Compact Antenna Test-Ranges (CATR) represent a high standard of state-of-the-art test facilities with a fast and real time measurement capability up to the sub-millimeter range. Future scientific observation instruments of ESA such as the Planck and Herschel telescopes are working within this frequency ranges and require a high measurement accuracy for large antenna apertures. This present paper shows the first modelling of a large CATR at millimetre frequency. The software used in this paper is GRASP8 (from TICRA), and specially the recent Multi-Reflector Geometrical Theory of Diffraction upgrade (MR-GTD thereafter).