How does the Instrument Landing System (ILS) work?

ILS provides precision guidance for landing in low visibility. It consists of: Localizer - VHF transmitter at runway end providing lateral guidance (left/right of centerline); Glideslope - UHF transmitter beside runway providing vertical guidance (typically 3-degree descent path); and Marker beacons - indicate distance from runway. Aircraft receivers display deviation from correct path. ILS categories: CAT I (200 ft decision height, 550m visibility), CAT II (100 ft, 350m), CAT III (lower or no limits with autoland). ILS has been the standard precision approach system for decades; it is being supplemented by GPS-based approaches (GLS).