Pushing the sensitivity limits of RTS-based continuous deformation monitoring of an alpine valley

Abstract

Monitoring applications may require operating robotic total stations (RTS) at the limit of their sensitivity with respect to target displacements. Thorough understanding and mitigation of systematic effects are required in order to reach or push this limit. We investigate some of these effects, in particular effects external to the total station, using data and experience gained from a continuously operating monitoring system installed at the terminus of the Great Aletsch Glacier in Switzerland. The system consists of two robotic total stations, about 60 prisms, four global navigation satellite system (GNSS) receivers, thermocouples, inclinometers, and meteo-sensors. The purpose of the monitoring is to study reversible deformations of the adjacent slopes, likely driven by snowmelt and mountain water level changes. The deformations reach the millimeter- to centimeter-level and shall be studied on time scales ranging from annual to sub-annual, and ideally even down to daily or sub-daily resolution. Our investigation focuses on four aspects: protective housing, pillar stability, refraction, and stability of orientation, all of which were found to affect the measurements on the milligon-level with lines-of-sight of up to 2 km. The results highlight signatures of apparent point displacements, and the discussion comprises approaches to bounding or mitigating these effects which may also be expected in similar monitoring situations at other locations.