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Abstract

The new realisation of the International Terrestrial Reference Frame, the ITRF2020, became publicly available in April 2022. For each of the four contributing space-geodetic techniques, it provides station positions and linear station velocities at a given epoch, as well as post-seismic deformation parametric models for stations subject to major earthquakes, and annual and semi-annual periodic terms for all stations. Discontinuities are introduced to take into consideration changes related to the equipment or to the station motion.During the evaluation of the ITRF2020, discrepancies were found in the temporal position modelling between two stations of the research facility of Ny-Ålesund on Svalbard (Norway). The possible impact that these discrepancies may have on the ITRF2020 is so far not well studied. In this study, we hypothesize that such discrepancies are the result of neglecting non-linear vertical motions resulting from contemporary ice cap melting in this area. The vertical motion at sites that only experience glacial isostatic adjustment (GIA) are well described by linear uplift rates. However, GNSS and GRACE time series show that several sites in polar areas that are still surrounded by ice caps experience accelerated rates of contemporary ice melting due to climate change.To be consistent with the idea of providing non-linear post-seismic deformation models at sites experiencing earthquakes, we suggest that sites experiencing non-linear motions due to contemporary ice-cap melting should also be provided with an adjusted uplift model. We assess the impact of this approach on terrestrial reference frame results, focusing particularly on two geodetic techniques, VLBI and GNSS.