false glide slope

A characteristic of the glide-slope portion of the ILS (instrument landing system), in which one or more false glide slopes at different angles to the horizontal occur well above the true glide slope. This is because of a radiation pattern of the antenna and the ground reflection of some of the transmitted energy, resulting in more than one overlapping lobe. The false glide slopes occur at odd multiples of the true glide-slope angle (typically 3°) (i.e., at 9° and 15°). At even multiples (6°, 12°), a centered glide-slope needle occurs, but this is because of a null signal; reverse sensing is present above and below these glide slopes. A pilot can easily recognize this false indication by the steeper-than-normal rate of descent. Pilots will not experience false glide slopes below the true glide-slope angle. Pilots can avoid encountering a false glide slope by following published approach procedures.