  GRAVITY is a second generation VLTI instrument, combining the light of four
telescopes and two objects simultaneously. The main goal is to obtain
astrometrically accurate information. Besides correctly measured stellar phases
this requires the knowledge of the instrumental differential phase, which has
to be measured optically during the astronomical observations. This is the
purpose of a dedicated metrology system. The GRAVITY metrology covers the full
optical path, from the beam combiners up to the reference points in the beam of
the primary telescope mirror, minimizing the systematic uncertainties and
providing a proper baseline in astrometric terms. Two laser beams with a fixed
phase relation travel backward the whole optical chain, creating a fringe
pattern in any plane close to a pupil. By temporal encoding the phase
information can be extracted at any point by means of flux measurements with
photo diodes. The reference points chosen sample the pupil at typical radii,
eliminating potential systematics due differential focus. We present the final
design and the performance estimate, which is in accordance with the overall
requirements for GRAVITY.
