This report presents a comparison between measured and calculated RCS data for a flat rectangular PEC plate standing on a fairly smooth ground surface. The purpose is to develop methods for the calculation of ground-surface interaction to meet future needs for reliable signature estimates and specifications. The ground surface is described by a large number of triangular facets to provide a deterministic coherent model. Fresnel reflection coefficients based on literature semi-empirical dielectric data for soils have been used. Calculated data have been separated into direct, double-bounce and triple bounce contributions. RCS measurements on a flat rectangular plate were performed during late autumn 2001 on a smooth ground surface of coarse wet sand. The measurements employed a CW method using X-band frequencies (8-12 GHz) and vertical polarisation. Depth resolution was achieved using Fourier transformation and software gating. Antenna height and range were varied as well as the inclination (-20 to +20°) of the plate. The comparison shows differences. The possible causes of these deviations are discussed. The results call for further development of methods for measurements as well as modelling.