## 2 Methodology  
To evaluate the impact of using various PCCs on the E5a signal, we prepared five solutions for absolute precise point positioning (PPP) and five for the relative double differences (DD) approach. Three solutions were prepared for Galileo and two for GPS data, respectively (Table 1). We used 30 s data collected between 30 June 2019 and 6 July 2019 (GPS week 2060) at stations belonging to EPN [27]. During this period, 198 stations provided Galileo data in the RINEX 3 format, which are subject to ongoing checks and validation by the EPN Central Bureau [28]. In the standard EPN processing of model antennas, we used receiver antenna corrections from individual calibrations (epncb.atx) as a priority (robot or chamber). If this was not possible, then we used the latest IGS-type mean model release (igs14.atx) [29]. In both cases, the calibration table contains corrections for both GPS frequencies; however, in the case of Galileo, only the chamber calibrations provided us the necessary corrections for the Galileo frequencies. Unfortunately, for the analyzed period, this applied to 25 antennas only (marked as red triangles in Figure 1). In our case, all of these antennas were calibrated by the University of Bonn and were the subject of ongoing analyses. A list of the stations and mounted antennas is shown in Table 2.  
The GNSS processing in the DD approach was done using the GNSS analysis software GAMIT [30], version 10.71. In this case, the analysis covered not only the selected stations (Table 3), but all of the EPN stations provided both GPS and Galileo data. The number of stations for these solutions was increased to reduce the potential impact of the different selection and process methods on the individual solutions of the 25 stations in the reference frame. The final set of 198 selected stations was divided into six clusters of a similar station density, including five tie stations per each cluster. The final number of stations per cluster ranged from 41 to 44. Observations in each cluster were processed according to the Guidelines for EPN Analysis Centres (ACs), independently for GPS and Galileo. The strategy was similar to the MUT's (Military University of Technology, Poland) official contribution to EUREF. Together with the input of other EPN ACs, a basis was formed for determining the most accurate and up-to-date EPN station positions. This is especially important as the EPN stations support the maintenance of the European Terrestrial Reference System 89 (ETRS89) [27]. In our DD processing, an ionosphere-free combination of the L1 and L2 (E1 and E5a) carrier phases was used as the basic observations. Solid earth tidal displacements were modeled according to the International Earth Rotation and Reference Systems Service (IERS) Conventions 2010. Absolute phase center corrections \begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & \multicolumn{4}{c}{**Galileo Solutions**} & \multicolumn{4}{c}{**GPS Solutions**} \\ \cline{2-6}  & **E1** & **E2** & **E3** & **G1** & **G2** \\ \hline PCCs & E01, E05 & E01, G02 & E01, G02 & G01, G02 & G01, G02 \\ Source & Bonn & Bonn & IGS & Bonn & IGS \\ \hline \hline \end{tabular} DD, double differences approach; PPP, precise point positioning; PCC, phase center corrections; IGS, IGS-type mean model (igs14.atx); Bonn, individual calibrations made by the University of Bonn (stored in epnc.atx).  
\end{table} 
Table 1: Solution types prepared for the DD and PPP approaches. For each solution receiver, the antennas were modeled using different sources of the PCCs. were applied for the satellite antennas (igs14_2062.atx) and for the receiver antennas (details shown in Table 1). The rest of the models used are summarized in Table 3.