## 3 Results  
### MMS Reference Trajectory and RKTLIB Results  
The differential solutions computed with the RNX2 RTKP CUI tool were achieved by using a local reference station different (see acknowledgment) from the one used for the reference trajectory (i.e., the MMS one). The reason for this was that the reference station data (free of charge) used for the MMS solution had not yet included the Galileo acquisitions. The mean distance between the reference GNSS station and the vehicle, mounting both the MMS POS/LV system and the GS14 receiver, was less than 5 km. Table 3 depicts the results of the application of Equation (6) used to select one solution (the \"best\" according to fj,k,c) for each tested combination. It is worth mentioning that, for the application of Equation (6), it was necessary to select, among the available nine different solutions produced for each satellite combination (see Section 2.3.1), only the one that minimized the objective function. Hence, the final comparisons of the three constellations were made only among the best solutions and not among all those produced. The number of RTKLIB remaining trajectories (\"n. of combinations\" column), the percentage of occurrence of the cut-off angles, and the percentage of occurrence of the ambiguity fix with the implemented methods achieved through the application of Equation (6) (see Section 2.3.3), are shown in Table 3 for the available combinations. For example, the \"best_gal_4\" row reports the previous data for the remaining 13 combinations (and thus trajectories), according to Equation (6), obtained with different sets of four Galileo satellites. Comparing the number of combinations shown in Table 3 with the ones featured in Table 1, an expected trend is evident, in which the percentage of selected combinations increases with the number of the considered satellites. Moreover, a clear difference between the Galileo, the GLONASS, and the GPS systems emerges from the cut-off angles associated to the best solution for each considered combination. This could be related to the increasing number of Galileo cycle slips (or loss of satellite tracking) occurred at lower elevation angles, whereas the opposite occurred for the other two constellations because of the availability of many combinations with satellites characterized by high elevation angles. Moreover, the experimental results for the GPS and GLONASS constellations showed a high occurrence of good solutions achieved through the \"fix and hold\" method. This experimental evidence showed, in this case, a quite stable behavior of the RTKLIB RNX2 RTKP CUI tool for the GPS and GLONASS constellations. The same did not occur for the Galileo constellation and there could be several reasons to justify these differences. First of all, because of the reduced operativeness status of the Galileo constellation, a very small number of combinations could be tested (see Tables 1-3). As such, because of the higher number of combinations, only the first row of Table 3 (\"best_gal_4\") \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Combination**} & \multicolumn{2}{c}{**n. of**} & \multicolumn{2}{c}{**Cut-Off**} & \multicolumn{3}{c}{**Ambiguity Fix**} \\ \cline{2-9}  & **Combinations** & **10\({}^{\circ}\)** & **15\({}^{\circ}\)** & **20\({}^{\circ}\)** & **Fix and Hold** & **Instantaneous** & **Continuous** \\ \hline best\_gal\_4 & 13 & 15\% & 85\% & — & 38\% & 24\% & 38\% \\ best\_gal\_5 & 4 & — & 100\% & — & 25\% & 50\% & 25\% \\ best\_gal\_6 & 1 & — & — & 100\% & 100\% & — & — \\ best\_glo\_4 & 29 & 73\% & 17\% & 10\% & 55\% & 38\% & 7\% \\ best\_glo\_5 & 82 & 65\% & 26\% & 9\% & 59\% & 38\% & 3\% \\ best\_glo\_6 & 62 & 52\% & 29\% & 19\% & 77\% & 16\% & 7\% \\ best\_gps\_4 & 77 & 68\% & 29\% & 3\% & 61\% & 18\% & 21\% \\ best\_gps\_5 & 264 & 60\% & 34\% & 6\% & 63\% & 15\% & 22\% \\ best\_gps\_6 & 373 & 54\% & 37\% & 9\% & 66\% & 11\% & 23\% \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Summary of the selected RKTLIB trajectories for the subsequent comparisons.