How Have Landing Aids Evolved Over The Years?

Light and radio systems were developed early on in aviation, and have become the ILS systems used today.


Photo: Max Delitsyn / Shutterstock

Right from the start of aviation, it was important to easily and accurately land an aircraft. This not only had vital safety implications, but it also boosted reliable and profitable operations. The development of various light systems and radio-based aids has led to the ILS systems that are standard and well-used today.

Landing in the early days of aviation

Airports and landings were very different in the earliest days of aviation. Aircraft used grass fields, often with multiple landing directions possible. Flight would usually be in daylight hours, and there was, of course, little other traffic to avoid or communicate with. Nevertheless, technology to assist with navigation and landing was quickly developed.

Approach lighting was the first landing aid introduced. Landing fields started to be marked with bright, often rotating, lights, so they could be found in the dark. In the early 1930s, lights were introduced that could show the pilot whether the aircraft was on the correct approach path or glide slope. Similar systems remain in use today, with runway lights and the precision approach path indicator (PAPI) system.

With the switch from grass landing fields to fixed runways, lights to guide pilots to the runway were soon developed too. The “slope-line approach system” introduced in the 1940s used two rows of lights to guide aircraft to the threshold of the runway.

Today’s systems are similar but much more accurate and usable from a greater height.

The introduction of radio navigation aids

Radio for navigation developed alongside the use of lights in the 1930s. The importance of locating an airfield and landing safely in poor weather or low visibility was quickly realized. Early radio-based aids had been experimented with during the First World War, but it was the importance of reliable mail services that really drove the development of such aids.

The first radio-based system to be developed was the so-called four-course radio range. This system used a series of morse code signals broadcast from different radio towers. The strength of the received morse code signals would indicate the distance from a tower, and intersection signals could be used to define a flight path. These systems worked well but had a very limited range and were subject to interference and distortion.

There were several other variations and developments based on similar, low-frequency radio signals. One of the most successful was the European-developed Lorenz beam, first used in Germany in the mid-1930s. This runway approach system used three towers broadcasting morse code-type signals on the same frequency. Receiving “dots” or “dashes” would indicate the aircraft was to the side of the centerline, and an intersecting tone signal would indicate a central approach. Further improvements came with the development of higher frequency signals, giving greater range and more ability to determine flight paths.

The Instrument Landing Systems (ILS)

These early morse code and Lorenz beams systems developed into the Instrument Landing Systems (ILS), still based on radio signals. This system is made up of two main components - the Localizer and the Glide Path or Glide Slope. The localizer guides the pilot and the aircraft in the lateral plane, while the glide slope gives vertical trajectory guidance. Frequencies are defined and fixed by ICAO in the VHF (Very High Frequency) and UHF (Ultra High Frequency) bands.

The first landing in the US using an ILS system is claimed to be in 1938. There have been improvements, of course, since, but the essential operation remains the same, based on the intersection and difference between two radio signals (now known as difference in depth of modulation).

Note that these ILS systems were developed separately from radar – and earlier. Radar technology was really developed during the Second World War and served more as a tool for tracking and separating aircraft than for onboard guidance (as it still does today).

ILS and “auto land.”

With the sophistication of navigation aids and autopilot systems today, most modern airliners can land automatically. This may not be the standard operating procedure for most airlines, but it is possible and used in certain situations.

Development of auto-land began soon after ILS was available. Not surprisingly, given its bad weather and common fog conditions, the early development of this was led by the UK (starting with military work from 1945 with the Blind Landing Experimental Unit). Auto-land systems today make use of ILS, as well as other systems, including the radio altimeter, auto-thrusts, and multiple independent auto-pilots.

New developments and satellite navigation

While ILS remains standard and well in use, other systems have been developed. Microwave landing systems (MLS) were developed in the 1980s and used a much higher frequency. These systems were a popular concept - they would allow more flexible approaches, with changes based on conditions, aircraft type, or to minimize noise disruption. MLS did not gain traction, though, with other developments taking the lead.

Satellite technology and GPS today work alongside traditional ILS systems. In the US, for example, the Wide Area Augmentation System (WAAS) has been developed, and Europe has implemented the European Geostationary Navigation Overlay Service (EGNOS).

These are augmentation-based systems - ICAO refers to them as Ground-Based Augmentation Systems (GBAS). GPS satellite data is used to track aircraft at all stages of the flight and to allow precision landing. Ultimately, this could replace ILS and allow landing at any airport without specific ground equipment.