An empirical analysis of the free core nutation period from VLBI-based series of celestial pole offsets

Currently, accurate observations of the Celestial Intermediate Pole (CIP) can only be obtained by the Very Long Baseline Interferometry (VLBI) technique. The differences among the observed CIP and that derived from IAU 2006/2000A precession-nutation model are the Celestial Pole Offsets (CPO). CPO contain the free core nutation (FCN), trends, and harmonics due to limitations of IAU precession-nutation model, geophysical excitations, etc. as well as the noise of observations. FCN is a free rotational mode of the Earth and, in principle, cannot be predicted. It can be determined from CPO time series with the use of different empirical models. The development of such models, and the enhancing of the existing ones, has been suggested in the IAU/IAG Joint Working Group ”Improving Theories and Models of the Earth’s Rotation (ITMER)”, since FCN contribution is the main one of CPO. The accurate estimation of the free core nutation (FCN) period of about 430 days is a challenging prospect. Comparison of the FCN period obtained by different authors and methods shows slight discrepancies. But, is there any evidence that the period of the FCN varies with time? If so, then this would complicate making a model of it. The FCN phase drift in 2000 could be related to that, but the is no clear evidence. In this study, we tested different FCN periods using several subsets of the observed nutations derived from VLBI analysis with the purpose of finding the optimal configuration that provides the lowest residuals. That is why a large quantity of empirical FCN models were also estimated and tested with different sliding window lengths and FCN periods. This analysis could bring us significantly closer to meet the accuracy goals pursued by the Global Geodetic Observing System (GGOS) of the International Association of Geodesy (IAG), i.e. 1 mm accuracy and 0.1 mm/year stability on global scales in terms of the ITRF defining parameters.

This research was supported partially by Generalitat Valenciana (SEJIGENT/2021/001) and the European Union—NextGenerationEU (ZAMBRANO 21-04) (S.B). This research was also supported by Spanish Projects PID2020-119383GB-I00 funded by Ministerio de Ciencia e Innovación (MCIN/AEI/10.13039/501100011033/) and  PROMETEO/2021/030 funded by Generalitat Valenciana (J.M.F., A.E.).