VLBI-scale of the ITRF2020-u2023

Zuheir Altamimi officially introduced the ITRF2020-u2023 at the AGU24 conference in Washington DC on Monday 9 December 2024. This marks the first update of the ITRF2020, showcasing a novel approach to annual updates of the International Terrestrial Reference Frames (ITRF) without requiring global reprocessing from the four space-geodetic Technique Centers. The update integrates the original ITRF2020 combined solution, which includes data up to December 2020, with an additional three years of observations (January 2021 to December 2023) for the inter-technique combination. For VLBI specifically, the IVS Combination Center supplied the ITRS Center with three years of new data, derived from the combined contributions of 12 IVS Analysis Centers using seven different analysis packages. This involved the analysis of nearly 700 sessions. Notably, these three years of data were independently analyzed by the respective Analysis Centers. Regarding the VLBI scale in the IVS contribution to ITRF2020-u2023, the scale drift exhibits distinct behavior over different periods. The drift from 2013.75 to 2021.00 differs significantly from the drift observed between 2021.00 and 2024.00. While modeling the uplift of NYALES20 and accounting for station events improved the positive drift during the earlier period, it had no effect on the drift observed over the past three years. To understand these differences, the IVS Combination Center's contribution to ITRF2020-u2023 is carefully compared with the initial ITRF2020 contribution. Additionally, the OSO Analysis Center contribution is examined under various analytical configurations. The primary objective of this study is to assess the strengths and limitations of this innovative approach to updating the ITRF.