New global solution of Earth orientation parameters from optical astrometry in 1900-1990.

Abstract

Astrometrically observed variations of latitude and Universal time with thirty instruments of different types, located at 19 observatories all over the world, are used to derive polar motion, UT1-TAI (since 1956) and celestial pole offset at 5-day intervals. The values based on the observations of individual stars are used, the model of least-squares global estimation contains a set of additional station parameters (such as corrections to the adopted station coordinates and their drifts, seasonal systematic deviations, or tidal parameter {LAMBDA}= 1+k-l). Plate tectonic motions of the observatories after NUVEL-1 model are subtracted from the observations, and (in the absence of Hipparcos catalog that we plan to use in the future), the Hipparcos Input Catalog (INCA) is used to bring local star catalogs into a common celestial reference system by means of removing the systematic differences of proper motions from the observed data. Oceanic tide-loading effects are also removed from the observations, as well as the short-periodic tidal variations in the Earth's rotation. Observations made by the method of equal altitudes (astrolabes) are additionally corrected for the local deformations of the apparent almucantar. More than three million observations lead to the solution which can be characterized by the formal standard errors of 'normal point' pole position decreasing from 0.02" at the beginning of the century to less than 0.01" in the last two decades; universal time UT1 is determined with formal standard errors mostly less than 0.001s.