The ILS is based on state-of-the-art technology that was developed in the 1930s, well before World War II broke out. It is still considered by pilots to be the best approach navigational aid available to date. But what does ILS stand for in aviation? The acronym stands for Instrument Landing System. It is a type of radio navigation system that provides precise guidance to aircraft approaching a runway.
ILS provides both horizontal and vertical guidance, helping pilots land at an airport with such accuracy that they don’t even need to look at the runway.
If you are interested in knowing what is ILS in aviation stick around.
The ILS Operating Principle
To operate smoothly, ILS needs several components to help guide the pilot. They include:
The Localizer
The localizer antenna is situated at the approach end of the runway. It sends out two signals in a lateral direction. Both signals produce lobe-like waves, with one lobe on the right side of the runway’s centre line and another on the left. The lobes overlap in the middle of the runway, directing planes as they get close to the runway in preparation for landing.
A frequency of 150 Hz is modulated into the right lobe and 90 Hz into the left lobe to distinguish between the two. The aircraft’s in-flight ILS receiver can then determine the lobe it is currently flying over.
As a plane veers away from the middle of the runway, the signal amplitude, or depth of modulation (DOM), rises. If the plane is flying in a leftward direction relative to the centreline, for example, it will pick up more of the 90 Hz signal and less of the 150 Hz signal. The discrepancy is transformed into an angular displacement by the aircraft instrument landing system, which is called the difference in depth of modulation (DDM). If the pilot sees this, it means they need to change their path to the right.
The Glide Slope
An antenna or transmitter for the glide slope is set up on the runway’s edge, about 300 metres (or 1000 feet) from the runway’s threshold. Typically, the glideslope shed will be located near the runway’s aim point markings, over 120 metres from the runway’s edge.
There are two lobes to it as well, transmitting perpendicularly to the runway. Similar to the localizer, the top beam is modulated at 90 Hz and the bottom beam at 150 Hz. This shows where the plane is on the proper glide path: where the lobes meet. The standard drop angle for gliding slopes is 3 degrees.
Using the detected DDM, the aircraft ILS system determines its position in relation to the calibrated glide. Indicators tell pilots to descend if they’re too high and to climb if they’re too low on the glide.
Approach Lighting
When flying in poor light, pilots may use the approach light system (ALS) to better see the runway and surrounding area. Its primary function is to assist pilots in determining the centreline of the runway and ease pilots’ transition from instrument to visual flight.
Marker Beacons
The first version of the ILS was created before the widespread availability of DME. This is why ILS approaches sometimes make use of marker beacons. With an audible tone and/or a visible light that lights in the cockpit, each beacon denotes a particular point on the approach.
Depending on the approach, you could see one, two, or perhaps all three types of marker beacons:
- Outer Marker: Lights up blue to indicate glideslope intercept or the Final Approach Fix.
- Middle Marker: Signals decision height with an amber light flash.
- Inner Marker: Determines the decision-making threshold for a CAT II ILS.
Classifications Used by the ILS System
Different landing categories are used in the ILS system according to Decision Height and Runway Visual Range (RVR). Most planes can land up to a Runway Visual Range of 550m and beyond thanks to the ILS systems installed at some airports, which fulfil the Category (CAT I) level requirement. However, when the weather conditions are severe and visibility is less than 550 meters landing becomes too dangerous. To guarantee safe flights in low visibility and bad weather, the International Civil Aviation Organization (ICAO) established Category II and Category III operations.
- Category I (CAT I) Operation: For a precise instrument approach and landing, the decision height (DH) must not be less than 60 meters (200 feet), and there must be at least 800 meters of visibility or 550 meters of runway visual range.
- Category II (CAT II) Operation: Is a precision instrument approach and landing system that requires a Runway Visual Range (RVR) of at least 300 meters and a Decision Height (DH) between 30m (100ft) and 60m (200ft).