## 3 Results
### Quality Analysis of BDS Observations  
The station YNSM is selected as the example to specify the status of observing and quality of observation for BDS data in CMONOC. The number of tracking satellites on one single epoch is 9-13 for BDS and 8-12 for GPS, respectively (Figure S1). In order to evaluate the quality of BDS data, the MPD and SNR of three sets of BDS satellites and GPS satellites are shown in Figure 3. The multipath delay includes two parts: The delay of satellite ends and the delay of ground station ends. The MPD in satellite ends is caused by the spacecraft internal multipath, and the impedance mismatch between antenna elements and power divider network causes reflection signals. Piecewise linear [30] or third-order polynomial [31] models can effectively mitigate or even eliminate the effects of MPD in satellite ends [32; 33; 34]. The magnitude of the MPD in station ends is related to the distance between the antenna and the object, the angle of the reflected signal, and the characteristics of the reflector. We used the MPD in station ends associated with observation condition to analyze the data quality in CMONOC. The SNR and MPD are highly correlated with the elevation angles of satellites, and the magnitude of MPD decreases with the increase of satellite elevation angle. The MPD for different frequencies are similar with each other, while the SNR are significantly distinct, especially for the GPS satellites, with larger SNR on L2 data comparing to L1 data.  
Figure 3: The multipath delay (MPD), signal to noise ratio (SNR) of BDS and GPS satellite systems at station YNSM, and the variation of their elevation angles with respect to time. (**a**) C01 (GEO), (**b**) C07 (IGSO), (**c**) C13 (MEO) and (**d**) G11 (GPS) are selected as typical examples on behalf of different satellite types. Different colors represent different frequencies. Red circle, black circle, and blue circle represent signal B1, B2, and B3, respectively, in (**a–c**). Red circles and Black circles represent the signal L1 and L2, respectively, in (**d**).  
The variation of MPD and SNR for B1 data and L1 data from four different satellites with respect to satellite elevation angle are shown in Figures S2 and S3. The MPD of B1 and L1 data decline with the increase of satellite elevation angle, and their quantities are at the same level. The specific RMS for C01 (GEO), C07 (IGSO), C12 (MEO) and G11 (GPS) are 0.26 m, 0.38 m, 0.46 m, and 0.47 m, respectively, and the last two values confirm the similarity on the status of observing and the quality of observations between GPS and MEO. The elevation angles of GEO satellites are always near 35\({}^{\circ}\), while they vary widely for IGSO and MEO satellites. The large RMS of MPD for MEO and IGSO, comparing with GEO, are attributed to their low elevation angles. Due to longer periods with the observing condition of low elevation angles for MEO, the corresponding RMS is larger than IGSO. Therefore, we can largely conclude that the RMS of MPD is directly correlated to the elevation angles of satellites in daily sessions. The SNR for IGSO, MEO, and GPS are positively correlated to the satellite elevation angle in similar correlation patterns. GEO satellites have smaller SNR comparing with IGSO, MEO and GPS on the same elevation angle, and possess almost stationary quantities due to their fixed elevation angles.