A Physical Theory of Diffraction Model for Predicting the Effects of Multipath on ILS and VOR Performance

Simbo Odunaiya

Abstract:	Since the Inception of the Instrument Landing System (ILS) in 1939 and the introduction of the VHF Omnirange (VOR) into the Federal Airways System in 1946, both systems have become the mainstay for navigation and precision approach to landing. Presently there are more than 1000 ILS installations and over 2000 VORs serving the flying public at airports throughout the United States and the rest of the world. While the National Airspace System (NAS) Navigation and Landing Transition Plan expects a final phase-out of these systems in 2010, it is likely that these systems will remain operational for as long as 15 to 20 years or more. The ILS will serve well as a backup system to GPS precision approaches at commercial jet terminals, and the VOR can do the same for enroute navigation. For this reason research must continue on the means of getting the best performance out of the ILS and the VOR so that they meet the future needs of contemporary navigation and precision approach guidance. One common factor that affects the precision of both the VOR and ILS information is the presence of multipath. In fact, multipath signals adversely affect the performance of most types of radio navigation systems. The ILS localizer and the VOR are especially susceptible to lateral multipath signals from large structures and power lines in the vicinity of the systems, while the ILS glide slope is highly susceptible to multipath from irregular terrain in front of the system antenna. The performance of these navigation and landing systems can sometimes be affected to the extent that they operate in out-of-tolerance conditions. In situations like these, a performance prediction model is required to evaluate the proper operation of these systems in areas where mulitpath is of major concern. Without such a model, a technique of trial and error wherein the system is first put in place and flight checks are performed to evaluate the system performance must be used. Needless to say, this technique is expensive and could result in excessive costs when the systems can not be adjusted to give acceptable performance in a particular operational environment. However, once the effect of multipath is quantified, there are ways in which these problems can be minimized. This paper presents the development of a state-of-the-art mathematical model which can predict the performance of Navigation and Landing Systems, e.g. ILS and VOR, when affected by lateral multipath. This mathematical model is based on the Physical Theory of Diffraction (PTD), and will eventually become part of a larger model that includes the effects of longitudinal multipath on the ILS and the VOR. In addition, the paper presents the theory and some model validations for both the ILS and the VOR.
Published in:	Proceedings of the 1999 National Technical Meeting of The Institute of Navigation January 25 - 27, 1999 Catamaran Resort Hotel San Diego, CA
Pages:	435 - 446
Cite this article:	Odunaiya, Simbo, "A Physical Theory of Diffraction Model for Predicting the Effects of Multipath on ILS and VOR Performance," Proceedings of the 1999 National Technical Meeting of The Institute of Navigation, San Diego, CA, January 1999, pp. 435-446.
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