Tilt monitoring to assess the stability of geodetic reference points in permafrost environment

Abstract

Comparison of station velocities of geodetic very long baseline interferometry (VLBI) and global positioning system (GPS) reference points at the Ny-Ålesund Space Geodetic Observatory on Spitsbergen has raised questions about the stability of the basements of the antennas over time spans of years. The antennas are roughly 100 m apart from each other. They are fixed to concrete basements erected in the permafrost ground. We have installed three continuously recording tiltmeters of resolution 0.1 μrad on the tops of these basements to monitor micro-movements over about one year, that is over a period where relevant changes in ground frost conditions take place. Several tests conducted during the initial phase of the set-up of the instruments have given insight into the magnitude of short-term tilt signals induced through movements of the VLBI antenna. Analysis of the long-term recordings suggests stable conditions for the basement of the GPS antenna on the 50 μrad level. For the basement of the VLBI antenna, the situation is less clear. There is evidence for permafrost induced instability in May–July 2002 at this place, however, with some doubts remaining due to partial malfunctioning and questionable temperature sensitivity in one of the components of the tiltmeter that was run here. The total observed tilt signal reflects movements of the entire basement, which do probably not exceed 1 mm.