A new method for extracting the rms height and correlation length of sand ripples on tidal flats from multi-frequency synthetic aperture radar (SAR) data is presented. The data used in this analysis were acquired by the 3-frequency SAR on board the space shuttle Endeavor during the Spaceborne Imaging Radar-C/X-Band SAR (SIR-C/X-SAR) missions in 1994 over tidal flats in the German Bight of the North Sea that had fallen dry during ebb tide. The normalized radar cross section (NRCS) of tidal flats that are covered with sand ripples is a function of the soil moisture as well as of the surface roughness. Since the water content of the sand on tidal flats is very high, the variation of the NRCS is dominated by variations of the surface roughness. By using the Integral Equation Model (IEM) we have inverted the simultaneously acquired X-, C, and L-band radar images into maps of the rms height and the correlation length of tidal flats.