The International Celestial Reference System (ICRS) is the
fundamental celestial reference system adopted by the
International Astronomical Union (IAU) for high-precision
positional astronomy. The ICRS, with its origin at the solar
system barycenter and "space fixed" axis directions, is meant to
represent the most appropriate coordinate system for expressing
reference data on the positions and motions of celestial
objects. The ICRS is a set of specifications defining a high-
precision coordinate system with its origin at the solar system
barycenter and "space fixed" (kinematically non-rotating) axes.
The specifications include a metric tensor, a prescription for
establishing and maintaining the axis directions, a list of
benchmark objects with precise coordinates for each one, and
standard models and algorithms that allow these coordinates to
be transformed into observable quantities for any location and
time. In this context, distinguishing between a reference system
and a reference frame, as used in astronom