Effects of glacial loading in Svalbard on the VLBI scale realisation

Abstract In the realisation of the International Terrestrial Reference System in 2000, and in the realisations since 2008, the scale of the International Terrestrial Reference Frame (ITRF) has been defined based on Very Long Baseline Interferometry (VLBI) and Satellite Laser Ranging (SLR) solutions, whereas ITRF2005 relied solely on VLBI. In the latest ITRF realisation, i.e. ITRF2020 with its first and second updates, VLBI sessions after September 2013 are excluded from the ITRF scale definition due to a detected drift in the VLBI solution with respect to the average scale trend computed from September 1997 until September 2013. Ny-Ålesund station in Svalbard, the northernmost fundamental geodetic station hosting all major techniques, plays a key role in ITRS realisation due to its location. Crustal motion measurements at Ny-Ålesund reveal nonlinear vertical motion, largely explained by elastic deformation caused by thinning glaciers. In the ITRF2020 updates (u2023 and u2024), a discontinuity in August 2016 was introduced for the VLBI station to account for the increasing uplift, reducing the detected scale drift relative to ITRF2020 by approximately 50%. In this study, we analyse the nonlinear vertical motion at Ny-Ålesund, compare it with predicted elastic loading signals arising from glacier thinning, and assess its impact on the VLBI scale estimates. Over the past 25 years, the land uplift rate increased from about 6 mm/year to 14 mm/year, indicating that the vertical displacement through time is not well described by a constant-rate model. The record also shows substantial interannual variability, of which the glacier loading model explains only about 50%. Including a nonlinear station motion model—based on Global Navigation Satellite System (GNSS) observations or loading predictions—for the VLBI station in Ny-Ålesund reduces the relative scale drift by 30–60%. Finally, we discuss alternative approaches to mitigate the negative effects of nonlinearity, such as modelling the elastic glacier signal or better integrating VLBI station motion with GNSS-derived displacement.