VLBI-scale of the ITRF2020-u2023

Paper i proceeding, 2025

Zuheir Altamimi officially introduced the ITRF2020-u2023 at the AGU24 conference in Washington DC on Monday, 9 December 2024. This marked the first update of ITRF2020, which showcases a novel approach to annual updates of the International Terrestrial Reference Frames (ITRF) without requiring global reprocessing from the four combination centers of space-geodetic technique that contribute to ITRF.
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. In particular, the three years of data were analyzed independently by the respective Analysis Centers, separate from the longer-term contribution to the ITRF2020, which spans 1979 to December 2021.
Regarding the VLBI scale in the IVS contribution to ITRF2020-u2023, the scale drift exhibited 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. Although 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 contribution of the IVS Combination Center to ITRF2020-u2023 was carefully compared with the initial contribution of ITRF2020. The primary objective of this study was to assess the strengths and limitations of this innovative approach to updating the ITRF, as well as to develop a strategy to update uniformly within all IVS Analysis Centers and the IVS Combination Center.