How the Instrument Landing System (ILS) Uses Localizer and Glideslope Signals to Guide Aircraft

The critical juncture of any low-visibility flight occurs at the decision height—a specific altitude where the flight crew must either establish visual contact with the runway environment or immediately abort the landing. For a standard Category I approach, this threshold sits at 200 feet above the ground. Reaching this exact point in space, often through dense cloud cover or heavy precipitation, relies entirely on the Instrument Landing System (ILS). Rather than relying on satellite positioning, the ILS uses ground-based radio frequency beams to construct a rigid, invisible corridor in the sky, guiding the aircraft down a precise geometric path until the runway lights pierce the gloom.[1][5]

The architecture of the ILS is divided into two distinct signal paths: the localizer for lateral alignment and the glideslope for vertical descent. The localizer antenna array sits beyond the departure end of the runway, projecting a signal outward along the extended centerline. It transmits on a VHF frequency between 108.1 and 111.95 MHz. This transmission is not a single beam, but two overlapping radio lobes. One lobe is modulated at 90 Hz and the other at 150 Hz.[5]

As an aircraft intercepts the localizer, its onboard receivers measure the relative strength of these two modulations. If the 90 Hz signal is stronger, the aircraft is drifting left of the centerline; if the 150 Hz signal dominates, the aircraft is to the right. When the receiver detects an equal strength of both frequencies, the aircraft is perfectly aligned with the runway. This overlap creates a horizontal corridor that is typically 700 feet wide at the runway threshold, narrowing continuously as the aircraft gets closer.[4]

Vertical guidance operates on the exact same principle, turned on its side. The glideslope antenna is positioned adjacent to the runway touchdown zone, typically 750 to 1,250 feet from the approach threshold. It transmits in the UHF band between 329.15 and 335 MHz, projecting a descent path that is usually angled at 3 degrees above the horizon. Like the localizer, the glideslope uses overlapping 90 Hz and 150 Hz lobes. The 90 Hz lobe sits above the desired glide path, while the 150 Hz lobe sits below it.[1]

The intersection of these vertical lobes dictates the aircraft's rate of descent. If the aircraft drops below the 3-degree path, the 150 Hz signal overpowers the 90 Hz signal, prompting the flight director or autopilot to pitch the nose up. Because the glideslope beam originates near the touchdown zone, the vertical corridor becomes highly sensitive as the aircraft nears the ground. A minor deviation that requires a slight correction at five miles out demands an aggressive maneuver at half a mile.[4][5]

This narrowing geometry explains why the decision height is the definitive node in the approach sequence. The ILS does not land the aircraft; it delivers the aircraft to a spatial window where a safe visual landing is guaranteed. The European Union Aviation Safety Agency (EASA) and the Federal Aviation Administration (FAA) strictly define these tolerances in their 2024 and 2025 regulatory frameworks. If the aircraft's receivers detect that the signal deviation exceeds safe limits before reaching the decision height, the approach must be abandoned.[1][2]

Signal integrity is paramount to this infrastructure. Because the ILS relies on line-of-sight radio waves, the beams can be distorted by physical obstacles. Large aircraft taxiing near the transmitting antennas, or even heavy snowfall accumulating in front of the arrays, can reflect the signals and create false courses. To prevent this, air traffic control establishes critical areas around the antennas. When weather conditions deteriorate, ground movements are restricted to ensure the localizer and glideslope signals remain undisturbed.[3][5]

Despite the proliferation of GPS and satellite-based augmentation systems, the ground-based ILS remains the global standard for precision approaches. Its closed-loop architecture provides a level of redundancy that aviation regulators require. As the Federal Aviation Administration states in its system documentation, the infrastructure is "designed to provide an approach path for exact alignment and descent of an aircraft on final approach to a runway." It converts abstract radio frequencies into a physical trajectory, ensuring that when the flight crew looks up at 200 feet, the runway is exactly where the instruments promised it would be.[1][4]