  GRAVITY acquisition camera implements four optical functions to track
multiple beams of Very Large Telescope Interferometer (VLTI): a) pupil tracker:
a $2 \times 2$ lenslet images four pupil reference lasers mounted on the
spiders of telescope secondary mirror; b) field tracker: images science object;
c) pupil imager: reimages telescope pupil; d) aberration tracker: images a
Shack-Hartmann. The estimation of beam stabilization parameters from the
acquisition camera detector image is carried out, for every 0.7 s, with a
dedicated data reduction software. The measured parameters are used in: a)
alignment of GRAVITY with the VLTI; b) active pupil and field stabilization; c)
defocus correction and engineering purposes. The instrument is now successfully
operational on-sky in closed loop. The relevant data reduction and on-sky
characterization results are reported.
