Tracing the kilometre to the SI metre

More and more often, surveyors or researchers in geosciences are facing the challenge of measuring distances over several hundreds of metres up to kilometres with uncertainties at the millimetre level and below: when constructing and monitoring current and future long‑distance tunnel projects or the gigantic accelerator facilities used in high energy physics, when establishing local ties between different geodetic instruments on geodetic fundamental stations, or when monitoring deformation networks at critical sites like nuclear power plants or future carbon dioxide repositories, for example.

 In general, two different measurement techniques are available for this task:

1.      electronic distance meters (EDM): the length measurement is based on the direct propagation of an electromagnetic wave between two reference points. In principle the range is limited by the visibility between these points.

2.      Global Navigation Satellite System (GNSS)‑based distance measurement: the distance between two antennas is calculated from 3D coordinates inferred from GNSS signal reception. In principle, there is no limitation of range.

Both approaches, however, are currently not capable of achieving traceability to the SI definition of the metre with one or even sub‑millimetre uncertainty over the respective long distances in air as needed. The accuracy of EDM predominantly suffers from the insufficient determination of the effective environmental conditions over the complete distance of hundreds of metres. GNSS‑based distance measurements are influenced by various effects, such as propagation delays in the ionosphere and troposphere, signal reflections by the ground, and by obstacles in the antenna neighbourhood, as well as antenna phase centre variations.

In view of common challenges, metrologists working in NMIs have united with geodesists who usually work in separate state‑funded research organizations and universities. In this joint research project the uncertainty of the scale in long‑distance metrology for measurements up to 1 kilometre shall be reduced and traceability to SI units shall be fostered. In the course of the JRP, novel technological and methodical solutions for calibration and long‑distance measurement will be developed. Most of these will be immediately available to improve traceability in surveying, geodesy and earth sciences.

Not all images were taken during the SIB60 project. ((c) for all images JRP SIB60 Consortium 2013, all rights reserved)