## 3 Results
### Ztd (Zenith Terospheric Delay)  
The ZTD analysis was performed by comparing the estimated zenith delays with the reference values provided by the IGS. The results are illustrated in Figure 4. The figure also highlights the 6 mm (target) and 3 cm (maximum acceptable) thresholds commonly required for real-time applications [10].  
The BRDM solution, based solely on broadcast ephemerides, demonstrated a somewhat lower performance, particularly for GPS signals. The mean ZTD errors reached  
Figure 4: Mean difference and STD of the ZTD estimates at all stations with respect to the IGS reference for all analyzed solutions.  
3.2 mm for GPS, 2.7 mm for Galileo, and 1.5 mm for the combined GPS+Galileo solution. Although these mean values are within the 6 mm target, the associated standard deviations were relatively high: \(\pm\)30.6 mm for GPS, \(\pm\)21.9 mm for Galileo, and \(\pm\)27.6 mm for GPS+Galileo. Notably, the standard deviation for GPS slightly exceeded the 30 mm threshold, while for Galileo and GPS+Galileo it remained within acceptable limits. This indicates that while the average ZTD estimation was satisfactory, the variability of the results, particularly for GPS, could impact reliability in real-time applications.  
In the case of the HAS solution, a larger variability was observed, especially for GPS signals. The mean ZTD errors were 5.0 mm for GPS, \(-\)0.2 mm for Galileo, and 7.5 mm for the combined GPS+Galileo solution. The corresponding standard deviations were \(\pm\)43.1 mm for GPS, \(\pm\)15.4 mm for Galileo, and \(\pm\)19.3 mm for GPS+Galileo. These results indicate that the use of Galileo signals, either alone or in combination with GPS, provides a noticeably better stability and precision compared to using GPS signals alone when applying HAS corrections.  
The CNES solution, based on real-time precise corrections, demonstrated outstanding performance in ZTD estimation. The mean ZTD errors were 0.7 mm for GPS, 1.3 mm for Galileo, and 0.7 mm for GPS+Galileo. The associated standard deviations were also exceptionally low: \(\pm\)4.6 mm for GPS, \(\pm\)8.0 mm for Galileo, and \(\pm\)4.4 mm for GPS+Galileo.  
Similarly, the CODE solution, based on post-processed precise products, yielded very high-quality results. The mean ZTD errors were 1.7 mm for GPS, 2.4 mm for Galileo, and 2.0 mm for GPS+Galileo. The standard deviations were also the lowest among all the solutions analyzed: \(\pm\)3.9 mm for GPS, \(\pm\)4.6 mm for Galileo, and \(\pm\)3.2 mm for GPS+Galileo.  
Overall, while BRDM and HAS demonstrated acceptable mean errors, their standard deviations--particularly for GPS signals--slightly exceeded the thresholds, which could pose challenges for high-reliability real-time applications. However, despite these slight exceedances, both solutions remain highly useful and can still be considered reliable for many applications. In contrast, the CNES and CODE solutions clearly outperformed the others, achieving both target mean errors and maintaining a low variability, ensuring robust and accurate ZTD estimations suitable for operational NWP systems. Additionally, as the Galileo HAS service continues to evolve, improvements in its performance are expected, which could further enhance its reliability and precision in the near future.