During the last decade, meteorology domain has lived many breakthroughs. In the field of remote sensing from space, one of the most significant advancement is in the use of Thermal Infrared sounders. Thales Alenia Space delivered sounders flagship METOP/IASI, resulting in the enhancement of meteorological events forecast and unprecedented 3D atmospheric knowledge. The next generation of European geostationary meteorological satellites, Meteosat Third Generation is currently being realized through the well-established and successful Cooperation between EUMETSAT and ESA with Thales Alenia Space being overall MTG prime contractor. MTG encompasses two kinds of satellites: an imaging mission MTG-I with the FCI imaging payload derived from the MSG-SEVIRI with a payload complement of a lightning detection system (LI as the Lightning Imager) and a sounding mission MTG-S with the payload complement including the thermal infrared sounding payload IRS (full disc coverage) and the Copernicus payload UVN/S4, a spectrometer in UV/VIS/NIR bands to retrieve air quality in Europe. Stemming from IASI, Thales Alenia Space is now developing the IRS interferometer as well as the detection and onboard processing chain. These key elements will be integrated by OHB which is the IRS instrument as well as MTG-S satellite Prime Contractor. Reaching performance from a geostationary orbit is a two-order of magnitude more stringent challenge compared to a LEO instrument. Thales Alenia Space did elaborate new and innovative technology to ensure sub nanometric interferometer metrology and thus to achieve real time on-board corrections. Two essential validation milestones have been achieved in 2019. • The interferometer engineering model was thoroughly tested successfully, especially in terms of spectral performance vs. microvibrations. • Thales Alenia Space has also tested the IRS “core spectrometer” engineering model, allowing to test the full spectrometer chain of the instrument by synchronizing the interferometer itself with the two 160x160 pixel detector arrays of the detection chain and the on-board data processing unit that is able to process the 2Gbit/s raw data flow generated by the instrument. The paper will focus on the two major outcomes of this test campaign: • The demonstration of the functionality of IRS new 3D metrology system, its associated processing and its benefits. • An overview of performances achieved during those two intensive test campaigns. In particular Spectral Response Function (SRF) knowledge/prediction, which is essential to support an unprecedented interpretation of the spectra and final products retrieval accuracy.