Radar echoes from planetary sounders often contain ambiguities between surface echoes (clutter) and subsurface reflections. Such problems severely constrain quantitative data analysis especially for rough terrains. We propose a physical optics approach to simulate planetary sounding radar surface echoes to address this specific issue. The method relies on the Huygens-Fresnel's principle which permits the recasting of Maxwell's equations in a surface integral formulation. To compute this integral, we describe the surface through a mesh composed of adjacent triangular elements for which we provide an analytical expression of the scattered electromagnetic fields. The main contribution of this work lies in the use of analytical integrals over triangular facet elements much larger than the wavelength of the electromagnetic field. Hence, the advantage of the proposed approach is its computation efficiency which reduces the computational requirements while maintaining the physical optics accuracy. This allows a systematic analysis of the continuously growing planetary sounding radar database. Equations and implementation are detailed in this paper as well as illustrations of obtained results for different instruments (namely, SHARAD and LRS). Our simulation results suggest that the method is able to accurately reproduce the observed clutter in both rough and smooth terrains of the planetary cases discussed in this paper.

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