Using the Earth orientation parameters to link the Hipparcos and VLBI reference frames

One of the methods, used to link the free Hipparcos reference system to the conventional extragalactic reference system, is the indirect method using the rapidly rotating terrestrial reference system as an intermediary. We solve the Earth orientation parameters, referred to the preliminary Hipparcos reference frame H37C, from the observations made by the classical methods of optical astrometry (latitude and universal time variations) in the interval 1899.7–1992.0. These parameters are then compared with the series of the Earth orientation parameters obtained by VLBI observations (thus referred to extragalactic objects) in the interval 1980.0–1992.0, and from their differences the two angles ε0x, ε0y (out of three) and their annual rates ωx, ωy are derived. The non-alignment of the terrestrial longitude systems used by optical astrometry and VLBI makes the determination of ε0z impossible, and the practical tests show that its obtained rate, ωz , is also worthless, very probably due to a mutual drift between the optical astrometry and VLBI terrestrial systems in longitude. The remaining angles ε0x, ε0y are determinedwith formal standard errors of about 0.9mas (for the mean epoch of the common observations of VLBI and optical astrometry, 1985.25) and their rateswith the accuracy of 0.3mas yr−1. The necessity of referring these angles to the mean epoch of the Hipparcos catalog, 1991.25 (as required by the Hipparcos Science Team) however raises their standard errors to 1.9 mas.