The materialization of a global celestial reference system is one of the fundamental tasks of astrometry. The era of Space Astrometry was ushered in by the Hipparcos mission, and it is now continuing with Gaia, which allows the production of catalogs of unprecedented size and accuracy. These high-accuracy catalogs are instrumental for a vast range of astrophysical applications, so it is important also to the final user to have an informed approach about its usage. This talk will give a brief overview of the approach adopted for the solution of the global astrometric sphere, and of the different issues that can introduce systematic errors in the catalog.i.e. the problem of the calibration and that of the correlations between different unknowns.