## 4 Measurement Quality Analysis
### MQA2: Double Difference Zero-Baseline Analysis  
Double difference zero-baseline analysis can be used to verify the precision of receiver measurements [29]. In this analysis, two GPS receivers were connected to the same antenna using a splitter (GPS Networking NALDCBS1X8 antenna splitter). When two receivers share an antenna, the error sources from the satellite signal (e.g., satellite clock, orbit, tropospheric, and ionospheric errors) are dependent. Because the signal paths are the same, the multipath errors will be the same for the two receivers. With one antenna used, the baseline is defined as zero. As a result, any nonzero result is generated by receiver noise. The quality of the zero-baseline result indicates the observation capability, measurement accuracy, and stability of the receiver.  
To calculate the short baseline double difference values, the satellite with the highest elevation was selected as the main satellite. The double difference results for the remaining in-view satellites were calculated with respect to the main satellite. Figures 15 and 16 show satellites with the most complete trends (from low elevation angle to high elevation angle and then to low elevation angle again) to show the zero-baseline results for GPS and BDS. Figure 15 shows the GPS PRN 22 zero-baseline result. The upper plot shows the zero-baseline result obtained using the code-phase pseudorange measurement from the two receivers. The two measurements from the same satellite go through a  
Figure 14: Relationship between carrier-phase pseudorange standard deviation and satellite elevation angle.  
\begin{table}
\begin{tabular}{l c c c c c c c c c} \hline \hline
**Ele.-\(\star\)** & 10 & 20 & 30 & 40 & 50 & 60 & 70 & 80 & 90 \\ \hline \multicolumn{10}{c}{**GPS L1 Code**} \\ \hline
**Mean** & 9.35 & 7.58 & 6.12 & 4.79 & 4.12 & 3.78 & 3.59 & 3.48 & 3.57 \\
**Std** & 1.53 & 1.33 & 0.97 & 0.59 & 0.44 & 0.27 & 0.18 & 0.24 & 0.38 \\ \hline \multicolumn{10}{c}{**GPS L1 Carrier**} \\ \hline
**Mean** & 1.52 & 1.21 & 0.99 & 0.79 & 0.66 & 0.62 & 0.58 & 0.57 & 0.58 \\
**Std** & 0.36 & 0.28 & 0.23 & 0.16 & 0.12 & 0.11 & 0.08 & 0.09 & 0.10 \\ \hline \multicolumn{10}{c}{**BDS L1 Code**} \\ \hline
**Mean** & 8.03 & 6.57 & 5.27 & 4.16 & 3.36 & 2.87 & 2.72 & 2.43 & 2.26 \\
**Std** & 1.78 & 1.42 & 0.95 & 0.81 & 0.52 & 0.33 & 0.23 & 0.15 & 0.10 \\ \hline \multicolumn{10}{c}{**BDS L1 Carrier**} \\ \hline
**Mean** & 2.13 & 1.75 & 1.38 & 1.16 & 0.90 & 0.80 & 0.76 & 0.68 & 0.63 \\
**Std** & 0.53 & 0.30 & 0.25 & 0.25 & 0.14 & 0.09 & 0.09 & 0.07 & 0.05 \\ \hline \hline \end{tabular}
\end{table}
Table 9: MQA1 statistical results.  
double difference calculation. The result is shown as the blue line. The green line is the elevation angle for this satellite. The lower plot shows the carrier-phase pseudorange measurement results. Both the code and carrier have higher noise when the satellite elevation angle is lower. However, the noise grades are not the same. For BDS, Figure 16 shows the BDS PRN 14 zero-baseline results. For a GPS satellite, the standard deviation of code-phase MQA2 results for all satellites is 29.46 cm and the carrier phase is 0.10 cm. The results are summarized in Table 10. The carrier-phase double difference zero-baseline results for the two systems are almost the same. However, in the code-phase pseudorange, the receiver generates more similar measurements for BDS.