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The combination of HIPPARCOS measurements with suitable ground-based astrometric data improves significantly the accuracy of the proper motions of bright stars. The comparison of both types of data allows us also to identify and to eliminate, at least partially, cosmic errors in the quasi-instantaneously measured HIPPARCOS data which are caused by undetected astrometric binaries. We describe a simple averaging method for the combination of two independent compilation catalogues. The combination of the basic FK5 with HIPPARCOS leads to the Sixth Catalogue of Fundamental Stars (FK6). The accuracy of the FK6 proper motions is higher than that of HIPPARCOS by a factor of about 2 in the single-star mode, and by a factor of more than 4 in the long-term prediction mode which takes cosmic errors into account. We present also the error budget for a combination of the Boss General Catalogue (GC) with HIPPARCOS data. We point out problems with known binaries, and identify an ensemble of astrometrically excellent stars.
The combination of ground-based astrometric compilation catalogues, such as the FK5 or the GC, with the results of the ESA Astrometric Satellite HIPPARCOS produces for many thousands of stars proper motions which are significantly more accurate than
The combination of ground-based astrometric compilation catalogues, such as the FK5 or the GC, with the results of the ESA Astrometric Satellite HIPPARCOS produces for many thousands of stars proper motions which are significantly more accurate than
The statistical test described by Wielen et al.(1994) is used to derive new zero-points of ground-based Cepheid period-luminosity (PL) and period-luminosity-colour (PLC) relations. Eleven relations are compared with the Hipparcos data. Our results ar
Stochastic field distortions caused by atmospheric turbulence are a fundamental limitation to the astrometric accuracy of ground-based imaging. This distortion field is measurable at the locations of stars with accurate positions provided by the Gaia
A comparison was made between $Gaia$ magnitudes and magnitudes obtained from ground-based observations for astrometric radio sources . The comparison showed that these magnitudes often not agree well. There may be several reasons for this disagreemen