Geodesy by radio interferometry: studies of the forced nutations of the earth 1. Data analysis

The analysis of 235 very long baseline interferometry observing sessions carried out between July 1980 and December 1984 has yielded estimates of corrections to the seven largest nutation series coefficients (with periods <=1 year) in the Wahr nutation series, the most complete such model available at this time. The largest correction is to the amplitude of the retrograde annual nutation: -1.80+/-0.18-i(0.42+/-0.18) milliarc seconds (mas), where the estimated standard deviations contain contributions from the effects of systematic as well as random errors. We interpret this correction as being due to an incompleteness in the model of the core-mantle coupling used by Wahr. We have also examined our results for evidence of any ``free core nutation'' and can place an upper bound of 1.0 mas (99.5% confidence interval) on its amplitude.