ITRF2020 updates: motivations, usage and uncertainty evaluation

The ITRF2020 marked considerable innovations compared with previous versions of the International Terrestrial Reference Frame (ITRF), by modeling nonlinear station motions (seasonal signals and post-seismic deformation for sites subject to major earthquakes). It was also demonstrated that the accuracy and long-term stability of the frame origin, based on the Earth’s center of mass as sensed by Satellite Laser Ranging (SLR), are, since ITRF2005, at the level of or better than 5 mm at epoch 2015.0 and 0.5 mm/yr. Moreover, for the first time in the ITRF history, the scale agreement between the SLR and Very Long Baseline Interferometry (VLBI) input solutions was at the level of 1 mm at the equator at epoch 2015.0, with no drift. Motivated by these results, and for a number of reasons exposed in this paper, the International Terrestrial Reference System (ITRS) Center decided to regularly (yearly) update ITRF2020. The first two ITRF2020 updates, named ITRF2020-u2023 and ITRF2020-u2024, were released in 2024 and 2025, respectively. They are aligned to ITRF2020, thus preserving the same frame parameters (origin, scale, and orientation), while providing improved and up-to-date station trajectory models. The results of the first two ITRF2020 updates are discussed, with a special focus on the evaluation of the uncertainties of the alignment over the frame physical parameters (origin and scale), as well as on Earth Orientation Parameters. We show in particular that the addition of 4 years of extended data from the four space geodetic techniques impacts the transfer of origin and scale between techniques at the level of only 1 mm, demonstrating a high-level of consistency between the frame parameters of ITRF2020 and its updates.