Design, establishment, analysis, and quality control of a high-precision reference frame in Cortes de Pallás (Spain)

Abstract

A high-precision geodetic reference frame was required in Cortes de Pallás (Spain) to undertake a long-term deformation monitoring project. Involving distances up to 2000 m, the aimed accuracy was 1 mm and 3 mm for horizontal and vertical components, respectively. Such a high precision is necessary to detect possible displacements of selected points on a critical rocky area in a short period of time, i.e., 2 or 3 years, and also to provide ground control for the integration of precise 3D models periodically obtained by remote sensing techniques. Considering the historical geotechnical problems of the area, the possible influence of the hydroelectric power plant, and the peculiar orography of the zone, a proper analysis of the stability is crucial if the reference frame is to be used for rigorous over-time deformation monitoring. This paper describes the deformation monitoring of a 10-pillar geodetic network which was measured from 2018 to 2020 by using a sub-millimetric Mekometer ME5000 (0.2 mm + 0.2 ppm) along with a demanding observing methodology which includes a network of data loggers for temperature, humidity, and air pressure as well as proper calibration of sensors and instruments to prevent potential inconsistencies between the scale of the network and the unit of length of the International System (SI meter). The results demonstrated that the chosen methodology yielded the aimed accuracy. However, using such a high precision methodology entails the problem that small displacements of only 1 or 2 mm are significantly detected as deformations by conventional deformation analysis and then it arises the problem of finding a subset of stable points for a rigorous datum realization when all the points seem to displace. This general problem is analyzed in the particular case of Cortes de Pallás, where a balanced mix of deformation analysis and technical decisions was eventually adopted to define a precise and stable reference frame for rigorous over-time deformation monitoring.

GBDM+: an improved methodology for a GNSS-based distance meter

“…In [32] we show the detailed results of the last deformation monitoring observation campaign (the initial campaign was already presented in [5]), including the agreement within 1 mm between the GNSS-derived distance (from 10 h during each two nights) and the ME5000-derived distance. Here we present the complete results of the uncertainty propagation from zero differences to double differences, first, and then to the final baseline distance for the relevant sources of error in our GBDM+ method.…”

“…We apply the method to the case of Cortes de Pallás deformation monitoring network [5], figure 5, which was observed with the submillimetric Mekometer ME5000 EDM during daytime and GNSS during nighttime in the 21-23 July 2020 observation campaign. Two Leica GS10 GNSS multiconstellation receivers (GLONASS, GPS and Galileo) were used with two Leica AR25 multi-constellation individually calibrated choke-ring antennas (calibrated with the anechoic chamber method) during two nights observing 10 h each night, roughly from 19:00 to 5:00 UTC, with data every 1 s although only epochs every 30 s were used for the computation with an elevation mask of 15 • (although the receivers tracked signals above 10 • ).…”

“…Absolute distance determination, or the determination of distances consistent with the SI meter definition, with an uncertainty of less than 1 part per million (1 ppm) is being increasingly required in the open air for different applications, including the determination of local ties in Fundamental Geodetic Observatories [2], high precision geodetic metrology in singular scientific projects, such as the European Organization for Nuclear Research (CERN) site for the survey and alignment of the components of accelerators, experiments and transfer lines [3], especially in views of the construction of the Future Circular Collider, a 100 km-length ring aiming at reaching collision energies of 100 TeV in the search for new physics, where geodetic surveillance with the highest accuracy will be required, and calibration baselines, such as the Nummela standard baseline [4], which, in turn, are used to accurately transfer the SI meter to engineering projects where a scale control is crucial, e.g. deformation monitoring of civil infrastructures, landslides and rockfalls [5]. The use of high precision electronic distance meters (EDMs) for this purpose is limited by the required determination of the index of refraction along the light path with an uncertainty below 1 ppm, which is a hard task in the open air even for lengths of only several hundred meters, since it has to be deduced from the accurate measurement of atmospheric parameters representative to the entire light path.…”

Long and close-range terrestrial photogrammetry for rocky landscape deformation monitoring

“…Further details about the geodetic measurements and subsequent computations are described in (García-Asenjo et al, 2019;García-Asenjo et al, 2022).…”

A sub-millimetre two-wavelength EDM that compensates the air refractive index: uncertainty and measurements up to 5 km

“…Deformations of a few millimetres or less per year over distances of several hundred meters or several kilometres remain difficult to detect, especially over short time periods. Measuring them is however of great importance for the monitoring of civil engineering structures such as bridges [1], power plants [2] and tunnels [3], or the study of geological formations such as glaciers [4], volcanoes and faults [5], e.g. for the prediction of earthquakes [6].…”