ESA is supporting an industrial study aimed at preliminary assessment of some critical aspects of the GAIA mission implementation: hereafter, our study approach is presented. The Fizeau interferometer of GAIA features a strict correlation between the conventional telescope and interferometer concepts and problems. The wide angle measurement concept is based on the stability of the basic angle between two macroscopic structures: the implementation is challenging. An experiment will address feasibility and performance of metrology at sub-nanometer level, over 0.5 m distance, controlling displacement and relative orientation `tip-tilt' of two platforms, simulating optical elements of GAIA. System definition is based on the scientfic rationales of GAIA; however, trade-off among competing requirements, if needed, should neither descope the mission nor push any critical parameter to its limiting values. Most of the critical aspects which arose in the Cambridge 1995 workshop on GAIA are being addressed, and some viable solutions seem to be taking shape: microarcsecond global astrometry is becoming closer to implementation