1996A&A...308.1001W

Corrections to the Earth's precession and nutation have been derived from VLBI observations of extragalactic sources carried out by JPL's Deep Space Network between 1978 and 1994. The analysis is based on the source right ascensions and declinations given in annual position catalogues referring to the J2000.0 equator and equinox. These catalogues result from the reduction of the VLBI observables by adopting the 1976 IAU convention on precession and one of the following nutation models: 1980 IAU Theory of Nutation, ZMOA 1990-2 and KSNRE. Differences of the J2000.0 positions of a source obtained at different epochs suggest the presence of imperfections in the precession and nutation terms. In contrast to the commonly practiced direct solutions, corrections to the luni-solar precession and the 18.6yr nutations in longitude and obliquity are determined by a least squares fit to the differences of positions of individual sources at different epochs. Using the 1980 IAU and KSNRE models gives sizable, largely similar corrections. The ZMOA 1990-2 model, on the other hand, is characterized by small corrections to the nutation terms. Each of the three data sets associated with one of the nutation models provides a solution in right ascension (RA) as well as in declination (Dec). The Dec solution is self-sufficient, whereas the RA solution requires some a priori knowledge of the precession and nutation quantities that are to be determined. The self-sufficient declination solutions for the 1980 IAU, the ZMOA 1990-2 and the KSNRE models yield almost identical corrections of the luni-solar precession, which are in turn -3.1±0.2mas/yr, -3.1±0.1mas/yr and -3.3±0.2mas/yr. For the 18.6yr nutations in longitude and obliquity the IAU and KSNRE models yield -5.4±1.1mas, 3.8±0.3mas and -6.7±0.8mas, 3.3±0.1mas, while the ZMOA model gives 0.0±0.4mas and 0.5±0.2mas, respectively.