The Global Sphere Reconstruction for the Gaia mission in the Astrometric Verification Unit

The core task of the Gaia mission is the solution of the Global Astrometric Sphere, which is providing the materialization of the astrometric reference frame for the catalog that will be the main outcome of the mission. Given the absolute character of the measurements, the Gaia Data Processing and Analysis Consortium (DPAC) has decided to replicate a dedicated version of this task, together with two other ones selected for their mission criticality, in an Astrometric Verification Unit (AVU). This task, named Global Sphere Reconstruction (GSR), focusses on the importance of having an implementation of the astrometric sphere solution from a well-defined subset of objects, based on an independent astrometric model as well as on a different solution algorithm. We analyze here these two aspects in the context of the GSR implementation at the Data Processing Center of Torino (DPCT) and the solution to implement the most computationally intensive part of the pipeline as a High-Performance Computing module.