# Development of a Submillimetric GNSS-Based Distance Meter for Length Metrology +
## 1 Introduction
The reporting document for the GBDM technique recommends the use of carrier-phase double differences (DD), and points out that GNSS signal multipath is the major limiting factor to get submillimetric accuracy, but no recommendation is given concerning a possible standard strategy for optimal GNSS processing and some metrological questions such as uncertainty determination or traceability are considered still open [22]. Consequently, some criticism still remains for the GNSS to be used as a submillimetric technique for length metrology. However, previous research conducted at the Department of Cartographic Engineering, Geodesy and Photogrammetry of the _Universitat Politecnica de Valencia_ (DICGF-UPV) showed the potential of GNSS techniques in terms of reproducibility for measuring distances over several hundred metres with uncertainties below one millimetre by using a functional model different from the traditional geodetic approach along with robust estimation [1; 23; 24]. The initial assumptions, the functional models and the intended computing approach are described in Section 2. As previously mentioned, the GNSS signal multipath error is considered the major limitation to use a GBDM in current metrologic facilities. The traditional geodetic approach to cope with this problem is to carefully select the GNSS sites and average out the multipath error by using long observation span times. However, current calibration baselines of reference [10; 25] were mostly set up decades ago, and they do not always have favourable conditions for GNSS measurements since they are normally surrounded by trees or buildings. As a result, L1/E1 DD residuals can easily reach up to several centimetres with no Gaussian distribution and consequently the obtained distance will be biased, thus impeding the desired submillimetric accuracy. Therefore, a consistent GBDM should include some processing strategy for multipath mitigation. Therefore, the last field experiment was specially designed to investigate and develop possible methods for multipath mitigation as applied to length metrology. This part of the investigation was conducted at the Space Geodesy and Navigation Laboratory of the University College London (SGNL-UCL), and their results are shown in Section 3. Nonetheless, not only does the GBDM technique have to be proven to be precise but it must also be accurate according to length metrology standards. Thus, the proposed GBDM technique was tested in four field experiments conducted at the UPV submillimetric calibration baseline from the years 2013 to 2016. Each experiment was a four-day campaign where both the GNSS and EDM techniques were applied. During the daylight hours, the inter-pillar distances of the calibration baseline were measured using a ME5000 in accordance to ISO17123-4 [5], and subsequently, GNSS antennas were set up to collect twelve-hour sessions overnight. The same operational scheme was repeated for four consecutive days. The experiment was designed in such a way that the obtained distances can be traced to the SI-metre. This paper is organised as follows. In Section 2, the general approach for GNSS data processing is described. Section 3 is devoted to the methods for multipath detection and mitigation that were developed and tested. The assessment of the resulting accuracy by comparison with distances derived from EDM traced to the SI-metre definition in a calibration baseline is explained in Section 4. Finally, some conclusions and future work are summarised in Section 5.