To date, more than 4000 extrasolar planets, or exoplanets, have been discovered using different detection techniques, including the transit photometry. This method measures the dimming of the light coming from the host star when the planet passes in front of it. The Characterising Exoplanet Satellite (CHEOPS) is a space mission designed to perform photometric observations of bright stars to obtain precise radii measurements of small transiting planets. To achieve such performances, CHEOPS must be well characterised and, for this purpose, an intense on-ground calibration campaign was run on the fully-assembled instrument. In this thesis, I present the different components specifically developed for the characterisation and calibration testing of CHEOPS. I detail several key results obtained from these measurements that suggest CHEOPS is meeting all its requirements. This work contributed to the project along various axes that are critical to understand the data and to achieve optimal scientific performances.