摘要：Astrometric ground-based catalogs usually suffer from varied systematic errors. These systematic errors were hard to detect because there was no independent reference catalog complete to very faint limiting magnitudes (∼20 mag). This situation has changed since the second data release of the Gaia mission (Gaia DR2). We aim to investigate positions and the proper-motion (PM) system of two ground-based catalogs, the UCAC5 and PPMXL, referring to the Gaia DR2. The individual position in the Gaia DR2 is transferred by its PM to the epoch of other catalogs for comparison. Systematic errors that depend on the magnitude, color, and sky regions in the UCAC5 and PPMXL could be clearly seen. A different behavior between the northern and southern sky is found in the PPMXL, which is possibly inherited from the imperfect calibration of the PM system. Besides, we perform a quantitative analysis of global differences for positions and PMs by the vector spherical harmonics method in terms of 3 rotation angles, 3 glide parameters, and 10 quadrupole parameters. We find a large glide component of ∼8mas along Z-axis and a rotation angle of ∼5mas about Z-axis for positional offsets between the PPMXL and Gaia DR2. These terms are found to be insignificant between the UCAC5 and Gaia DR2. We show that the position and PM system of the UCAC5, a new reduction of ground-based observations in the frame of the Gaia reference system, has been largely improved. This indicates that systematic errors in positions and PMs obtained from ground-based observations are mostly impacted by a relatively poor reference catalog. But these observations can be reconstructed in the frame of a space-based reference catalog. In this sense, our results justify the tradition of space-calibrated ground-based astrometric catalogs.
关键词：astrometry;catalogs;proper motions;reference systems
