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).