The Gaia-ESO Survey: processing of the FLAMES-UVES spectra
  The Gaia-ESO Survey is a large public spectroscopic survey that aims to
derive radial velocities and fundamental parameters of about 10^5 Milky Way
stars in the field and in clusters. Observations are carried out with the
multi-object optical spectrograph FLAMES, using simultaneously the medium
resolution (R~20,000) GIRAFFE spectrograph and the high resolution (R~47,000)
UVES spectrograph. In this paper, we describe the methods and the software used
for the data reduction, the derivation of the radial velocities, and the
quality control of the FLAMES-UVES spectra. Data reduction has been performed
using a workflow specifically developed for this project. This workflow runs
the ESO public pipeline optimizing the data reduction for the Gaia-ESO Survey,
performs automatically sky subtraction, barycentric correction and
normalisation, and calculates radial velocities and a first guess of the
rotational velocities. The quality control is performed using the output
parameters from the ESO pipeline, by a visual inspection of the spectra and by
the analysis of the signal-to-noise ratio of the spectra.
  Using the observations of the first 18 months, specifically targets observed
multiple times at different epochs, stars observed with both GIRAFFE and UVES,
and observations of radial velocity standards, we estimated the precision and
the accuracy of the radial velocities. The statistical error on the radial
velocities is sigma~0.4 km s^-1 and is mainly due to uncertainties in the zero
point of the wavelength calibration. However, we found a systematic bias with
respect to the GIRAFFE spectra (~0.9 km s^-1) and to the radial velocities of
the standard stars (~0.5 kms^-1) retrieved from the literature. This bias will
be corrected in the future data releases, when a common zero point for all the
setups and instruments used for the survey will be established.
