Most of the rough surfaces are generally multiscale in nature as observed by radar even at narrow-band. In this paper, we apply the modulation concept to study the characteristics of backscattering from such a multiscale surface, using the IEM scattering model. The modulated surface is special kind of multiscale surface in that a random surface is superimposed upon a periodic type surface. The modulation is rationed from the fact the measured correlation function usually exhibits zero-crossing which equally states the fact that the maxima of surface spectra is shifted toward the higher wavenumber. The amount of the frequency shift depends on the modulation wavenumber. It changes the spectral decay rates behind the peak valued. The normal incidence angle can be significantly shifted as well. It is also found the maximum modulation wavenumber is related by the inverse of base-band correlation length. In all cases studied, it was found that the modulation effects significantly changes the backscattering behavior, both its angular trend and level, and is polarization dependent. It is inferred from this study that the actual surface correlation length is very difficult to measure and may not be defined by a single parameter. In fact, radar can only observe effective correlation length under an operation frequency, which in turns depends on modulation wavenumber. In particular, in terms of retrieval, the correlation length measured by the radar and that by field measured may have quite different meanings.
