## 3 Results
### Differences between the GPS and Galileo Solutions  
Before comparing solutions in the DD approach, we checked whether the G and E solutions could be directly compared. As already mentioned, we used \(22\) IGS stations that provide Galileo data. The mean coordinate difference at the reference stations was \(0.0\)/\(0.0\)/\(1.6\) mm (north/east/up), and at all stations was \(-0.1\)/\(-0.1\)/\(-0.2\) mm (north/east/up). We did not see any visible network effect or scale issue either (Figure 6.). For individual stations, we observed a large (up to cm) difference in the horizontal component, but in spite of this, we concluded that the solutions were in the same frame and that there were no contradictions for further analysis. Figure 4: Coordinate differences between the E1 and E2 solutions and their standard deviations. Darker columns refer to the DD approach. Values are presented in Table A1 (Appendix A). Figure 5: Differences in PCV between the E05 and G02 values for the LEIAR25.R4 LEIT antennas (on the left) and the resulting height differences for the DD approach (E2–E1). The mean differences for the horizontal component was approximately 2 mm for the PPP approach and approximately 0.5 mm for the DD approach for all of the compared solutions (i.e., E1-G1, E2-G1 and E3-G2). The discrepancies in the estimated differences between both approaches reached a few millimeters. Detailed results for each station are presented in Figures 7-9, and the corresponding values in Appendix A (Tables A3-A5). The JAV_RINGANT_G3T antenna showed the biggest differences; a positive bias can be observed in both approaches in the east component, as well as a positive bias in the north component for the E2 (Figure 8) and E3 (Figure 9) solutions. Therefore, we can conclude that for one type of antenna, using proper PCCs for the E5a signal (solution E1) ensures the better compatibility of the Galileo solutions with the GPS solutions in the horizontal plane. When we used the G02 values instead of the E05 value (solutions E2 and E3), we introduced a bias of up to 1 cm. For the rest of the antennas, one can conclude that the consistency in the horizontal plane is satisfactory regardless of the used PCC values. Figure 6: Coordinate differences between solutions E1 and G1 for the DD approach. Figure 7: Coordinate differences between the E1 and G1 solutions and their standard deviations. Darker columns refer to the DD approach. Values are presented in Table A3. The situation is different for the estimated height. For the first pair (E1 and G1), a clear positive bias can be observed \(-6.5\) mm for DD and \(10.8\) mm for PPP (Figure 7). Only for the AUBG station did we notice a negative (\(-5.5\) mm and \(-7.1\) mm) bias for both approaches. There was no visible dependency between the antenna models and the received differences. Therefore, this should be considered as a systematic error between the GPS and Galileo solutions. As shown in the work of [14], using full chamber calibrations in conjunction with the calibrations provided by the GSA for satellite antennas causes an inter-system GPS-Galileo bias. The authors indicated that this bias was present in the up component only and reached \(9\) mm. Our outcome confirms the results of the above work: the lower value for the DD approach (visible also in Figure 4) probably resulted from the absorption part of the common signals, which is characteristic of the regional solution [38]. Figure 8: Coordinate differences between the E2 and G1 solutions and their standard deviations. Darker columns refer to the DD approach. Values are presented in Table A4. Figure 9: Coordinate differences between the E3 and G2 solutions and their standard deviations. Darker columns refer to the DD approach. Values are presented in Table A5. Interesting here are the results for the E2 and E3 solutions. In these solutions, we skipped the E05 corrections and used G02 (individual or mean-type values) instead. The mean difference in the height for E2-G1 was 2.4 mm for PPP and 0.3 mm for DD (Figure 8).