ilS

ILS (Instrument Landing System) is a radio navigation aid that provides precision guidance to aircraft approaching a runway, enabling safe landings in low‑visibility conditions. The system transmits two coherent signals: the localizer, which supplies lateral alignment with the runway centreline, and the glide slope, which provides vertical guidance through a defined descent angle, typically 3 degrees. An additional component, the marker beacon, indicates distance from the runway threshold, though many modern installations use distance measuring equipment (DME) or satellite‑based augmentation instead.

The localizer antenna array is situated at the opposite end of the runway and emits a directional pattern that the aircraft’s receiver interprets as a deviation signal, moving the cockpit indicator left or right. The glide slope antenna is positioned alongside the runway and creates a vertical signal that the pilot uses to maintain the proper descent path. Both signals are modulated at specific frequencies within the VHF band (108–112 MHz), and the aircraft’s avionics decipher them to display guidance on the instrument panel, often as a coupled ILS display.

ILS categories (CAT I, CAT II, and CAT III) define the minimum visibility and decision heights for operation, with CAT III allowing landings in near‑zero visibility using additional systems such as auto‑land. Maintenance of the system requires regular calibration to ensure accuracy, and any malfunction results in the approach being downgraded to a non‑precision or visual procedure.

While satellite‑based navigation (e.g., GPS with augmentation) is increasingly common, ILS remains a globally standardized, highly reliable method for precision approaches, especially at major airports where high traffic volumes demand consistent performance.