ILS localiser coverage and the protected zone

The localiser is part of an Instrument Landing System (ILS) and is used by pilots to guide an aircraft during final approach. It provides lateral guidance helping to align an aircraft with the runway centreline.

Localisers emit a highly directional radio signal transmitted from a fixed antenna system. This video shows a sample profile of the pattern emitted by the localiser which envelopes a defined protected zone.

This protected zone commonly defines an area of ±10° at a range of 25 nautical miles (NM) and ±35° at a range of 17 NM, guaranteeing an area which is free from lateral lobe interference of side lobes. ILS guidance should not be enabled outside of this zone.

Also shown is the area of reduced localiser coverage, which helps reduce signal reflections from nearby buildings. This begins at ±15° at a range of 17 NM and raises the height above threshold (HAT) from 2,000ft up to 4,500ft at the ±35° limit. A HAT of 1,000ft above the elevation of the highest obstacle is used when required.

Although these shapes would not normally be visible when using a training simulator, it can be useful for visualising the real-world scale of the signal and also for validating that flight instruments are correctly calibrated.

Learn more about the realtime flight simulator project built with Unreal Engine.

Note that this interactive realtime model shows a general overview of localiser coverage, rather than a specific airport setup. Individual installations will typically be determined by local authorities.

An area of London is displayed only to provide a general sense of scale.

Surface textures copyright Mapbox and OpenStreetMap.