## 3 Signal Performance  
### Pseudorange Code Bias and Signal Noise  
The pseudorange code bias in the BD2 satellites was first detected and verified by multipath (MP) combination, which was presented by [PERSON] and [PERSON] and applied in the TEQC toolkit to edit GPS and GLONASS observation data [22]. This combination utilizes dual-frequency observation data. MP combination is GEO-free, which is not influenced by errors of satellites and site location, troposphere delay and first-order ionosphere delay. The original combination series only contained phase ambiguity, multipath and noise influence. The phase ambiguity can be eliminated by the arithmetic average. The multipath delay in phase, which is much smaller than pseudorange, can be ignored. Thus, MP combination can be used to evaluate the pseudorange code multipath and noise level. In general, the combination should be expressed as  
\[MP_{i}=P_{i}-\frac{f_{i}^{2}+f_{j}^{2}}{f_{i}^{2}-f_{j}^{2}}\lambda_{i}\varphi_ {i}+\frac{2f_{j}^{2}}{f_{i}^{2}+f_{j}^{2}}\lambda_{i}\varphi_{j}-B_{i} \tag{1}\]  
where \(f\) represents the signal frequency, \(P\) and \(\varphi\) represent the pseudorange code and carrier phase observation, \(\lambda\) is the wavelength of the phase and \(B\) stands for the phase ambiguity bias, and the subscripts \(i\) and \(j\) represent the frequency of the combined signals. In the study, we collected all the available data in the iGMAS network. The observation data of the BD3 open service signals at DOY 070 in 2019 was adopted for test and analysis. MP combination series versus elevation are presented in Figure 2.  
In Figure 2, the B1I and B3I observation signals transmitted by BD2 MEO Satellites are presented together with the BD3 MEO FOC B1I, B1C, B2a and B3I signals. Seen from Figure 2, there is an obvious bias in the B1I and B3I signals transmitted by BD2 satellites, whilst the variation of the mean multipath does not distribute around zero. Moreover, an obvious tendency with elevation can be found from the positive value (low elevation) to negative values (high elevation). It is noteworthy that this pseudorange code bias has almost been eliminated in the BD3 MEO FOC satellites in the B1I, B1C, B2a  
Figure 2: The BD2 and BD3 medium Earth orbit (MEO) satellites’ multipath combination versus elevation. and B3I signals. As a result, the MP combination variation is smooth when the elevation is high but fluctuates largely in low elevation, and the combination series is an almost zero mean distribution. According to our analysis, the Root Mean Square (RMS) of the BD3 MP combination is about 0.1 m when the elevation is in the vicinity of zenith. As shown in Figure 2, the data in \"BD3-B1C\" shows a slower convergence and it is more discrete than the other three BD3 frequencies. After comparing the four frequencies of open service signals, the noise level of the B1C signal is much larger than the other three frequencies' observation data, which may indicate that the B1C signal is more sensitive to the multipath and noise. Furthermore, we demonstrate the MP combination versus elevation and azimuth in the station local coordinate frame, where the orientation of the azimuth is toward the north and clockwise rotation. The MP combination series of BD2 and BD3 together with GPS and Galileo satellites are shown in Figure 3, where only B1I in BD2, B1C in BD3, L1 in GPS and E1 in Galileo are presented, and only the multipath combination data range between \(\pm\)2 meters is displayed. Seen from Figure 3, the MP combination series shows the symmetric distribution with the azimuth in the four satellites, and only the BD2 satellites present an elevation-dependent bias. In addition, we estimated the Signal to Noise (S/N) data recorded by the iGMAS HMNS station at DOY 070, 2019. The site is located in Hermanus, South Africa, and the type of the receiver equipped on HMNS is \"gnss_ggr\", which can track all the open service signals transmitted by the BD3 satellites.