  Context. The Gaia ESA mission will estimate the astrometric and physical data
of more than one billion objects, providing the largest and most precise
catalog of absolute astrometry in the history of Astronomy. The core of this
process, the so-called global sphere reconstruction, is represented by the
reduction of a subset of these objects which will be used to define the
celestial reference frame. As the Hipparcos mission showed, and as is inherent
to all kinds of absolute measurements, possible errors in the data reduction
can hardly be identified from the catalog, thus potentially introducing
systematic errors in all derived work. Aims. Following up on the lessons
learned from Hipparcos, our aim is thus to develop an independent sphere
reconstruction method that contributes to guarantee the quality of the
astrometric results without fully reproducing the main processing chain.
Methods. Indeed, given the unfeasibility of a complete replica of the data
reduction pipeline, an astrometric verification unit (AVU) was instituted by
the Gaia Data Processing and Analysis Consortium (DPAC). One of its jobs is to
implement and operate an independent global sphere reconstruction (GSR),
parallel to the baseline one (AGIS, namely Astrometric Global Iterative
Solution) but limited to the primary stars and for validation purposes, to
compare the two results, and to report on any significant differences. Results.
Tests performed on simulated data show that GSR is able to reproduce at the
sub-$\mu$as level the results of the AGIS demonstration run presented in
Lindegren et al. (2012). Conclusions. Further development is ongoing to improve
on the treatment of real data and on the software modules that compare the AGIS
and GSR solutions to identify possible discrepancies above the tolerance level
set by the accuracy of the Gaia catalog.
