  The Characterising Exoplanet Satellite (CHEOPS) is a space mission designed
to perform photometric observations of bright stars to obtain precise radii
measurements of transiting planets. The high-precision photometry of CHEOPS
relies on careful on-ground calibration of its payload. For that purpose,
intensive pre-launch campaigns of measurements were carried out to calibrate
the instrument and characterise its photometric performances. We report on main
results of these campaigns, provide a complete analysis of data sets and
estimate in-flight photometric performance by mean of end-to-end simulation.
The on-ground photometric stability of the instrument is found to be of the
order of 15 parts per million over 5 hours. Our end-to-end simulation shows
that measurements of planet-to-star radii ratio with CHEOPS can be determined
with a precision of 2% for a Neptune-size planet transiting a K-dwarf star and
5% for an Earth-size planet orbiting a Sun-like star. It corresponds to
signal-to-noise ratios on the transit depths of 25 and 10 respectively,
allowing the characterisation and detection of these planets. The pre-launch
CHEOPS performances are shown to be compliant with the mission requirements.
