# Development of a Submillimetric GNSS-Based Distance Meter for Length Metrology +
## 5 Conclusions and Future Work  
We have proposed and tested new methods to develop a submillimetric GNSS-Based Distance Meter (GBDM) for application to length metrology in open air for distances up to 1 km, as an alternative to more expensive approaches to the problem like two-colour telemeters or laser trackers, which are currently under development. We present the corresponding functional model as well as specific methods to mitigate the main source of error in the GBDM determination of length for baselines up to 1 km: the multipath error. Consistency with the unit of length (SI-metre) has been shown within few tenths of a millimetre by using a calibration baseline of reference having distances traced to the SI-metre and a ME5000 distance meter.  
The future application of the GBDM to longer baselines, in the range of 1 to 5 km, can be expected as soon as some difficulties are circumvented: mainly, the correction and corresponding assessment of uncertainty of the impact on the baseline length of the residual tropospheric delay, which should not be neglected especially in the case of relatively large height differences. The evaluation of different absolute antenna calibration models for use in the problem at hand and the corresponding uncertainty assessment is also a question to be addressed in the future.  
The present study has been realized with the main focus on the GPS constellation, and the marginal use of satellites from other constellations (Galileo and GLONASS). An optimized, fully multi-constellation, approach needs also to be further developed.  
Conceptualization, L.G.-A., S.B. and C.A.; data curation, L.G.-A.; formal analysis, C.A. and P.G.; funding acquisition, L.G.-A.; investigation, L.G.-A., S.B. and C.A.; methodology, L.G.-A., S.B. and C.A.; resources, P.G.; software, L.G.-A. and S.B.; supervision, L.G.-A.; writing--original draft, L.G.-A. and S.B.; writing--review and editing, C.A. and P.G. All authors have read and agreed to the published version of the manuscript.  
This work was partly performed within the 18 SIB01 GeoMetre project of the European Metrology Programme for Innovation and Research (EMPIR). This project has received funding from the EMPIR programme co-financed by the Participating States and from the European Union's Horizon 2020 research and innovation programme. This research was also partly funded by the Spanish Ministry of Education, Culture and Sports (PRX17/00371).  
Not applicable.  
All data included in this study are available upon request by contact with the corresponding author.  
We especially acknowledge the Universidad Complutense de Madrid (UCM) for the long-term collaboration agreement that makes it possible to use the Mekometer ME5000. We are also grateful to [PERSON] for his support to calibrate the frequency of the Mekometer, thermometers and barometers at the Calibration Laboratory of the UPV. We are thankful to the Centro Espanol de Metrologia (CEM) for their support as observers to our research to develop a submillimetric GBDM. Finally, we acknowledge IGS and MGEX for their efforts to disseminate products available to the global community.  
The authors declare no conflict of interest.