The Meteosat Third Generation (MTG) series of future European geostationary meteorological satellites consists of two types of satellites, the imaging satellites (MTG-I) and the sounding satellites (MTG-S). The Infrared Sounder (IRS) is one of the two instruments hosted on board the MTG-S satellites. The scope of the IRS mission is to provide the user community with information on time evolution of humidity and temperature distribution, as function of latitude, longitude and altitude. Regarding time and space sampling, the entire Earth disk will be covered, with particular focus on Europe, which will be revisited every 30 minutes. This paper presents a synthetic overview of the mission and the instrument, and will go through the level 1 processing chain which takes instrument raw data to obtain spectrally and radiometrically calibrated and geolocalised radiances, called level 1b products. A discussion will be presented around the radiances uniformisation in space, spectral range and time and its impact for the users community.