Impact of Free Core Nutation Modeling on the Estimation of Earth Rotation Parameters from Different VLBI Session Types

Abstract

Free Core Nutation (FCN) arises from complex geophysical processes causing misalignment between the mantle and the liquid core, and exhibits a retrograde motion with a period of about 431 days as observed by Very Long Baseline Interferometry (VLBI) as part of the celestial pole offsets (CPO). This study assesses the influence of using an empirical model of FCN on estimating Earth Rotation Parameters (ERP) from different types of VLBI sessions, i.e., 24-hour S/X sessions (2001–2022), 24-hour VGOS sessions (2019–2022), and Intensive sessions (2001–2022). To evaluate the impact, a priori values of CPO from the IERS Bulletin A series and the FCN model by Belda et al. (2016) are used, and the estimated polar motion and UT1-UTC values are compared against the IERS 20 C04 EOP solution. The results indicate that the sole application of the empirical FCN model does not degrade the WRMS values but introduces time-dependent systematic differences in ERP. The comparison of S/X and VGOS sessions indicates that ERP estimated using the Belda model in VGOS sessions demonstrate slightly lower WRMS values.