Simulation analysis on resonance and direct approaches for determining free core nutation parameters with celestial pole offsets

Diurnal tidal oscillations in the coupled atmosphere–ocean system generate important contributions to the Earth’s free core nutation (FCN) and annual and sub-annual components of forced nutation in the celestial pole offsets. The determination of FCN parameters cannot avoid the influence of geophysical fluid excitation neither with the direct analysis of FCN signal (direct approaches) nor with the resonance analysis of forced nutation (resonance approaches). There is a significant difference in the FCN parameters obtained with resonance and direct approaches from celestial pole offsets observed through very long baseline interferometry (VLBI). The source of the difference between the two lacks quantitative analysis, which causes difficulties in interpreting the validity of the derived FCN parameters. Using both approaches, we conducted a simulation of celestial pole offsets to quantitatively demonstrate how geophysical fluid excitation affects the determination of FCN parameters from VLBI observations. Using the same excitation source, the FCN period obtained by the direct approach deviated from the set value (430.21 d) by more than 10 d, while the FCN period obtained by the resonance approach showed no deviation from the set value by more than 1 d. The results indicate that the resonance approach more accurately reflects the intrinsic period of the FCN. The impact of atmospheric and oceanic contributions on the determination of the FCN period with the resonance approach was within 2 d. Numerical simulation shows that discrepancies in FCN parameters caused by geophysical excitation were nonnegligible in constructing accurate FCN models.