# Development of a Submillimetric GNSS-Based Distance Meter for Length Metrology +
## 4 Assessment of the GNSS-Based Distances Accuracy
### Description of the Experiment
\begin{table}
\begin{tabular}{c c c c c c} \hline
**Year** & **Dates** & **Sessions** & **GNSS Receivers** & **GNSS Antennas** & **Interval (s)** \\ \hline
2013 & 25 Jun-28 Jun (day) & 4 & Trimble 5700 & Leica AR25 R4 & 15 \\
2014 & 24 Jun-27 Jun (day) & 4 & Leica GS10 & Leica AR25 R4 & 15 \\
2015 & 21 Jul-24 Jun (day) & 4 & Leica GS10 & Leica AR25 R4 & 15 \\
2016 & 12 November–16 & 4 & Leica GS10 & Leica AR25 R4 & 1 \\  & November (day) & & & & \\ \hline \end{tabular}
\end{table} 
Table 5: GNSS instruments used in every campaign and time interval of data registration. The corresponding differences among the GBDM distances, the ME5000 distances, as well as the original calibrated distance traced to the metre-SI (which is displayed in Table 2) are shown in Table 7.  
The GNSS measurements in 2013 were collected using four single-constellation (GPS-only) Trimble 5700 receivers and two signal splitters. They were performed in the daytime shortly before the corresponding monitoring campaign using the UCM Mekometer ME5000. The differences between the computed GNSS distances and their corresponding FGI-certified and UCM-Mekometer ME5000 were respectively \(+0.12\) mm and \(-0.13\) mm. Taking into account their respective uncertainty (standard deviation for GNSS), the difference can be considered statistically negligible. In addition, there is no significant evidence of pillar displacements from 2012 to 2013. In 2014, the GNSS measurements were collected using two single-constellation Leica GS10 receivers and they were performed in the nighttime whereas the corresponding monitoring campaign using the UCM Mekometer ME5000 was carried out during the same days in the daylight. The comparison between the GNSS distance and the EDM-derived for this epoch yielded a difference of 0.28 mm, which can be considered statistically negligible taking into account their respective uncertainties (standard deviation for GNSS). In addition, non-significant is the baseline length variation in this epoch with respect to the previous epoch. This agrees with the conclusion obtained by a dedicated deformation approach for the entire network [47], which concluded no significant displacements for the baseline. The GNSS measurements in 2015 were collected in the nighttime using two multi-constellation Leica GS10 receivers. The spantime was eight hours and the receivers measured carrier phase observables for the following number of satellites: 21 GPS, 19 GLONASS and 4 Galileo. The corresponding monitoring campaign using the UCM Mekometer ME5000 was carried out during the same days in the daylight. Since both EDM-based distances, i.e., FGI-certified and the one obtained by using the UCM Mekometer ME5000, agreed well, it is also concluded that there were no significant displacements. Once again, the difference between the obtained GNSS distance and the corresponding using the UCM Mekometer ME5000 is satisfactorily small. Similarly, the GNSS measurements in 2016 were collected in nighttime by using two multi-constellation Leica GS10 receivers whereas the measurements using the UCM Mekometer ME5000 were carried out during the same days in daytime. No significant discrepancies could also be found among any of the types of distances. Nevertheless, all these distances constitute but a limited sample so that additional measurements need to be made to draw definitive conclusions and further research has \begin{table}
\begin{tabular}{c c c} \hline \hline
**Year** & **GNSS Distance** & **\(\sigma\) (mm)** \\ \hline
2013 & 94.40133 m & 0.036 mm \\
2014 & 94.40085 m & 0.034 mm \\
2015 & 94.40110 m & 0.040 mm \\
2016 & 94.40126 m & 0.052 mm \\ \hline \hline \end{tabular}
\end{table} 
Table 6: GNSS-Based Distance Meter (GBDM) distances obtained in years 2013, 2014, 2015 and 2016, and their corresponding standard deviations.