# Comparison Analysis on the Accuracy of Galileo PPP Using Different Frequency Combinations in Europe
## 2 Methods
### Evaluation of the Accuracy of Dual-Frequency Galileo Static PPP  
Table 2 lists the receiver and antenna types for Galileo PPP. The stations are selected by determining that they can ensure the continuous PPP and multi-frequency observations for the selected period from the day of year (DOY) 120 to 130 in 2020. Figure 1 shows the number of available satellites and the global dilution of precision (GDOP) value for GPS and GAL at station SKE8. More than six satellites can be observed for Galileo, which is less than GPS.  
The reference coordinates of the stations were collected from the EUREF weekly solution. The positioning errors are the differences between the estimated coordinates and reference coordinates. The accuracy measured by root mean square (RMS) for GPS and GAL static PPP are listed on the right panel of Figure 2, with the geographical distribution of stations shown in the left panel. The data were processed in a 24-h session, and the sampling rate was 30 s for each station. It was observed that the stations are located in the northern European area, and no obvious graphical related systematic errors were observed. The average RMS for GPS and GAL at 10 successive days are 3.9 mm/2.2 mm/6.5 mm and 1.6 mm/2.3 mm/5.7 mm on the north/east/up component, respectively. The horizontal positioning accuracy for Galileo is better than GPS, especially in the north direction, and this can be clearly observed on the right panel of Figure 2. The repeatability of RMS for station OSK6 shown in Figure 3 confirms that the repeatability of GAL PPP on the horizontal direction is better than that of GPS.  
\begin{table}
\begin{tabular}{l l} \hline \hline
**Model** & **Settings** \\ \hline Sample rate & 30 s \\ Elevation cutoff angle & 7\({}^{\circ}\) \\ Phase wind-up & Phase wind-up correction \\ Satellite and receiver & Using js14\_2101.atx \\ antenna phase center & Corrected \\ Relativity & Yaw-Steering \\ Satellite attitude model & Corrected with GFZ multi-GNSS final products \\ DCBs & Corrected with products provided by DLR \\ Earth tides correction & IERS2010 \\ Station coordinates & Estimated as static and kinematic \\  & ZHD: Saastamoinen model [21] \\  & ZWD: Estimated as the random walk process parameter for \\  & each station \\  & Mapping function: Niell mapping function \\ Receiver clocks & Solved for at each epoch as white noise \\ Inter frequency bias (IFB) & Modeled as constant \\ Filter method & Kalman filter \\ \hline \hline \end{tabular}
\end{table}
Table 1: Data modeling strategies for PPP.  
Figure 1: Number of observed satellites and the global dilution of position (GDOP) for GPS and GAL on station SKE8 on DOY 120, 2020.  
Figure 2: The RMS on north (**top**), east (**middle**), and up (**bottom**) components for GPS (**left**), and its comparison with the GAL dual-frequency static PPP (**right**).  
\begin{table}
\begin{tabular}{l l l} \hline
**Receiver Type** & **Antenna Type** & **Stations** \\ \hline \multirow{2}{*}{TRIMBLE NETR9} & LEIAR25.R3 & ARJ6 JON6 KAD6 LOV6 NOR7 \\  & LEIT & OSK6 OST6 OVE6 SKE8 LEK6 \\ \hline \multirow{2}{*}{JAVAD TRE\_3 DELTA} & JAVRINGANT\_DM & KEV2 KIV2 KILF FINS JOE2 \\  & SCIS & OKL2 OUL2 ROM2 SAVU \\ \hline \end{tabular}
\end{table}
Table 2: Receiver types and antenna types for the Galileo tracking stations.  
Figure 4 shows the RMS of static GPS and BDS dual-frequency PPP for the selected stations. The left panel indicates the distribution of RMS, and no clear location-related distribution is observed. The right pane shows the RMS comparison of each station. The average RMS is 3.9 mm, 2.2 mm, and 6.5 mm for GPS and 2.7 mm, 2.4 mm, and 9.3 mm for BDS on the north/east/up component, respectively. The BDS can improve the RMS on the north direction by 30.7%; however, the RMS on the up direction is worse than that of GPS.