Quantitative estimates of the surface roughness of planets at various spatial scales are particularly important for the develop- ment of some remote-sensing instruments. This is the case of the imaging and sounding radars operating from the Moon and Mars orbit that are sensitive to roughness from decameter to hect- ometer scales. This is also the case for laser altimeters themselves because the width and shape of the return pulse are affected by the local topography inside the spot illuminated by the laser, which typically has a diameter of a few decameters.