## 3 Results and Discussion
### Real-Time Approach  
First of all, we decided to compare the solutions obtained by these two receivers considering the geodetic antenna: this choice was made in order to establish which is the best accuracy available with these types of receivers. In this case we have considered only the VRS correction. In Table 13, we show the performances of NVS and u-blox receivers in a NRTK positioning mode. In order to make more clearer the results, remember that in the NMEA message:  
* stand-alone positioning
* DGPS positioning
* RTK positioning with FIX phase ambiguity
* RTK positioning with FLOAT phase ambiguity  
Despite the similar results in terms of accuracy (Tables 13 and 14), the u-blox receiver allows us to fix the ambiguity phase twice with respect to the NVS receiver considering the VRS correction and a geodetic antenna. If a low-cost antenna is considered (Table 15) the u-blox receiver is able to fix the ambiguity phase at 97% of the epochs and because the NVS cannot fix any epoch, 100% of the epochs are defined as a float. As shown in Figure 4, the float solutions are not good solutions because they are about 50 cm from the reference position. In order to verify whether it is the GLONASS constellation is \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multicolumn{6}{c}{**NVS (LEIAR10 Antenna, only GPS, Session Length = 15 min)**} \\ \hline  & **\#E [m]** & **\#N [m]** & **\#h [m]** & **\#E [m]** & **\#N [m]** & **\#h [m]** & \\ \hline
5 km & 0.081 & 0.003 & -0.103 & 0.005 & 0.002 & 0.010 & FIX \\
10 km & 0.214 & 0.192 & -0.098 & 0.011 & 0.012 & 0.023 & FLT \\
25 km & 0.131 & 0.082 & 0.112 & 0.014 & 0.011 & 0.034 & FLT \\ \hline \hline \end{tabular}
\end{table} 
Table 12: Difference between estimated and reference positioning using a VR at various distances for a session length of 15 min, considering the LEIAR10 antenna splitted, and only the GPS constellation. the problem (in the sense that the software has problems fixing the ambiguity phase of GLONASS satellites), some tests are designed to consider only the GPS satellites. Analysing Tables 14 and 15, it is possible to observe that the geodetic antenna improves the u-blox NRTK results considerably, but not the NVS ones if we use this as a double constellation receiver. In fact, in both cases, no fixed epochs are reached with this last receiver. If this receiver is used in a single constellation mode, a small improvement can be obtained but, in spite of all, the u-blox results are the best that it can be obtained (Table 16). In general it is possible to affirm that the best choice is to use only the GPS constellation also for NRTK positioning. \begin{table}
\begin{tabular}{c c c} \hline  & **NVS** & **u-blox** \\ \hline Estimated-reference coordinates & & \\ East [m] & 0.003 & 0.002 \\ North [m] & \(-\)0.012 & \(-\)0.012 \\ h [m] & 0.177 & 0.164 \\ Planimetric residual at 95\% [m] & 0.017 & 0.014 \\ Altimetric residual at 95\% [m] & 0.186 & 0.168 \\ Percentage of FIX epochs & 33\% & 67.30\% \\ \hline \end{tabular}
\end{table} 
Table 14: Planimetric errors obtained with the LEIAR10 antenna and VRS correction. \begin{table}
\begin{tabular}{c c c} \hline  & **NVS** & **u-blox** \\ \hline Estimated-reference coordinates & & \\ East [m] & N/A * & \(-\)0.001 \\ North [m] & N/A * & \(-\)0.008 \\ h [m] & N/A * & 0.234 \\ Planimetric residual at 95\% [m] & 0.797 & 0.009 \\ Altimetric residual at 95\% [m] & 0.504 & 0.237 \\ Percentage of FIX epochs & 0.0\% & 97.2\% \\ \hline \end{tabular} * N/A = no fixed epochs available.  
\end{table} 
Table 15: Planimetric errors considering the Garmin antenna and VRS correction. Also considering a low cost antenna and only the GPS constellation (Table 17) we have obtained unsatisfactory results; the fixing percentage is very low (less than 8%) and the quality of float positions