2017ApJS..232....4T

We combine Gaia DR1, PS1, Sloan Digital Sky Survey (SDSS), and 2MASS astrometry to measure proper motions for 350 million sources across three-fourths of the sky down to a magnitude of m_r∼20. Using positions of galaxies from PS1, we build a common reference frame for the multi-epoch PS1, single-epoch SDSS and 2MASS data, and calibrate the data in small angular patches to this frame. As the Gaia DR1 excludes resolved galaxy images, we choose a different approach to calibrate its positions to this reference frame: we exploit the fact that the proper motions of stars in these patches are linear. By simultaneously fitting the positions of stars at different epochs of-Gaia DR1, PS1, SDSS, and 2MASS-we construct an extensive catalog of proper motions dubbed GPS1. GPS1 has a characteristic systematic error of less than 0.3 mas yr^–1 and a typical precision of 1.5-2.0 mas yr^–1. The proper motions have been validated using galaxies, open clusters, distant giant stars, and QSOs. In comparison with other published faint proper motion catalogs, GPS1's systematic error ( 0.3 mas yr^–1) should be nearly an order of magnitude better than that of PPMXL and UCAC4 ( 2.0 mas yr^–1). Similarly, its precision (∼1.5 mas yr^–1) is a four-fold improvement relative to PPMXL and UCAC4 (∼6.0 mas yr^–1). For QSOs, the precision of GPS1 is found to be worse (∼2.0-3.0 mas yr^–1), possibly due to their particular differential chromatic refraction. The GPS1 catalog will be released online and be available via the VizieR Service and VO Service.