# GNSS Real-Time Precise Point Positioning in Arctic Northeast Passage
## 3 Results  
### GNSS Signal Quality in NEP  
Satellite signal quality is closely related to positioning accuracy, therefore, we analyzed the maximum satellite elevations, NSAT, PDOP, SNR, and multipath errors of GPS, Galileo, and BDS.  
GNSS uses a specific orbital inclination to cover densely populated mid- and low-latitude areas, which means that the satellite elevation angle will be lower when sailing in polar regions [5]. Consequently, this may produce poor satellite geometry. Furthermore, the complex environment in the polar region may affect the GNSS signal and positioning accuracy. Therefore, herein, we analyze the spatial structures and signal quality of GPS, Galileo, and BDS-2 (because CLK93 products only provided BDS-2 correction information).  
Figure 4 shows the variation between the maximum satellite elevation in a day and the ship's average latitude in the NEP. The maximum GPS and Galileo satellite elevations vary considerably with latitude. The maximum GPS and Galileo satellite elevations are approximately 75\({}^{\circ}\) when the latitude of the ship is approximately 68\({}^{\circ}\) N, while they are 60\({}^{\circ}\) when the ship reaches 78\({}^{\circ}\) N. Conversely, the maximum BDS satellite elevation is not sensitive to latitude changes in the NEP, which remains at approximately 65\({}^{\circ}\)-70\({}^{\circ}\) because of the contributions of inclined geo-synchronous orbit (IGSO) BDS satellites. Although they have the same orbital inclinations as medium-Earth-orbit (MEO) satellites, their orbital altitudes are about twice as much.  
Figure 5 shows the variation between the GNSSs' NSAT and the ship position in the NEP. We found that the NSAT did not significantly decrease when the ship entered high latitudes (approximately 78\({}^{\circ}\) N). There are more than 10 and 7 satellites that can be observed for GPS and Galileo, respectively, most of the time, while the BDS has less than 4. After the combination, the NSAT remained above 16.  
Figure 6 further illustrates the availability of GNSS in the NEP, and shows the one-day average NSAT, PDOP, SNR, and multipath errors for GPS, Galileo, and BDS. Table 1 summarizes the 11-day mean values of NSAT and PDOP for GPS, Galileo, BDS, and multi-GNSS from 10 September to 20 September 2019, and Table 2 shows the average SNR and multipath errors of each GNSS signal in the NEP.  
Figure 3: Journey of M/V TIANHUI in NEP from 10 September to 20 September 2019.  
Figure 4: Relationship between satellite maximum elevation angle and latitude of the ship’s position.  
Figure 5: Variation between NSAT for GPS, Galileo, BDS, G+E+C, and the ship position in the NEP.  
Figure 6: Daily average NSAT, PDOP, SNR, and multipath errors for GPS, Galileo, and BDS, respectively.  
Figure 6 shows the NSAT is inversely proportional to the PDOP. The NSAT of BDS is the least proportional, with an average of 3.766, because BDS-2 is a regional satellite navigation system that mainly covers the Asia-Pacific region. While GPS and Galileo have more satellites than BDS (11.556 and 8.729, respectively), their PDOP values are relatively stable. Moreover, the sum number of satellites in all systems can reach approximately 24.050, and the multi-GNSS PDOP value is 1.113. For the SNR, we found that the change of daily-average SNR values is not significant, and there is a slight gap between the different frequencies. Galileo E5b has the highest SNR (49.897 dB); however, Galileo E1's SNR is the lowest (43.946 dB). The GPS L1 and L2 SNR values are approximately equal (45.769 dB and 45.158 dB, respectively). The SNR value of the BDS B2 signal is higher than that of the B1 signal, and the mean values are 48.744 dB and 43.966 dB, respectively. For multipath errors, one can notice that it considerably changes with environment variation. In general, the GPS L1 signal has the strongest ability to resist multipath interference, which is about two times better than that of the GPS L2, Galileo E1, and BDS B1 signals, and approximately 1.5 times better than that of the Galileo E5b signal.