Comparison Of The PPMXL And UCAC5 Catalogs With The Gaia DR2

Astrometric ground-based catalogs usually suffer from varied systematic errors.These systematics were hard to be detected since 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 proper motions(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 the PM of Gaia DR2 to the epoch of other catalogs for comparison.Sys-tematic errors that depend on the magnitude,color,and sky regions in the UCAC5 and PPMXL could be clearly seen.A different behaviour between the northern and south-ern sky is found in the PPMXL,which possibly inherits 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 three rotation angles,three glide parameters,and ten quadrupole parameters.We find a large glide component of～8 mas along Z-axis and a rotation angle of～5 mas about Z-axis for positional offsets between the PPMXL and Gaia DR2.These terms are found insignif-icant 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 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.