ILS False GP Location RNAV

All directional antenna (including the parabolic aerial used for the ILSILS —Instrument Landing System) suffer from sidelobes, an unhelpful but unavoidable phenomenon where waves are emitted in directions other than the one the designer wants the wave to go in.

One of the consequences of this is false glideslopes, where the aircraft could lock onto a signal bringing it in on much too steep a path. As all the false glideslopes are above the correct one, the way to avoid them is to intercept from below - that way, the first glideslope you hit will be the correct one.