Half-Plane Scattering Applied to ILS Grading Criteria

Abstract

A technique for calculating the derogation in the difference in depth of modulation (DDM) signal, used for aircraft glide slope guidance, due to slightly rough ground surfaces is developed. The contributions to the scattered signal from diffuse and specular scatter are treated separately. The diffuse derogation is calculated based on empirical data, and an average variation is presented. The specular contributions to the variation in the signal is calculated based on expanding the half-plane diffraction solution in terms of the ground roughness. A specular scattered deviation about the mean signal is then obtained by statistically averaging over normally distributed ground heights. Results of the analysis indicate that substantial relaxation in grading criteria can be effected.