# Assessment of GNSS Galileo Contribution to the Modernization of CROPOS's Services
## 4 Discussion
### Analysis and Discussion of the VPPS Results
Northing components obtained with GGG and GAL-only observations have shown a great level of similarity (matching), leading to the conclusion that Northing components obtained with 'GAL-only' observation can be regarded as equally accurate and reliable as those obtained with GGG observations, even though 'GAL-only' results have shown a greater level of  
Figure 13: Box and whisker charts for Easting (**left**), Northing (**middle**), and Height (**right**) results obtained with VPPS (GGG), VPPS (GAL-only), and static GGGB daily solution (REF). variability. When the results are averaged and compared to the REFERENCE value, there is a negligible difference \(<\) 1 mm. * Height (Figure 13 right). When it comes to the height components, there is no significant difference between MEDIAN values (GGG: 0.7806 m vs. GAL-only: 0.7799 m) and AVERAGE values (GGG: 0.7803 m vs GAL-only: 0.7794 m). On the other hand, there is a greater RANGE for 'GAL-only' results (0.0217 m) compared to GGG results (0.0171 m), that has consequences for the differences in IQR values (GGG: 0.0040 m vs. GAL-only: 0.0051 m). Small differences between the average values (\(<\)1 mm) compared to the REFERENCE value give the difference \(<\) 4 mm, providing a confirmation that calculating the average results improves the accuracy and reliability of the final value. Four outlier values have been found: one outlier in GGG results and the other three outliers in GAL-only results. Usually, the outliers are found by considering the IQR, Q1, and Q3 values, where the lower and upper outlier gates are calculated with the following expressions: \[\text{Q1}-1.5\times\text{IQR: Lower outlier gate}.\] \[\text{Q3}+1.5\times\text{IQR: Upper outlier gate}.\]  
For the lower gate of GGG results, it applies that \(\text{Q1}-1.5\times\text{IQR}=163.7782-1.5\times 0.0040=163.7722\), which is greater than the value of 163.7711 which is, in its turn, identified as the outlier shown in Figure 13 (right). The height identified as an outlier was obtained as the first observation in the second series (A-2-1); comparing the subsequent results in the same series, those values have changed and came closer to the REFERENCE value (see Figure 11). That outlier could be related to some initialization uncertainties; subsequent series in the same session (A-2) with independent initializations have provided values closer to the REFERENCE. Lower and upper outlier gates for GAL-only heights are given as follows: \(\text{Q1}-1.5\times\text{IQR}=163.7689\) m, \(\text{Q3}+1.5\times\text{IQR}=163.7892\) m. When all GAL-only heights are compared with the lower and upper gate values, two outliers below the lower gate and one outlier above the upper gate are identified, as shown in Figure 13 (right). Two outliers below the lower gate came from the series A-9-1 and A-9-2 in the last session; the outlier above the upper gate came from the series A-1-3 in the first session (see Figure 13 right). Two outliers below the lower gate are related to the high DOP values and low visibility of Galileo satellites (Table 2). It can be concluded that GAL-only results have provided accurate and reliable results, although with a greater range and spread, pointing out that several sessions providing redundant measurements are needed to obtain more accurate and reliable results. If all VPPS results (GGG and GAL-only) along with the GPPS (GGGB) are considered, and if the MIN and MAX values are analyzed, then the range values can be derived and displayed in Table 5.  
Considering the range values, it can be clearly seen that the GPPS results are generally better than the VPPS results. Considering the VPPS results, a smaller range has been reached for solutions based on GGG than those based on GAL-only observations. A similar conclusion can be drawn for GPPS solutions: GGGB solutions show smaller range values than solutions based on GAL-only.