IRES is a two axis infrared Earth horizon sensor for accurate measurement of pitch and roll attitude angles with respect to the Earth disk centre. It is used in three-axes stabilised spacecrafts operating in GEO (Geostationary Earth Orbit) and MEO (Medium Earth Orbit). Due to its extended acquisition and operational capabilities, IRES allows meeting transfer orbit operations of different launchers. The operating principle is based on electro-mechanical modulation of the radiation coming from the Earth horizon in the 14-16.25μm band. Four IR (Infrared) beams in a single telescope swung by a mirror along a scan path of 45° Earth latitude North and South, generates Earth/Space and Space/Earth pulses. These are compared in phase with the internal encoder reference to derive Pitch and Roll measurements. IRES consists of an optical head packaged with the processing electronics in a single housing for the two axis measurement. Pitch and Roll are computed inside the sensor by means of a dedicated ASIC.