Tools for electromagnetic simulation, such as SPECRAY EM / FERMAT, make RCS and ISAR image computations down to millimetre wavelength possible from the CAD of targets. The higher the frequency is, the greater the influence of the precision and the realism of the CAD models on results are. In millimetre wavelength, whatever the precision of the CAD, the roughness of surface and the flatness of sheets of construction of terrestrial military vehicles cannot be reproduced by the geometry. Nevertheless, a realistic prediction of RCS can be carried out by taking into account the statistical surface qualities, which are superposed, on the deterministic effects of the geometry of the CAD. The calculations carried out on the CAD 600 and 80 kfacets of the ZSU 23-4 illustrate these effects, without degrading the efficiency of computations.