## 2 Quality Analysis of BeiDou Measurements
### Analysis of Multipath and Noise  
The multipath combination (MPC) is often used to assess the multipath and code noise level of a receiver [4; 5; 6]. The MPCs eliminate the first-order ionosphere delay and the geometry distance between satellites and stations. The systematic error and the noise of carrier-phase usually could be neglected compared with that of the code observations. Then the MPCs contain a constant ambiguity term of combined carrier phases, systematic hardware delay and thermal noise plus noise of code measurements. The MPCs for the receiver and satellites on two frequencies can be expressed as [4; 5; 6]:  
\[\textit{MPCl}_{r}^{s}=P_{\
u}^{s}-(a+1)\cdot B_{\
u}^{s}+a\cdot B_{2\
u}^{s} \tag{1}\]  
\[\textit{MPCl}_{r}^{2s}=P_{2\
u}^{s}-b\cdot B_{\
u}^{s}+(b-1)\cdot B_{2\
u}^{s} \tag{2}\]  
where \(a=2f_{2}^{s}/(f_{1}^{s}-f_{2}^{s})\), \(b=2f_{1}^{s}/(f_{1}^{s}-f_{2}^{s})\), \(P\) and \(B\) are the BeiDou code and phase observations, respectively; \(f_{1}\) and \(f_{2}\) are the frequencies of \(B_{1}\) and \(B_{2}\).  
We processed the data of the CHDU station on September 24, 2012 to analyze the multipath and noise error. The station is equipped with the Unicore UB240-CORS receiver and UA240 antenna. The mask elevation is 7\({}^{\circ}\) and the interval is 30 s. Figure 4 displays the MPCs of the B1 and B2 codes and the variation of the elevations of satellite C01 at CHDU on September 24, 2012. It can be seen that the variations of the elevations of C01 are within 35\({}^{\circ}\)-39\({}^{\circ}\). The MPCs of B1 and B2 codes of satellite C01 have small changes with the variations of the elevations. The MPCs of B1 codes are steadier than that  
Figure 3: Sky plots (azimuth _vs._ elevation) of five IGSO and four GEO satellites at CHDU on September 24, 2012.  
of B2 codes and the standard deviation of the MPCs of B1 codes is smaller than that of B2 codes. Furthermore, both the MPCs of the B1 codes and B2 codes are within \(\pm\)1 m.  
Figure 5 shows the standard deviations of the MPCs of the B1 and B2 codes of the BeiDou satellites. The standard deviations of MPCs of B1 and B2 codes of the BeiDou satellites are smaller than 0.5 m, except for C04, C11 and C12 and the average values are 0.458 and 0.504 m, respectively. The standard deviations of MPCs of B1 codes are smaller than that of B2 codes, except for C04. We also can see that the MPCs are different for different types of satellites. The GEO satellites, except for C04, have the best performance while the MEO satellites have the worst performance.  
Figure 6 shows the standard deviations of the MPCs of C/A and P2 codes of GPS satellites. The standard deviations of C/A and P2 codes of all the GPS satellites are less than 0.50 m except for the G18 satellite. The average values of the standard deviations of the C/A codes of all the GPS satellites are 0.315 m, which is smaller than that of the B1 codes of BeiDou satellites. The average standard  
Figure 4: MPCs of the B1 and B2 codes, as well as the variation of the elevations of satellite C01 (GEO) at CHDU station on September 24, 2012.  
Figure 5: Standard deviations of MPCs of B1 and B2 codes of BeiDou satellites at CHDU station on September 24, 2012.  
deviations of P2 codes are 0.401 m, smaller than that of the BeiDou B2 codes. The results above show that the accuracy of GPS code observations at the CHDU station is better than that of the BeiDou code ones. The values do not mean that BeiDou code observations are worse than GPS ones. The reason may be that the MPCs derived based on Equations (1) and (2) were composite values that not only include the pseudo-range multipath error but also other errors, such as the pseudo-range and phase noise, and the hardware delay in the receivers and satellites. In addition the BeiDou system constellation is not complete, but the GPS statistics contain all satellites of the constellation.