  The ongoing improvements of the advanced gravitational wave (GW) detectors
are setting challenging requirements on instrument calibration. We report tests
of a calibration technique, based on the well-known gravitation force, which
has been applied for the first time on a large interferometer. The results
obtained with Advanced Virgo are in good agreement with the predictions and
with the usual calibration method. This technique is expected to lead to
accurate absolute calibration at the sub-percent level in the coming years,
matching the needs of the blooming GW science.
