Exploring reasons for the ITRF2020 VLBI scale drift

Journal article, 2026

In the recent realization of the International Terrestrial Reference System (ITRF2020), it was found that the VLBI scale parameter showed a positive drift after 2013.75. Although several possible reasons for this apparent VLBI scale drift are being discussed by the International VLBI Service for Geodesy and Astrometry community, a clear explanation for this issue has not yet been identified. In this study, we investigated the reasons for the apparent VLBI scale drift in the ITRF2020 using CATREF software, applying the same approach as that used for producing the ITRF2020. We estimated the impact of various models and methods used in the ITRF2020 on the VLBI scale drift, such as discontinuities applied to VLBI station positions and velocities, gravitational deformation models, and thermal deformation corrections. The analysis revealed that there is no simple explanation for the scale drift; however, the station position modeling of NYALES20 is one of the major contributors to it, and a combination of different deformation models and equipment changes for some stations can explain most of the drift observed in the scale parameter.