When cloud, rain, darkness or fog hides the runway, pilots do not simply point the aircraft toward an airport and descend until the lights appear. A precision Instrument Landing System, or ILS, can provide independent lateral and vertical radio guidance aligned with a specific runway. The technology dates from an earlier era of navigation, yet it remains fundamental at major airports because it is precise, standardised and independent of satellite navigation. The FAA states that ILS continues to support Category I, II and III operations even as performance-based navigation expands.[1]
The short answer
An ILS combines two principal ground transmitters. The localizer tells the aircraft whether it is left or right of the runway centreline. The glide slope tells it whether it is above or below the intended descent path. Cockpit receivers compare the radio signals and display the resulting guidance to the pilots or feed it to the automatic flight system. The aircraft follows the intersection of those two electronic planes toward the runway.[1][2]
The localizer provides lateral guidance
The localizer antenna is normally positioned beyond the far end of the runway so its signal projects outward along the extended centreline toward approaching aircraft. FAA material specifies ILS localizer channels in the VHF range from 108.10 to 111.95 MHz. The transmitted pattern creates a precisely defined course. Airborne equipment detects whether the aircraft is displaced to one side and shows the correction required.[1][2]
The glide slope provides vertical guidance
The glide-slope transmitter operates in UHF, from 329.15 to 335.00 MHz according to FAA documentation. Its antenna is located beside the runway near the approach end. The resulting radio pattern defines the descent path. A typical published ILS approach is close to three degrees, although the exact angle is procedure-specific. If the aircraft is above the path the indication commands descent toward it; below the path, it commands the opposite correction.[1][2]
Why two independent transmitters are used
A runway is a three-dimensional target. Centreline alignment alone does not tell pilots whether the aircraft is descending at an appropriate angle, while vertical guidance alone cannot keep the aircraft aligned laterally. Localizer and glide slope therefore solve different geometric problems. Their signals are generated independently but interpreted together in the cockpit to create a precision approach path.
What the pilots actually see
Traditional instruments used crossed needles: one for localizer and one for glide slope. Modern flight displays present equivalent deviation information electronically, often on the primary flight display. When both deviations are centred, the aircraft is on the ILS course and glide path. Flight directors can convert the deviations into pitch and roll commands, and certified autopilot systems may follow them automatically.
Capturing the localizer
Air traffic control normally vectors an arriving aircraft onto an intercept heading or the flight-management system flies a published transition. The aircraft approaches the localizer from an angle. Once the receiver senses the course moving toward centre, the flight director or autopilot can capture it and turn onto the final approach course. Intercept geometry and clearances are controlled so the aircraft joins the signal predictably.
Capturing the glide slope
The aircraft normally intercepts the glide slope from below. As it reaches the vertical path, the autopilot or pilot lowers the nose sufficiently to begin the stabilised descent. Capturing from below helps avoid misleading higher-angle signal lobes. FAA guidance specifically warns that false glide-slope signals can exist outside the intended region, which is why published procedures and interception techniques matter.[2]
Why an ILS frequency tunes two systems
Pilots select the published ILS frequency, but the localizer and glide-slope transmitters operate in different radio bands. ILS channels are paired so selecting the VHF localizer frequency causes compatible airborne equipment to use the associated UHF glide-slope channel automatically. This reduces cockpit workload and prevents arbitrary mismatching of lateral and vertical facilities.
Distance information
ILS itself fundamentally provides angular guidance rather than a continuously measured distance to touchdown. Distance may come from DME associated with the approach, GNSS position, or defined fixes. Historically, marker beacons provided position cues at points along the approach. FAA documentation notes that DME, RNAV/GPS fixes and other authorised sources can substitute for an outer marker.[2]
Approach lights are part of the system around the system
Radio guidance can bring the aircraft accurately toward the runway, but the transition to visual references is equally important. Precision runways may have approach-light systems, touchdown-zone lights, centreline lights and high-intensity runway edge lights. These are not the localizer or glide slope, but FAA guidance treats visual information as a functional component of the complete precision-approach environment.[2]
Decision altitude and decision height
On an instrument approach, the crew cannot descend indefinitely hoping to see the runway. Published minima define the point at which required visual references must be available for continuation, subject to the applicable procedure and rules. If they are not, the aircraft executes the missed approach. Lower-category operations require greater visual acquisition than properly approved higher-category operations.
Category I
Category I ILS provides precision guidance to conventional instrument minima. The exact decision altitude and runway visual range depend on the approach, aircraft, operator approval and regulatory framework. It is misleading to attach one universal visibility number to every Category I approach. Terrain, lighting, equipment and procedure design all affect published minima.
Category II and III
Category II and III operations permit approaches in progressively lower visibility when the runway, ground equipment, aircraft, crew training, maintenance programme and operator approval all satisfy the required standard. Category III is not simply “a better ILS frequency.” It is an entire operational system involving redundancy, monitoring, runway protection, lighting and certified airborne equipment.[1]
Autoland
Some transport aircraft can use multiple autopilot channels and radio-altimeter inputs to continue from ILS tracking through flare and, depending on aircraft capability, rollout guidance. Autoland is not synonymous with ILS: many ILS approaches are hand-flown, and an aircraft can use ILS guidance without conducting an automatic landing. Conversely, advanced landing systems can incorporate other approved navigation technologies.
Why pilots still monitor an autoland
Automation changes the pilot’s task from continuously manipulating the controls to supervising system performance and being prepared for defined failures. Crews confirm the correct approach is selected, monitor localizer and glide-slope capture, check aircraft configuration, verify annunciations and compare the aircraft’s path with required parameters. Automatic does not mean unattended.
Runway protection becomes critical in low visibility
Large vehicles or aircraft near ILS antennas can distort the transmitted signal. Airports therefore protect sensitive and critical areas, particularly during low-visibility operations. Taxi clearances and holding points may keep aircraft farther from the runway than passengers expect. FAA siting material notes that large structures can reflect or re-radiate localizer energy and that surfaces near glide-slope installations can create harmful reflections.[3]
Why an aircraft on the ground can disturb an approach
A metal airliner is a substantial radio reflector. If it taxis through a protected area while another aircraft is relying on precision guidance, the signal reaching the approaching receiver can be distorted. Low-visibility procedures therefore coordinate air traffic control, airport ground movement and navigation-facility protection as one system.
ILS is angular guidance
An important detail is that localizer deviation represents angle rather than a fixed number of metres. Far from the runway, a given angular error corresponds to a large lateral displacement. Close to the threshold, the same angular error represents a much smaller distance. The indication therefore becomes increasingly sensitive in terms of metres as the aircraft approaches the runway.
The localizer course is deliberately narrow
FAA aeronautical information describes localizer course width as adjusted so full-scale fly-left to full-scale fly-right corresponds to about 700 feet at the runway threshold.[4] That geometry gives pilots increasingly precise alignment as touchdown approaches. It also explains why aggressive corrections close to the runway are undesirable: small needle movement can represent a meaningful change in commanded flight path.
Why the glide slope antenna is beside the runway
FAA guidance places a typical glide-slope transmitter roughly 750 to 1,250 feet beyond the approach threshold and 250 to 650 feet to the side of the runway centreline.[2] Exact siting is engineered for the airport. The signal geometry must create the intended path while accounting for terrain, structures and radio-reflection effects.
What happens if one part fails?
Navigation facilities are monitored. If the glide slope is unavailable but the localizer remains usable, a published localizer-only procedure may sometimes be available with higher minima. If the localizer is unavailable, the ILS precision approach itself cannot provide its fundamental lateral guidance. Notices to air missions and air traffic services communicate facility status, and crews use another authorised approach or runway.
Why GPS has not made ILS obsolete
Satellite-based RNAV and augmentation systems provide highly capable approaches, but ILS remains valuable because it is an independent terrestrial system with decades of operational standardisation. The FAA explicitly states that ILS will continue providing GPS-independent Category I, II and III vertically guided services and notes that ILS remains the approved system supporting Category II/III operations in its current infrastructure.[1]
ILS versus RNAV
RNAV describes an aircraft’s ability to navigate along defined paths rather than being restricted to flying directly between ground beacons. GNSS-based approaches can provide lateral and, with suitable augmentation, vertical guidance. An ILS instead derives its final-approach guidance from runway-associated radio transmitters. Both can be precise, but their infrastructure, failure modes and operational approvals differ.
Why pilots sometimes use ILS in clear weather
An ILS is not reserved for fog. Crews may use it at night or in visual conditions to reinforce centreline and vertical-path awareness, subject to ATC clearance and procedure. FAA material specifically notes frequent use under visual and night conditions to help aircraft adhere to runway alignment.[1] The radio guidance becomes another cross-check rather than a substitute for looking outside.
The runway still has to be suitable
Installing an ILS does not automatically create unrestricted all-weather capability. Runway lighting, obstacle environment, missed-approach design, terrain, electrical resilience, facility monitoring and airport low-visibility procedures all influence what minima can be approved. Aircraft and operator capability impose additional constraints.
The human side
Low-visibility approaches are highly procedural. Crews brief the approach, confirm navigation identifiers and minima, configure the aircraft at defined points, monitor stabilised-approach criteria and know exactly what indications require a go-around. Recurrent simulator training exposes pilots to failures that would be inappropriate to practise routinely in real low visibility.
Common misconception: the ILS lands the aircraft
The ground ILS does not control the aircraft. It broadcasts guidance signals. The pilot or aircraft automatic-flight system responds to those signals. Even in an autoland, the landing capability resides in the combined airborne and ground system, with pilots monitoring it. Saying “the ILS lands the plane” hides the distinction between guidance, flight-control automation and crew responsibility.
Conclusion
An Instrument Landing System turns a runway into a precisely defined radio target. The localizer establishes lateral alignment; the glide slope establishes vertical descent; cockpit systems combine the two; airport lighting supports the visual transition; and protected areas prevent ground traffic from corrupting the signal. Its frequencies and antenna technology may look old beside satellite navigation, but its independence, precision and mature low-visibility procedures explain why ILS remains central to modern airline operations.
Sources / Technical References
- [1] Federal Aviation Administration, Instrument Landing System overview — https://www.faa.gov/about/office_org/headquarters_offices/ato/service_units/techops/navservices/gbng/ils
- [2] FAA Aeronautical Information Manual, Chapter 1, ILS — https://www.faa.gov/air_traffic/publications/atpubs/aim_html/chap1_section_1.html
- [3] FAA Procedures for Handling Airspace Matters, ILS siting/reflection considerations — https://www.faa.gov/air_traffic/publications/atpubs/pham_html/chap6_section_3.html
- [4] FAA AIP, ENR 4.1 Navigation Aids — https://www.faa.gov/air_traffic/publications/atpubs/aip_html/part2_enr_section_4.1.html
Disclaimer: This article is general aviation education, not operational guidance. Approach minima, equipment requirements and procedures vary by runway, aircraft, operator and authority. Current charts, aircraft manuals, operator procedures and ATC instructions always take precedence.
