Accurate Estimation of ILS Glideslope in the Presence of Terrain Irregularities

Abstract

The primary emphasis of this paper is to evolve a general analytical method for the accurate prediction of glideslope aberrations in aircraft instrument landing systems (ILS) in the presence of known terrain features. Methods of mathematical modelling for electric fields in the presence of arbitrary reflecting boundaries are critically analysed, and graded according to their applicability to the evaluation of glideslopes in ILS. Using a multiple plate model of the terrain, a series of computer programs is developed to compute glideslopes using the Uniform Theory of Diffraction (UTD) and, for the first time, Uniform Asymptotic Theory (UAT). The programs are most general in terms of handling combinations of rays of various orders. An innovative algorithm for direct determination of the point of diffraction at an edge is incorporated. Results have been generated for several airports in India and those pertaining to Madras airport are presented in this paper. The results, computed for conditions corresponding to actual flight tests, are in excellent agreement with flight measured data. This establishes the model developed here as a practical and economical method of accurately predicting site behaviour of proposed ILS glideslope equipment.