  Presented here are the details of the astrometric reductions from the x,y
data to mean Right Ascension (RA), Declination (Dec) coordinates of the third
U.S. Naval Observatory (USNO) CCD Astrograph Catalog (UCAC3). For these new
reductions we used over 216,000 CCD exposures. The Two-Micron All-Sky Survey
(2MASS) data are extensively used to probe for coordinate and coma-like
systematic errors in UCAC data mainly caused by the poor charge transfer
efficiency (CTE) of the 4K CCD. Errors up to about 200 mas have been corrected
using complex look-up tables handling multiple dependencies derived from the
residuals. Similarly, field distortions and sub-pixel phase errors have also
been evaluated using the residuals with respect to 2MASS. The overall magnitude
equation is derived from UCAC calibration field observations alone, independent
of external catalogs. Systematic errors of positions at UCAC observing epoch as
presented in UCAC3 are better corrected than in the previous catalogs for most
stars. The Tycho-2 catalog is used to obtain final positions on the
International Celestial Reference Frame (ICRF). Residuals of the Tycho-2
reference stars show a small magnitude equation (depending on declination zone)
that might be inherent in the Tycho-2 catalog.
