The astrometric global solution is a principal component of the final star catalogue. It
is the solution of a relatively complex least-squares problem related to Gaia’s design
that can only be solved using an iterative process. Hopefully we found an efficient
converging algorithm with a robust stopping criterion. Nonetheless the usage of an
iterative algorithm imposes some limitations on our knowledge of the statistical
properties of the solution such as the variance which need to be investigated further
through simulation techniques.