  New astrometric reductions of the US Naval Observatory CCD Astrograph Catalog
(UCAC) all-sky observations were performed from first principles using the TGAS
stars in the 8 to 11 magnitude range as reference star catalog. Significant
improvements in the astrometric solutions were obtained and the UCAC5 catalog
of mean positions at a mean epoch near 2001 was generated. By combining UCAC5
with Gaia DR1 data new proper motions on the Gaia coordinate system for over
107 million stars were obtained with typical accuracies of 1 to 2 mas/yr (R =
11 to 15 mag), and about 5 mas/yr at 16th mag. Proper motions of most TGAS
stars are improved over their Gaia data and the precision level of TGAS proper
motions is extended to many millions more, fainter stars. External comparisons
were made using stellar cluster fields and extragalactic sources. The TGAS data
allow us to derive the limiting precision of the UCAC x,y data, which is
significantly better than 1/100 pixel.
