Radio Waves to Runway: How ILS Works

A thick layer of clouds had swallowed the airport. To the pilots, there was no visual reference to the runway ahead. The aircraft continued its descent, engines roaring, ailerons making diminutive corrections, and flaps extending steadily. The pilots remained fixated at their instruments, maintaining a flight path that, to the human eye, did not exist. Suddenly, the clouds dissipated, and the runway lights emerged out of nowhere- exactly where the instruments had predicted they would be.

The technology responsible for a zero-visibility landing such as this one is the Instrument Landing System, or ILS for short.

The ILS is a precision approach aid based on two radio signals, which provide the pilots with lateral and vertical guidance. A standard ILS approach follows a 3-degree glideslope, which creates a controlled and manageable descent towards the runway.

Localiser

The localiser (LOC) provides lateral guidance, thus indicating whether the aircraft is to the left or right of the runway centre line. It is a ground-based navigational aid that transmits VHF beams. The localiser's antennas are located beyond the far end of the runway, transmitting a signal along the extended centre line.
On the cockpit display, the pilot sees a localiser needle. When it is centred, the aircraft is aligned with the runway.

Glideslope

The glideslope (GS) provides vertical guidance and normally establishes the aircraft on a 3-degree descent path. Its antenna is positioned beside the runway, near the touchdown zone.
Just like the localiser, the glideslope is displayed as an indicator in the cockpit. If centred, the aircraft is on the correct vertical path.

Radio Signals

ILS signals use two modulation frequencies- 90Hz and 150Hz. The aircraft's ILS receiver compares the relative strength of these two signals to determine its position relative to the localiser or the glideslope. The result is reflected on cockpit indicators, thus allowing pilots to make continuous corrections and adjustments.

ILS Identifiers and Frequencies

ILS localisers operate in the 108.1 - 111.95 MHz frequency range. Each ILS also transmits a morse-code identifier, allowing pilots to verify that they have tuned the correct navigational aid. This is essential when multiple approaches and navigational facilities are operating in the same area.

ILS Categories

ILS approaches are categorised as CAT I, CAT II, or CAT III, primarily depending on DH (decision height) and RVR (Runway Visual Range). The decision height is the height at which pilots must decide whether to continue the approach. Different categories of ILS have different DHs. 

From the cockpit, an ILS approach can look deceptively simple- two needles, an impending runway, and a steady descent. Behind those indicators, however, lies an incredibly intricate and organised system of antennas, radio signals, precise calculations, and clear communication that enables an aircraft to fly through zero-visibility conditions safely and steadily, reflecting the remarkable precision that epitomises modern aviation.