# Comparison Analysis on the Accuracy of Galileo PPP Using Different Frequency Combinations in Europe
## 2 Methods
### Evaluation of the Accuracy of Dual- and Triple-Frequency Galileo Static PPP  
To evaluate the performance of different dual- and triple-frequency observations on PPP accuracy improvements, five different experiments were designed and are listed in Table 3. The multi-frequency Galileo observations were processed in the raw model and  
Figure 4: The RMS on north (**top**), east (**middle**), and up (**bottom**) component for GPS (**left**), and its comparison to the BDS dual-frequency PPP.  
Figure 3: Repeatability of static PPP RMS for station OSK6 from DOY120 to DOY129, 2020.  
then compared to the ionosphere-free PPP. As there are no Galileo E6 signal observations from the \"TRIMBLE NETR9\" receiver, it is excluded from the comparison.  
Figure 5 shows the average RMS of different combinations over ten successive days. Overall, Galileo can achieve better accuracy than that of GPS and BDS PPP, with the horizontal accuracy better than 3 mm and the vertical accuracy better than 6 mm. However, there is exception for the processing mode \"GAL-R17\" using the E1/E5b combinations, whose positioning accuracy are observably worse than the other frequencies. It might be caused by the satellite clocks biases that exist between the E1/E5a and E1/E5b observations, which are about 0.15 ns [22]. The accuracy of PPP in the up direction can be improved when three- and four-frequency is used, reaching 4.8 mm for the triple- and four-frequency observations.  
To characterize the site-specific reasons affecting the positioning accuracy, Figure 6 shows the average RMS of all stations in different processing options. It is observed that the accuracy in the up direction is more sensitive to the observation quality, especially for the \"BDS-IF\" and \"GAL-R17\" modes. Overall, the processing option \"GAL-R18\" and \"GAL-R157\" showed the relatively better three-dimensional accuracy.  
The accuracy of hourly static PPP from different combinations were then analyzed, with a total of 4560 epochs used for the RMS statistics. Figure 7 shows the average RMS of PPP from different processing combinations. First, it was observed that the accuracy of PPP can be improved when multi-GNSS observations are used for the ionosphere-free combinations. The GPS/Galileo combinations performed better than that of the GPS/BDS combination, and the highest accuracy of 6.6 mm /8.9 mm/13.7 mm could be achieved for the \"IF-GERC\" multi-GNSS PPP. The raw PPP could achieve a comparable accuracy to the IF combinations, though the ionospheric-delay parameters were additionally estimated. The best positioning accuracy was achieved for the \"IF-GERC\" combination, reaching 5.6 mm/9.2 mm/12.6 mm on the north/east/up direction, respectively. Figure 8 shows the accuracy for different stations, and it is observed that the good accuracy of \"RAW-GERC\" is obvious for most stations.  
\begin{table}
\begin{tabular}{l l l} \hline \hline
**Group** & **Strategies** & \\ \hline GPS-IF & L1/L2 & IF PPP \\ BDS-IF & B1/B3 & IF PPP \\ GAL-IF & E1/E5a & IF PPP \\ GAL-R15 & E1/E5a & RAW PPP \\ GAL-R17 & E1/E5b & RAW PPP \\ GAL-R18 & E1/E5 & RAW PPP \\ GAL-R157 & E1/E5a/E5b & RAW PPP \\ GAL-R1578 & E1/E5a/E5b/E5 & RAW PPP \\ \hline \hline \end{tabular}
\end{table}
Table 3: PPP solutions used in this study and their abbreviations.  
Figure 5: Average positioning accuracy for different observation combinations over ten days.  
Figure 8: Average RMS of static PPP from different processing modes at different stations.  
Figure 6: Site-specific RMS for different processing modes.  
Figure 7: Average RMS of static PPP from different processing modes.