \\
**CHDU** & 6.4 & 7.5 & 9.9 & 1.7 & 1.1 & 1.5 \\
**DHAB** & 7.4 & 8.5 & 7.8 & 1.8 & 2.2 & 1.8 \\
**HKTU** & 7.4 & 7.5 & 11.9 & 1.8 & 1.3 & 1.5 \\
**HRBN** & 5.9 & 6.9 & 8.8 & 1.9 & 1.3 & 1.2 \\
**JOHA** & 7.6 & 9.3 & 7.2 & 4.1 & 3.1 & 2.4 \\
**LASA** & 6.4 & 7.5 & 8.6 & 1.9 & 1.7 & 2.0 \\
**LEID** & 8.1 & 6.9 & 6.5 & 4.0 & 1.9 & 1.5 \\
**NTSC** & 7.0 & 7.4 & 10.4 & 2.0 & 1.4 & 1.6 \\
**PETH** & 8.3 & 9.4 & 10.5 & 2.0 & 1.9 & 2.0 \\
**SHAO** & 7.1 & 7.1 & 11.2 & 1.2 & 1.6 & 2.1 \\
**SIGP** & 7.6 & 8.1 & 11.4 & 2.9 & 1.5 & 1.6 \\
**XIAN** & 6.9 & 7.1 & 10.6 & 1.8 & 0.7 & 0.8 \\
**MEAN** & 7.1 & 7.9 & 9.6 & 2.2 & 1.6 & 1.6 \\ \hline \end{tabular}
\end{table}
Table 4: Averaged RMS of the ionosphere-free phase (LC) and range (PC) observation residuals for each station-satellite pair.  
Figure 6: (**a**) The averaged RMS of LC observation residuals for each station-satellite pair.  
(**b**) The averaged RMS of PC observation residuals for each station-satellite pair.  
From Table 4 or Figure 6, phase residuals of GEO, IGSO and MEO satellites are very similar and increase from 7 mm for GEOs to 10 mm for MEOs and the differences among stations are also very slight. However, the range observations of the stations located outside of China have a larger noise. JOHA has the largest RMS of about 4.1 m, 3.1 m and 2.4 m for GEO, IGSO and MEO satellites, respectively, and LEID has a very similar performance. The other three overseas stations DHAB, PETH and SIGP are rather close to China and show only a slightly larger RMS. Comparing with the results of Galileo [7], the phase accuracy of BeiDou is of a comparable quality, whereas range is slightly noisier.  
The distance-dependent range noise might be caused by the lower elevations of the GEO and IGSO satellites for the stations far away from China. As an example, Figure 7 provides a sky plot of the tracked GEO and IGSO satellites over one day at four particular stations for comparison: SHAO in China and three overseas stations: LEID, JOHA and DHAB. For the farthest station LEID the satellites come rarely above an elevation higher than 30 degrees and satellites C01, C03 and C04 are almost invisible. For station JOHA, the situation is slightly improved. At DHAB all satellites are tracked and even with a much higher elevation, but all satellites are on the east edge of the sky. This special satellite distribution and the low elevation might be the reason of the larger range RMS due to larger multi-path effects and inaccurate modeling of atmospheric delays.  
Figure 7: Sky plot of tracked satellites at four particular stations, LEID (51.9 \({}^{\circ}\), 44.0 \({}^{\circ}\)), JOHA (-25.8 \({}^{\circ}\), 27.9 \({}^{\circ}\)), DHAB (24.2 \({}^{\circ}\), 54.5 \({}^{\circ}\)), and SHAO (121.5 \({}^{\circ}\), 30.9 \({}^{\circ}\)) on day 171 in 2012. It is denoted by azimuth and zenith for four GEOs: C01, C03, C04, C05 and five IGSOs: C06, C07, C08, C09, C10. For some of the overseas stations, satellites can only be tracked on a rather low elevation and some of them are even not visible compared to station SHAO.