  We describe development and application of a Global Astrometric Solution
(GAS) to the problem of Pan-STARRS1 (PS1) astrometry. Current PS1 astrometry is
based on differential astrometric measurements using 2MASS reference stars,
thus PS1 astrometry inherits the errors of the 2MASS catalog. The GAS, based on
a single, least squares adjustment to approximately 750k grid stars using over
3000 extragalactic objects as reference objects, avoids this catalog-to-catalog
propagation of errors to a great extent. The GAS uses a relatively small number
of Quasi-Stellar Objects (QSOs, or distant AGN) with very accurate (<1 mas)
radio positions, referenced to the ICRF2. These QSOs provide a hard constraint
in the global least squares adjustment. Solving such a system provides absolute
astrometry for all the stars simultaneously. The concept is much cleaner than
conventional astrometry but is not easy to perform for large catalogs. In this
paper we describe our method and its application to Pan-STARRS1 data. We show
that large-scale systematic errors are easily corrected but our solution
residuals for position (~60 mas) are still larger than expected based on
simulations (~10 mas). We provide a likely explanation for the reason the
small-scale residual errors are not corrected in our solution as would be
expected.
