Overview

The US Naval Observatory (USNO) has a long history of providing accurate astrometric data for millions of stars from their own observations plus other data. The USNO CCD Astrograph Catalog (UCAC) project utiized the "redlens" 20 cm aperture astrograph in an all-sky observing program between 1997 and 2004 (CTIO in the south, NOFS in the north) with a limiting magnitude of about R = 16.5. The previous release, UCAC4, became available in 2012.

The 1st Gaia data release provides proper motions for only about 2 million stars (TGAS subset of the Tycho-2 stars) in the mainly 6 to 11.5 magnitude range. Gaia DR2 which will contain proper motions of about a billion stars is scheduled for release in April 2018. In the meantime the astronomical community would benefit from proper motions of millions of stars fainter than the Tycho-2 limit, if a substantial improvement in precision and accuraccy could be made beyond what was available in the pre-Gaia era.

Re-reduction of UCAC + combine with Gaia DR1 provides proper motions for over 107 million stars on the 1 to 5 mas/yr level, strongly depending on magnitude. UCAC observations (mean epoch 2001) provide positions with 10 to 70 mas precision, and about 14 years of epoch difference to Gaia DR1.
Please see also the paper, Zacharias, Finch, & Frouard (2017), which contains several usful figures and validation results for the UCAC5 data. Full UCAC5 documentation is available here.