# GNSS Real-Time Precise Point Positioning in Arctic Northeast Passage
## 3 Results
### RT-PPP Performance in NEP  
#### 3.2.1 Performance of Real-Time CLK93 Products  
For RT-PPP, the positioning performance is highly related to the quality of real-time SSR corrections. Therefore, we took GFZ GBM products as references to evaluate the CLK93 SSR product. Additionally, we used BKG NTRIP Client (BNC) software to receive the CLK93 stream, and collected data with 5 s intervals from 10 to 20 September 2019. First, we evaluated the real-time orbit accuracy of GPS, GLONASS, Galileo, and BDS corrected by CLK93 SSR products. Figure 7 illustrates the 11-day average RMS values of orbit in X/Y/Z components, as well as in a three-dimensional (3D) direction. For GPS, we can see that almost all satellite accuracies are better than 0.05 m, while they are better than 0.08 m in 3D except the PRN G09, which has a significant error (worse than 0.15 m in the X/Y/Z directions, and 0.25 m in 3D). For GLONASS, the orbit accuracy is generally between 0.03 and 0.08 m in the X/Y/Z directions and approximately 0.1 m in 3D; however, the orbital error of the PRN R01 satellite is relatively large, and its 3D accuracy is approximately 0.21 m. Galileo has the highest satellite accuracy in the X/Y/Z directions (better than 0.05 m), while in 3D, it is better than 0.08 m. For BDS satellites, we excluded GEO satellites (PRN C01 to C05) due to their poor accuracy (lower than 1 m); furthermore, their real-time orbit is better than 0.10 m in the X/Y/Z directions, and between 0.08 and 0.16 m in the 3D direction, except for the PRN C06 satellite. \begin{table}
\begin{tabular}{l l l l l l} \hline  & **GPS** & **Galileo** & **BDS** & **G+E+C** \\ \hline NSAT & 11.556 & 8.729 & 3.766 & 24.050 \\ PDOP & 1.753 & 2.009 & 6.940 & 1.113 \\ \hline \end{tabular}
\end{table} 
Table 1: Mean values of NSAT and PDOP for GPS, Galileo, BDS, and multi-GNSS (G+E+C) from 10 September to 20 September 2019. \begin{table}
\begin{tabular}{l l l l l l l} \hline
**System** & **GPS** & \multicolumn{3}{c}{**Galileo**} & \multicolumn{2}{c}{**BDS**} \\
**Frequency** & **L1** & **L2** & **E1** & **E5b** & **B1** & **B2** \\ \hline SNR (dB) & 45.769 & 45.158 & 43.946 & 49.897 & 43.966 & 48.744 \\ Multipath (m) & 0.234 & 0.478 & 0.458 & 0.348 & 0.394 & 0.295 \\ \hline \end{tabular}
\end{table} 
Table 2: Mean SNR and multipath errors of each GNSS signal from 10 September to 20 September 2019. Table 3 shows the mean RMS values of the four GNSSs' corrected orbits. Galileo has the highest orbit accuracy, and its 3D RMS is 0.052 m. The GPS orbital accuracy, which is 0.063 m, is a bit lower than Galileo's. The RMS of the GLONASS is 0.092 m, which ranks third. As for the BDS, the mean RMS of the 3D orbit is 0.142 m, which is three times worse than Galileo. Similarly, we analyzed the real-time clock errors corrected by CLK93 products. When comparing different IGS clock products, it is necessary to select a reference satellite in each GNSS to eliminate systematic biases [23]. For this reason, we selected GPS G32, GLONASS R24, Galileo E36, and BDS C14 satellites in each system. We computed both the standard deviation (STD) and RMS of each satellite clock to reflect the accuracy and consistency of clock biases. Figure 8 shows the STD and RMS of each satellite clock, and the mean values are displayed in Table 4. Regarding the STD, the Galileo clock has the highest accuracy and considerable stability; furthermore, all satellite clock STDs are better than 0.15 ns, and the average value is 0.084 ns (approximately 2.5 cm). The accuracy of the GPS clock is slightly lower than that of Galileo, the STD values are better than 0.3 ns except for G09, and the mean value is 0.13 ns (approximately 4.0 cm). Compared with the GPS and Galileo clock, the accuracy of the real-time clock error of the BDS satellite is considerably lower, and its average STD is 0.346 ns (approximately 10.0 cm). The precision of the GLONASS clock is the lowest, with all satellite clock STDs worse than 0.3 ns, and the mean value is 0.623 ns (approximately 18.0 cm).