New estimates of retrograde free core nutation parameters

Based on the available celestial intermediate pole (CIP) coordinate determinations from VLBI observations over the last 25 years, we have constructed a new highly accurate combined series of CIP coordinates. An amplitude-phase analysis of this series has allowed us to improve the previously estimated retrograde free core nutation (RFCN) parameters as functions of time. During this interval, the RFCN period changed several times. It was −418.1 ± 0.2 days before 1992.1 and then −431.6 ± 0.2 days until 1999.0. In 1999, the CIP oscillations damped out almost completely and the processwas restructured. Since 2000, the amplitude of these oscillations has increased and their period has become 450.7± 0.1 days.