The VLBI Scale Drift: Current Status and Remaining Challenges

Since the release of the International Terrestrial Reference Frame ITRF2020, significant efforts have been made to understand the positive scale drift observed in Very Long Baseline Interferometry (VLBI) solutions after epoch 2013.75 and its abrupt stop after 2021.0. Recent studies have demonstrated that introducing additional discontinuity intervals for the station NYALES20 (Svalbard, Norway), originating from the co-located Global Navigation Satellite Systems (GNSS) stations NYAL and NYA1, reduces this trend. Following these findings, the ITRF2020 update (ITRF2020-u2023) incorporates two additional discontinuity intervals for NYALES20. But can these adjustments fully capture the station's complex motion and reduce the VLBI scale drift? In this study, we assess the effectiveness of the ITRF2020-u2023 modifications concerning NYALES20's station movement model in mitigating the VLBI scale drift and investigate the abrupt stop of the drift after 2021.0. Furthermore, we analyze the influence of non-tidal loading models on the time series of VLBI scale factors. Our findings provide insight into the current status of the VLBI scale and potential suggestions for further refinements.