We propose a simple technique to estimate a roughness parameter, RMS gradient, of a planetary surface. The technique is practiced on A-scope data of radio sounding observation of planet with a simple formula. The formula was derived analytically for the case of Gaussian random rough surface using a geometrical optics model. The technique was examined by simulations of planetary sounding observations which is based on physical optics, and demonstrated to estimate the RMS gradient of the planetary surface in the range from some fraction of degrees to about ten degrees. Finally, the upper limit of valid range of RMS gradient was formulized as a function of observation altitude and the maximum detection range.