How the Instrument Landing System Guides Planes Down
The Instrument Landing System sends two radio beams that tell a pilot whether the plane is left or right of the runway and above or below the glide path.
In this article
The Instrument Landing System, or ILS, is a pair of radio beams that tell a pilot how far left or right of a runway the aircraft is, and how far above or below the correct descent path. The pilot, or the autopilot, keeps two needles centred and the aircraft arrives at the runway even when cloud or fog hides it. It is the most common precision approach aid at large airports.
Two beams, two jobs
An ILS has two separate transmitters. The localiser gives sideways guidance. Its antennas normally stand beyond the far end of the runway and send a signal along the extended centreline. The glideslope gives up and down guidance. Its antennas stand beside the runway near the touchdown area.
SKYbrary, the aviation safety knowledge base, explains that each beam is really two overlapping beams. For the localiser, one lies slightly to the right of the centreline and one slightly to the left; they cross exactly on the centreline. For the glideslope, one lies slightly above the wanted path and one slightly below. The aircraft's receiver compares the two and shows the result on a cockpit needle. Both needles centred means the plane is on the line.
How the receiver tells left from right
Each beam carries two audio tones, 90 Hz and 150 Hz, mixed into the radio signal. Per Wikipedia's ILS article, the transmitter arranges things so that one tone is stronger on one side of the centreline and the other tone is stronger on the other side. Along the centreline they are equal.
The receiver measures the difference in how strongly each tone modulates the carrier. This is called the difference in depth of modulation. When it is zero, the needle is centred. When one tone dominates, the needle moves toward the side the pilot should fly to. Because the receiver compares the two tones rather than measuring signal strength, the reading does not depend much on distance from the antenna.
Frequencies and the glide path
Localisers use 40 channels between 108.10 and 111.95 MHz, and glideslopes use 40 paired channels in the 328.6 to 335.4 MHz range, according to Wikipedia. That article gives the glideslope band slightly differently in different places, so treat the exact edges as approximate. Each localiser channel is paired with a particular glideslope channel, so the pilot tunes one frequency and the receiver picks up both.
The usual glide path is 3 degrees above horizontal, and SKYbrary says the glideslope is normally set to put the aircraft about 50 feet above the runway threshold. Airports can choose other angles because of terrain or noise. Wikipedia gives London City Airport, which uses 5.5 degrees, as an unusually steep example.
Distance markers
Pilots also need to know how far along the approach they are. Traditionally, marker beacons did this: SKYbrary says the outer marker is typically about 5 nautical miles from touchdown and the middle marker about 1 nautical mile. Many airports now use distance measuring equipment (DME) instead, which gives a distance reading directly.
Categories: how low can you go
Not every ILS allows the same low-visibility landings. A pilot flying an approach reaches a decision height: if the runway lights are not in sight by then, the pilot must go around. Higher categories allow lower decision heights and lower visibility. The figures below are the ICAO/EASA values as summarised on Wikipedia; the FAA's wording differs slightly.
- Category I: decision height not below 200 feet, and runway visual range (how far lights can be seen along the runway) of at least 550 metres.
- Category II: decision height between 100 and 200 feet, and runway visual range of at least 300 metres.
- Category III: decision height below 100 feet or none, and runway visual range under 300 metres or none.
SKYbrary adds that Category III is split further into IIIA, IIIB and IIIC, and that IIIC, a landing with no height or visibility limit, is not routinely available, mainly because of difficulty moving around the airport afterwards. Categories II and III need special aircraft equipment, trained crews and extra airfield installations, including a backup power supply. Wikipedia notes that Category III requires an automatic landing system or an approved head-up display, and that the ground equipment must shut down faster on a fault: a Category I localiser within 10 seconds, a Category III localiser within 2.
Where it came from
Early blind-landing work appeared in the 1920s and 1930s. The Lorenz beam was first used in Berlin in 1932, and in 1929 Jimmy Doolittle flew a takeoff and landing guided entirely by instruments in the United States. Wikipedia reports that the first scheduled U.S. airliner landing using an ILS took place on 26 January 1938. After the Second World War, the new International Civil Aviation Organization chose ILS in 1947 as its first standard precision approach system, and the specification was published in 1950.
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