Earth Orientation Parameters 1899.7-1992.0 In The Hipparcos Reference Frame

Abstract

The optical astrometry observations of latitude/universal time variations made with 48 instruments at 31 observatories are used to determine the Earth orientation parameters (EOP) since the beginning of the century. The Hipparcos Catalogue is used to bring more than four million individual observations, made in the interval 1899.7-1992.0, into the International Celestial Reference System. The Earth orientation parameters (polar motion, celestial pole offsets and, since 1956.0, also universal time UT1) are determined at 5-day intervals, with average uncertainties ranging from 8 mas (in the eighties) to about 40 mas (in the forties). Making use of very long series of ground-based observations, the solution also leads to the improvement of proper motions of about ten per cent of the observed Hipparcos stars, with precision of ±0.2 — 0.5 mas/yr. In addition, 474 auxiliary parameters, describing the rheological properties of the Earth and seasonal deviations of the observations at contributing observatories, are found. The new solution provides the EOP series suitable for further analyses, e.g., for studying long-periodic polar motion, length-of-day changes or precession/nutation.

Astrometric star catalogues as combination of Hipparcos/Tycho catalogues with ground-based observations

“…Thus the solutions OA97 (Vondrák et al 1998), OA99 (Vondrák et al 2000), and finally OA00 (Ron and Vondrák 2001) were worked out. The main differences among these solutions consisted in number of participating observatories, in the different approach to how the data acquired at the same observatory were merged, and the treatment of obviously systematic deviations of some Hipparcos stars.…”

Difficulties of Preserving the Leap Second

“…The five Earth Orientation Parameters (EOP) -two components of polar motion x, y, two components of celestial pole offsets ∆ψ, ∆ǫ, and universal time U T 1 (that is nothing else but the angle of Earth's rotation around its spin axis) -are now analyzed and routinely derived from the observations at several world's centers, combined and regularly published in IERS bulletins. The most recent system of constants and algorithms (McCarthy, 1996, Vondrák, 1998 are used . The past solutions based on optical astrometry were merged with the combined solutions from the modern techniques (Vondrák, 2001).…”

Corrections of proper motions in declination by using ILS data

“…The value (t − t 1 ) is in centuries, and t 1 is counted from 32000 MJD. The codes (CA, CI, GT, KZ, MZZ, TS and UK) are from the monograph by Vondrák et al (1998). The longitude λ W is west of the zero meridian.…”

“…About 15 years elapsed since the HIPPARCOS ESA mission (ESA 1997(ESA ) observations (1991 is the epoch of this catalogue), and with the errors of the Hipparcos proper motions close to 1 mas/yr, the error of apparent places of stars is now more than 15 mas; so that it exceeds the error of the Hipparcos positions by one order of magnitude (which is about 1 mas). Also, for some Hipparcos stars, the errors of proper motions are much larger than the averaged value itself, even not realistic at all (Vondrák et al 1998); the Hipparcos astronomical satellite mission lasted less than four years, not enough to get a sufficient accuracy of the proper motions. To improve the accuracy of the proper motions for some Hipparcos stars, the ground -based data were used and some new catalogues were published (such as ARIHIP, EOC-2, etc.)…”

“…Za neke Hiparkos zvezde, grexka sopstvenih kretanja je mnogo ve a od proseqne vrednosti ili nije realna (Vondrák et al 1998), jer su astrometrijska posmatranja Hiparkos satelitom trajala kra e od qetiri godine (nedovoljno za rexavanje nekih problema dobijanja sopstvenih kretanja, xto se sada uoqava). Za dobijanje taqnijih sopstvenih kretanja nekih Hiparkos zvezda, koriste se klasiqna posmatranja sa Zemlje, i tokom protekle decenije pojavili su se novi katalozi (kao xto su ARIHIP, EOC-2 i dr.).…”