  The VLTI instrument GRAVITY combines the beams from four telescopes and
provides phase-referenced imaging as well as precision-astrometry of order 10
microarcseconds by observing two celestial objects in dual-field mode. Their
angular separation can be determined from their differential OPD (dOPD) when
the internal dOPDs in the interferometer are known. Here, we present the
general overview of the novel metrology system which performs these
measurements. The metrology consists of a three-beam laser system and a
homodyne detection scheme for three-beam interference using phase-shifting
interferometry in combination with lock-in amplifiers. Via this approach the
metrology system measures dOPDs on a nanometer-level.
