The Earth's core parameters as seen by the VLBI

Context. From the VLBI nutation series, one can derive information on the fluid core, such as the resonant frequency associated with the retrograde free core nutation (RFCN). This frequency has already been determined, along with other parameters, and then later confirmed with a longer data set. Aims. This paper looks at the sensitivity of the RFCN frequency with respect to three factors not considered before: the state-of-the-art VLBI analysis strategy, possible change of the RFCN period with time, and the effects of the atmosphere contaminating the determination. Methods. Using several geodetic VLBI nutation data sets, we estimate amplitudes of various forced nutations, from which we deduce the resonant RFCN frequency consisting in the real frequency and a damping factor. The time stability is checked by using a sliding window. The atmospheric contribution is assessed using the 6-h NCEP/NCAR Reanalysis data. Results. We show that the resonant period is stable within half a day from one VLBI data set to another. The atmospheric angular momentum contribution is found to have no noticeable effect. However, we point out that, although small, inconsistencies between VLBI data sets do show up and could have an impact, if not corrected, in future determination of geophysical parameters, if one wants to reach a higher accuracy than at present.