GRAVITY active subsystems involving the astrometric measurement. In red is the optical path of the reference star, in blue the optical path of the science star. The dotted lines are free space-light propagation, and the solid lines correspond to the single-mode fibers. The reference star is used to correct the atmospheric piston in real time by activating the internal piezo and/or the VLTI delay lines. In green is represented the metrology system, injected into the two integrated optics beam combiners (IO), and measured thanks to 4 metrology diodes inside the cryostat, and 4 metrology diodes on each telescope. The metrology measurements are used to control the GRAVITY differential delay lines to position the fringes on the science camera. Yellow represents the acquisition camera control loop that commands the tip-tilt and the pupil position. Orange is the open loop polarization control system.