## 4 Conclusions  
Our research was aimed at checking the effect of using GPS-based ground antenna phase center corrections for the Galileo E5a frequency. For many of the antennas, we did not have the full calibrations containing Galileo-specified values, and we were forced to use substitutes. In the case of the most commonly used second L-band frequencies (L2 and E5a), this is especially important, because both the frequencies and the corresponding phase center corrections are unequal and may affect the estimated values. We showed that, except for the JAV_RINGANT_G3T NONE antennas, such an approach does not cause significant positional differences in the horizontal component. However, it should be noted here that we tested only six types of antenna. Perhaps there are other models of antennas that have the same problem, but at the moment, this cannot be verified. As shown, using G02 values introduced an average bias in the height of -7.7 mm, with a standard deviation of 3.8 mm. For the latest calibrations, this bias was lower (-4.8 mm), with a standard deviation of 1.6 mm (Table 4). It can be assumed here that the stations providing Galileo observations and equipped with uncalibrated individual antennas are affected in a similar way. In our case, this applies to 172 stations. Publication of new IGS products containing receiver antenna corrections for Galileo will help this matter; however, it is very important to further calibrate the antennas for Galileo frequencies. Figure 11: Dependence between height differences (G2–G1) and time of the calibrations. Figure 10: Coordinate differences between the G2 and G1 solutions and their standard deviations. Darker columns refer to the DD approach. Values are presented in Table 6.  
On the other hand, Table 5 clearly shows that using the E05 values made the Galileo solutions less consistent with the GPS solutions (E1-G1) than using G02 values (E2-G1 and E3-G2). The differences show a bias of 6.5 mm for DD and 10.8 mm for PPP approaches respectively for the E1-G1 comparison, while for E2-G1 and E3-G2 the differences are not significant. This issue was already indicated by the IGS Antenna Working Group and is related to the incompatibility between the calibrations for the satellite antennas provided by the GSA and the current IGS products, which causes an additional inter-system GPS-Galileo bias in height. However, the IGS assumes that the new release of IGS products should solve this problem. Herein, both errors compensated for one another, which resulted in a higher consistency between the GPS and Galileo solutions. However, the resulting high agreement between the GPS and Galileo solutions does not reflect the true consistency. Only when the true consistency is reflected will it be possible to assess the real agreement between the GPS and Galileo estimates. A supplementary comparison of the chamber and robot calibrations for the GPS solutions showed good agreement between the final coordinates. The individual differences below 3 mm and mean values at the sub-millimeter level indicate that both methods were very consistent. Slightly worse results pertaining to height for individual stations applied only to calibrations carried out long ago. For the latest calibrations carried out after 2016, the mean difference was 1.7 mm, with a standard deviation of 1.6 mm, clearly showing good compatibility between the robot and chamber calibrations. Compared to the Galileo issues, this should be treated as a minor concern (Table 5). **Author Contributions:** Conceptualization, A.A.; formal analysis, A.A. and D.K. ; investigation, A.A. and D.K. ; methodology, A.A.; visualization, D.K. ; writing--original draft, A.A. All authors have read and agreed to the published version of the manuscript. **Funding:** This research was funded by statutory funds from the Institute of Spatial Engineering and Geodesy, Faculty of Civil Engineering and Geodesy, Military University of Technology.