ILS Localizer Operation Beyond Line-of-Sight

Richard H. McFarland

Abstract:	Many believe that VHF signals only propagate line of sight. It turns out because of this mistaken concept some specifications have been written by the Government which mandate that placement of VHF transmitting facilities be located so as to provide unobstructed line of sight with the user. The unfortunate consequence is that engineers referring to the specifications end up producing designs which have large, and unnecessary costs. In one case these costs amounted to over 1 million dollars. This paper specifically addresses the contemporary Instrument Landing System Very High Frequency Localizer performance and needs. The Federal Aviation Administration issues specifications for siting these aircraft navigation aids that are common to all major airports in the United States and all over the world. Great care must, of course, be taken to insure that signals are adequate to provide safety of aircraft operations. Historically over specification has been common. Money has been spent unnecessarily which seemed to be more acceptable in the past than it is today. Today’s tight budget constraints make it crucial that only needed site preparation be accomplished. One intent of this paper is to help engineers focus on the important aspects of site designs for localizers. Many of the new localizers now are destined for non-federal sites and this makes it even more critical to have minimum site preparation costs. Some FAA regional engineers are inexperienced; they turn to their published specifications which are inconsistent with contemporary knowledge, and end up coercing engineering design frms working for local government entities to over design. Data are presented showing localizer performance when a mountain exists between the localizer and the user aircraft. Discussions of localizer performance when the transmitting antennas are below a hump in the runway, are behind a diffracting screen, or below ground level of the runway. The ILS localizer is a safe, reliable means for an aircraft to land at some airports without the pilot having to see the runway. Good propagation of these VHF localizer signals is crucial for these low-visibility approaches to be accomplished successfully. Understanding the limits is important.
Published in:	Proceedings of the 1997 National Technical Meeting of The Institute of Navigation January 14 - 16, 1997 Loews Santa Monica Hotel Santa Monica, CA
Pages:	129 - 138
Cite this article:	McFarland, Richard H., "ILS Localizer Operation Beyond Line-of-Sight," Proceedings of the 1997 National Technical Meeting of The Institute of Navigation, Santa Monica, CA, January 1997, pp. 129-138.
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