# Performance of BDS-3: Measurement Quality Analysis, Precise Orbit and Clock Determination
## 3 Results
### Measurement Quality Analysis
#### 3.1.2 Multipath Combination  
The multipath (MP) combination is a measure of the combined results of multipath, receiver noise, and bias variations between pseudo-range and carrier-phase measurements. The combination is constructed using a single-frequency code measurement and dual-frequency phase measurements of a pass that one receiver is continuously tracking for one GNSS satellite without cycle slips [31]. The constant biases, such as ambiguities, hardware delay in satellite and receiver are eliminated by averaging the MP series of a pass. The residual series are dominated by the multipath and noises of pseudo-range measurements since the carrier phase multipath and noises are much smaller in magnitude. Figure 4 shows MP time series of the B1 signal for BDS-2 (C10, C12) and BDS-3 (C31, C32, C33, C34) satellites during a pass as a function of time and a function of elevation angle, sampled at 30 s, at station BJF1 according to different satellite types, i.e., IGSO (a) and MEO (b) satellites. In general, the continuous tracking duration is about 18-20 h and 7-8 h for IGSO and MEO satellites. Note that C34 satellite tracking data of about one hour at the beginning is lost. As expected, signals observed at low elevation angles are affected more severely by multipath conditions. Moreover, we can see from MP series that BDS-2 satellites (red) contain additional systematic biases (drifts), namely, the satellite-induced code biases, which are obviously elevation-dependent. The code bias variations could reach up to about 1 m at high elevation angles, particularly for BDS-2 MEO satellite. These results of BDS-2 MP combinations are very consistent with those discussed in [PERSON], et al. [5] and [PERSON] and Beer [19], while for BDS-3 satellites (blue), these code biases found in BDS-2 signals have been greatly reduced, comparable to those of other GNSS systems [32]. Figure 5 shows the results for MP combinations of the B3 signal for BDS-2 and BDS-3 satellites. We can see that the multipath errors and noise level of B3 signal are smaller than those of B1 for each satellite. Similarly, there are almost no code biases in BDS-3 measurements for B3 signal. Figure 3: Mean C/N0 values as a function of elevation for BDS-2 and BDS-3 IGSO and MEO satellites signals at site BJF1 equipped with GMR-4011 receiver with LEIAR25.R4 antenna (left) and site KUNU equipped with POLAR5 receiver with JAVRINGANT_DM antenna (right) during DOY 204-221, 2016. The upper and bottom panels show the results of B1 and B3 frequencies, respectively. In order to further analyze and compare pseudo-range noise, multipath errors and satellite-induced code biases, we plot results of the MP combination for B1 and B3 measurements for two individual stations, as shown in Figure 6. All values of MP combination are grouped according to their corresponding elevations, and the mean and standard deviation are computed for each satellite type (IGSO and MEO). The results for the CETC-54 GMR-4011 receiver (The 54 th Research Institute of China Electronics Technology Group Corporation, Shijiazhuang, China) at site BJF1 are depicted in the left panels, and the SEPT POLARX5 receiver (Septentrio, Leuven, Belgium) at site KUNU is shown in the right panels. Again, observations from the same test period between DOY 204 and DOY 221 (2016) are used. We can see that the variation of the mean and standard deviation with elevation reveals a similar characteristic for the two stations. For the mean values, BDS-3 exhibits a typical stochastic error (zero-mean), whereas an obvious systematic bias can be observed in BDS-2 for each receiver and each frequency. The elevation-dependent bias for B1 signal is more pronounced than B3, and IGSO is smaller than MEO. The standard deviation does not include the systematic bias component. For B1 signal, a standard deviation of about 0.8 m at low elevations angles and about 0.2 m close to zenith can be seen for BDS-2 and BDS-3.