Calibration of the Gaia Radial Velocity Spectrometer from ground-based
  observations of candidate standard stars
  The Radial Velocity Spectrometer (RVS) on board of Gaia will perform a large
spectroscopic survey to determine the radial velocities of some 1.5x10^8 stars.
We present the status of ground-based observations of a sample of 1420
candidate standard stars designed to calibrate the RVS. Each candidate star has
to be observed several times before Gaia launch (and at least once during the
mission) to ensure that its radial velocity remains stable during the whole
mission. Observations are performed with the high-resolution spectrographs
SOPHIE, NARVAL and CORALIE, completed with archival data of the ELODIE and
HARPS instruments. The analysis shows that about 7% of the current catalogue
exhibits variations larger than the adopted threshold of 300 m/s. Consequently,
those stars should be rejected as reference targets, due to the expected
accuracy of the Gaia RVS. Emphasis is also put here on our observations of
bright asteroids to calibrate the ground-based velocities by a direct
comparison with celestial mechanics. It is shown that the radial velocity zero
points of SOPHIE, NARVAL and CORALIE are consistent with each other, within the
uncertainties. Despite some scatter, their temporal variations remain small
with respect to our adopted stability criterion.
