# GNSS Real-Time Precise Point Positioning in Arctic Northeast Passage
## 3 Results
### RT-PPP Performance in NEP
#### 3.2.3 Performance of DF RT-PPP in the NEP  
Because the U-blox F9P can receive GPS, Galileo, and BDS dual-frequency signals, we used a DF RT-PPP model to further verify the performance of RT-PPP in the NEP. However, some GPS satellites, such as G02, G11, G16, G19, and G20, only contain single-frequency data because the carrier phase on L2 is derived from the mixed channel in the U-blox F9P receiver ([[https://igs.org/formats-and-standards/](https://igs.org/formats-and-standards/)]([https://igs.org/formats-and-standards/](https://igs.org/formats-and-standards/)), accessed on 1 February 2022). We used the IF-LC code and carrier-phase observations in the DF RT-PPP model. Furthermore, we used GPS L1/L2, Galileo E1/E5a, and BDS-2 B1/B2 dual-frequency signals. Moreover, our other strategies are similar to those in Table 5.  
Figure 11a illustrates east/north/up coordinates with DF RT-PPP solutions for GPS and multi-GNSS from 10:00 to 22:00 on 12 September 2019, and Figure 11b shows the NSAT and PDOP values. Figure 11a shows that both GPS and multi-GNSS can achieve decimeter- and centimeter-level accuracy after convergence. However, it takes a long time for GPS to converge, and it often diverges after convergence. Figure 11b shows that large PDOP values are highly consistent with the divergence for GPS RT-PPP positioning results. Furthermore, the number of GPS satellites decreased considerably compared with the SF RT-PPP because dual-frequency cannot be observed by some GPS satellites. Conversely, the multi-GNSS RT-PPP solution tends to be more stable because of the increased number of satellites and a more stable spatial structure than the single GPS.  
Figure 12 represents the daily RMS values in each of the five components for the GPS and GPS/Galileo/BDS combinations during the 11-day testing period when the M/V TIANHUI passed through the NEP. The GPS RT-PPP solution is shown in blue, while the combination solution is shown in red. The GPS can only achieve decimeter-level positioning accuracy in the horizontal and up directions, and sub-meter-level accuracy in the 3D direction because of the small number of GPS satellites. However, sub-decimeter or centimeter accuracy can be realized after the combination of GPS, Galileo, and BDS in each direction.  
\begin{table}
\begin{tabular}{l l l l l l} \hline  & **E (m)** & **N (m)** & **2D (m)** & **U (m)** & **3D (m)** \\ \hline GPS & 1.956 & 4.048 & 4.869 & 4.609 & 7.268 \\ G+E+C & 1.553 & 2.931 & 3.596 & 3.110 & 5.138 \\ \hline \end{tabular}
\end{table}
Table 6: Mean RMS values of SF RT-PPP performance in the east, north, 2D, up, and 3D directions for GPS and multi-GNSS from 10 to 20 September 2019.  
Figure 11: (**a**) DF RT-PPP position errors in the east, north, and up directions for GPS and multi-GNSS; and (**b**) NSAT and PDOP values for GPS and multi-GNSS from 10:00 to 22:00 on 12 September 2019.  
Table 7 summarizes the average RMS values in each direction. The accuracy of the north direction is still slightly lower than that of the east in the DF RT-PPP model. The combined results are improved by 73.3% in the horizontal direction compared with single GPS. In terms of the up direction, the positioning accuracy changes from 0.257 m to 0.057 m, and an improvement of approximately 77.8% is achieved when performing the combination DF RT-PPP model. Concerning the 3D component, the accuracy is of approximately 0.434 m for GPS DF RT-PPP, while 0.107 m is achieved by the combination, indicating an improvement of approximately 75.3%.