ILS — Instrument Landing System

How ILS Guides an Aircraft

An Instrument Landing System combines two separate radio beams to guide an aircraft precisely down to a runway when visibility is too poor for a visual approach. The localizer provides lateral guidance — it tells the aircraft whether it's left, right, or centered on the extended runway centerline. The glideslope provides vertical guidance — it tells the aircraft whether it's above, below, or on the correct descent path, typically a 3-degree angle down to the touchdown point. Together, lateral and vertical guidance let an aircraft (or its autopilot, in an autoland) fly a precise three-dimensional path to the runway using radio signals alone, without the crew needing to see it until very late in the approach.

Frequency and Signal Structure

The localizer transmits on frequencies from 108.10 to 111.95 MHz, using only the odd-tenths channels within that range (108.10, 108.15, 108.30, and so on) — a deliberate interleaving that lets the adjacent VOR band share the same frequency range without the two systems interfering with each other. Both the localizer and glideslope work on the same underlying principle: two overlapping signal lobes, modulated at 90 Hz and 150 Hz, that are received in exactly equal strength only when the aircraft sits precisely on the correct course or glidepath — any deviation shifts the balance between the two tones, which the aircraft's receiver converts into the needle deflection (or flight-director guidance) the crew sees in the cockpit.

ILS Categories and Minima

Not every ILS installation — and not every aircraft or crew — is certified to the same minimum weather conditions. Category I is the baseline: a decision height of 200 feet and a runway visual range (RVR) of 2,400 feet, reducible to 1,800 feet RVR with touchdown-zone and centerline lighting or an autopilot/flight-director/HUD system. Category II lowers that to a 100-foot decision height and 1,200 feet RVR (reducible to 1,000 feet with autoland or HUD). Category IIIa permits no decision height, or one below 100 feet, down to 700 feet RVR. Category IIIb goes further still — no decision height, or below 50 feet, down to as little as 150 feet RVR. Category IIIc — full autoland with no RVR limitation at all — exists in the standard but is essentially unused operationally, since taxiing safely in zero visibility after landing remains the practical limiting factor. Higher categories require correspondingly more capable ground equipment, aircraft systems and crew certification — a CAT I-only runway simply cannot support a CAT III approach no matter how equipped the aircraft is.

What Happens Above Minima

If an aircraft reaches its decision height (or decision altitude) without the required visual reference to continue, the crew flies a missed approach — a published go-around procedure that climbs the aircraft away from terrain and back to a safe point to either try again or divert. This isn't a rare emergency procedure; it's a routine, expected outcome that ILS approaches are specifically designed around, and crews brief for it on every approach regardless of how likely it actually is.

ILS Alongside Satellite-Based Approaches

Satellite-based approach procedures (RNP, GPS-based approaches) have increasingly supplemented ILS at many airports, offering precision guidance without requiring the same ground infrastructure — useful at runways or airports where installing a full ILS isn't practical. ILS nonetheless remains the dominant precision-approach standard worldwide, particularly for CAT II/III operations at major hub airports, precisely because of its long operational track record and the deep infrastructure already built around it.