Distributed metrology and active control for according stabilization of space telescopes

The paper is concerned with the active angular stabilization of the astrometric instrument of the scientific satellite GAIA (Global Astrometric Interferometer for Astrophysics). The ESA satellite, being an evolution of the HIPPARCOS mission, aims to provide a precise astrometric map of the Galaxy. The main mission requirement is the time stability of the basic angle between the line-of-sights of the two on-board telescopes. The residual angular perturbation shall be less than few picoradian. Only laser interferometric technology, and specifically active Fabry-Perot (FP) interferometry, is expected to favour such a requirement. The paper first mentions two alternative stabilization strategies, passive and active. Then it focuses to the active one, which is under development by a team led by Alenia Spazio, Italy. Active stabilization, operating in closed-loop, needs appropriate instruments to measure the basic angle variations and actuators to force them to zero. The former ones are provided by one-meter length FP interferometers locked to a frequency-stable laser beam and capable of picometer precision, the latter by piezo-electric translators. A complex digital control unit manages a set of 12 interferometers in addition to laser frequency and basic angle stabilization.