# Development of a Submillimetric GNSS-Based Distance Meter for Length Metrology +
## 4 Assessment of the GNSS-Based Distances Accuracy  
### The UPV Calibration Baseline of Reference  
To validate the accuracy of the GNSS-based distances they have to be compared with distances accurately traced to SI-metre in an outdoor metrological infrastructure. For practical reasons, we opted for the UPV calibration baseline. This baseline, which was set up in November 2007 by the Department of Cartographic Engineering, Geodesy and Photogrammetry (DICGF), was originally planned to evaluate the uncertainty of geodetic instruments according to ISO 17123 series [44]. In 2012, absolute scale was transferred from de Nummela Standard Baseline (NSB) by the Finnish Geospatial Research Institute (FGI) so as to carry out the experiment to validate the accuracy of the GNSS-based distances.  
As shown in Figure 10, the UPV metrological infrastructure is a triangle-shaped test field with seven observation pillars which includes an Heerbrugg-type EDM calibration baseline [12]. Oriented north-west, the calibration baseline consists of six observation pillars (No.1 to No.6) approximately in line at 0, 28, 94, 198, 282 and 330 m.  
The pillars have a diameter of 22 cm and height 1.20 m above ground level made of two insulated steel pipes. The inner one covers a concrete structure with an square metre foundation that extends to a depth of 60 cm. The outer steel pipe prevents the inner pillar from differential dilations due to meteorological effects. To install measuring instruments on the top they have a double forced-centring mount system: the standard 5/8\" fixing screw and a Kern-type trivet system.  
Three measurement campaigns were carried out from December 2007 to February 2008 using the following total stations: Topcon GTS605, Leica TPS 1200+ and Leica TDA5005. All of them proved to be compatible within a combined least squares adjustment and the coordinates given in Table 1 were adopted as first set of approximate local coordinates. The coordinates are referred to a local right handed Cartesian system whose origin is the top of the pillar No.1, the \(x\)-axis points to pillar No.6 and \(z\)-axis is positive upwards.  
Figure 10: UPV submillimetric calibration test field.  
The horizontal and vertical stability of the pillars over time has been periodically monitored [45; 46; 47]. Since no significative displacement was detected, this first set of coordinates have been safely regarded for geometrical reduction computations in subsequent campaigns.  
The field campaign to transfer scale from the Nummela Standard Baseline (NSB) in 2012 was carried out from 28 May to 1 June by the Finnish Geospatial Research Institute (FGI). The measurements were done using the Kern ME5000 No. 357094 of the Aalto University along with the reflector No.374414 which were calibrated at Nummela before and after the transferring campaign. Dry and wet temperatures were measured using calibrated Thies Clima Assmann-Type psychometers with an estimated \(\pm\)0.3 \({}^{\circ}\)C uncertainty and air pressure was measured using two calibrated Thommen 3B4.01.1 aneroid instruments with an estimated \(\pm\)0.3 hPa uncertainty. The EDM and the reflector were installed in the 5/8\" fixing screws of the pillars using Leica GDF321 tribrachs. As the observational scheme consisted in four sets of 'double-in all-combinations'distances, every distance was consequently measured 16 times. This observational scheme was strictly repeated in the subsequent four field campaigns.  
Therefore, the traceability chain for the UPV baseline distances that were to be use as ground truth was: SI definition of the metre, quartz gauge bar, Vaisala interferometer, FGI Nummela Standard Baseline, FGI Mekometer ME5000 and UPV calibration baseline. The resulting nominal distances were obtained with expanded total uncertainties \(U_{k=2}\) of 0.1-0.3 mm (Table 2).  
\begin{table}
\begin{tabular}{c c c c c c} \hline
**Pillar** & \(x\)**(mm)** & \(\sigma_{x}\)**(mm)** & **y(mm)** & \(\sigma_{y}\)**(mm)** & **z(mm)** &