Instrument landing system (ILS) is a ground-based precision radio navigation system that gives pilots short-range guidance, and it is the most commonly used precision instrument approach today. Working through two primary components - the localizer and the glideslope - the system supplies the cockpit with continuous lateral and vertical information, letting crews track both the runway centerline and a steady 3 degree descent angle. By delivering this simultaneous precision lateral and vertical guidance, ILS allows aircraft to approach safely at night or in bad weather.
Expert behind this article
Jim Goodrich
Jim Goodrich is a pilot, aviation expert and founder of Tsunami Air.
What is ILS in aviation?
ILS refers to Instrument Landing System, which is a precision radio navigation system that operates as a ground-based instrument approach system and provides precision lateral and vertical guidance to an aircraft approaching and landing on a runway. ILS provides short-range guidance to aircraft to allow them to approach a runway at night or in bad weather and enables a safe landing during instrument meteorological conditions (IMC).
ILS stands for Instrument Landing System, a precision instrument approach procedure that provides short range guidance to aircraft during the final stages of flight. The Instrument Landing System sends information to instruments in the cockpit, enabling pilots to maintain a predetermined flight path from the en-route structure to the runway threshold. It combines lateral guidance from the localizer, which provides lateral course guidance, with vertical guidance from the glideslope, which offers a typical descent angle of 3 degrees. This guidance allows pilots to transition smoothly from instrument flying to visual flying as they approach the runway at night or in instrument meteorological conditions that include reduced visibility due to fog. Because ILS is the only system currently approved for Category II/III operations, it is used frequently under visual and night conditions and is generally paired with a DME to establish other fixes on the localizer course.
What is the ILS antenna?
The ILS antenna is a type of antenna used to provide precision guidance for aircraft during the landing phase of a flight. It is designed to transmit and receive signals from an aircraft's navigation system, linking the aircraft to the ILS ground station. Through these exchanges, the ILS antenna provides horizontal and vertical guidance, enabling the aircraft to detect and follow the ILS signal to a safe landing. Typically mounted on the aircraft's nose or wing, the antenna is engineered to avoid shadowing caused by structures like the vertical stabilizer or landing-gear doors. A single nav antenna almost always feeds multiple nav receivers and sometimes the glideslope as well, consolidating guidance data within the cockpit. The antenna must be electrically bonded to the airframe to suppress interference and maintain signal integrity, while its placement represents a compromise between solid mounting, aerodynamic drag, and clear line-of-sight to ground transmitters. By continuously interpreting beam pairs transmitted from the localizer and glideslope aerials, the airborne equipment provides information showing the aircraft's displacement from the runway centreline and its position relative to the ideal descent path.
What are the components of ILS in aviation?
The components of ILS in aviation are listed below.
- Inner Marker (IM) is a component of ILS for Category II and Category III
- Localizer is a component of ILS
- Outer Marker (OM) is a component of ILS
- Glide-slope is a component of ILS
The basic components of an ILS are the localizer, glide slope, Outer Marker, and Inner Marker when installed for use with Category II or Category III instrument approach procedures. Guidance information includes the localizer and glide slope, which together give vertical and horizontal guidance by sending two directional radio signals on paired VHF and UHF frequencies. Localizer transmits between 108 and 111.95 MHz on one of 40 channels, while glide slope operates on 329.15 to 335 MHz and carries two signals at 90 Hz and 150 Hz. Range information is supplied by the Outer Marker, Inner Marker, Distance Measuring Equipment, and compass locators. Marker beacons and DME let the pilot know how far the aircraft is from the runway. Visual information adds approach lights, touchdown lights, centerline lights, and runway lights to help the pilot switch from instrument to visual flying. The whole system is divided functionally into guidance information, range information, and visual information, and is generally paired with a DME for distance measuring on the same frequency pairing.
What is the difference between ILS and ALS in aviation?
The difference between ILS and ALS is that ILS is a precision radio-navigation aid that laterally and vertically guides the aircraft down to the runway whereas ALS is only a pattern of lights installed near the runway threshold. Aircraft can continue the approach to 100 ft (30.48 m) above the runway when the ILS is used and ALS provides visual reference. ALS serves a runway that has an instrument approach procedure (IAP) associated with it and ILS is often that very IAP. ILS gives continuous azimuth and glide-slope data inside the cockpit, while ALS gives no cockpit signal and is simply a configuration of signal lights placed symmetrically on both sides of the runway extended centreline starting at the landing threshold and extending outward into the approach zone. ILS remains effective in zero visibility, whereas ALS is designed to help pilots transition from instrument flying to visual flying and is useless when clouds still obscure the lights.
Approach lighting system (ALS) is a series of lightbars, strobe lights, or a combination of the two that extends outward from the runway end 1,400-1,500 feet (426.7-457.2 meters) for non-precision instrument runways and 2,400-3,000 feet (731.5-914.4 meters) for precision instrument runways, the latter equipped with ILS. ALS provides visual information about runway alignment, height perception, roll guidance, horizon references and limited distance-to-go information. ILS gives electronic steering data that makes an ALS visible from farther away. ALS types like Medium Intensity Approach Lighting System (MALS) or High Intensity ALS with Sequenced Flashers (ALSF-2) vary only in light intensity and flashers, whereas ILS categories (I, II, III) vary in decision height and runway visual range and require additional pilot training.
Jim GoodrichPilot, Airplane Broker and Founder of Tsunami Air
What is the difference between ILS and VOR in aviation?
The differences between ILS and VOR in aviation are given in the table below.
| ILS | VOR |
|---|---|
| More sensitive localizer | Less sensitive radial |
| Provides more accurate lateral | Provides horizontal guidance |
| guidance | |
| Short-range guidance | Facilitates navigation on any bearing around them |
| Provides vertical guidance | Provides only horizontal guidance |
| Localizer supports a single, | Accurate guidance by following specific radials |
| specific direction | |
| Lateral and vertical guidance | Lateral guidance only No vertical guidance |
| Precision approach | Non-precision approach |
| Can serve as a stand-alone | Acts as a backup procedure when airports do not have an ILS |
| localizer-only procedure | approach |
| Shares the same navigation radio | Uses a rotating variable signal and a stationary master signal |
| and display equipment with VOR | Operates within 108.0 to 117.95 MHz frequency band Receivers compare the difference between the VOR's variable and reference phase VOR/DME approach provides lateral guidance and range information via DME |
The difference between ILS and VOR is that the Instrument Landing System (ILS) delivers both lateral and vertical guidance, enabling a precision approach, whereas the VHF Omnidirectional Range (VOR) provides only lateral guidance, making its associated VOR/DME procedure non-precision. The ILS localizer is more sensitive than a VOR radial, so it supports a single, specific runway direction, while VORs facilitate navigation on any bearing from 0 to 359 degrees. Although the two systems share the same navigation radio and display equipment in the flight deck, the localizer signal increases in sensitivity as the aircraft approaches the runway, a feature absent in the constant-sensitivity VOR. When an airport lacks an ILS approach, the VOR/DME approach serves as a backup procedure, pilots fly it only when cleared by ATC, using DME for range information and the VOR radials for lateral guidance.
How does ILS work in aviation?
ILS works as a ground-based instrument approach system; the two separate facilities, the localizer and the glideslope, work together. The localizer sends a VHF signal between 108.1-111.95 MHz to provide lateral guidance, and the glideslope, located near the approach end of the runway, transmits a UHF signal between 329.15-335.0 MHz to provide vertical guidance information.
The pilot tunes in the correct radio frequency on the airplane's navigational radio to pick up the guidance: the localizer modulates 90-Hz and 150-Hz signals so the overlap determines which side of the localizer course the airplane is flying on, while the UHF signal is simultaneously modulated by 90-Hz and 150-Hz signals so intersecting beams define the ‘on GS’ indication.
Cockpit gauges show the pilot relevant information to stay on profile: a horizontal needle shows the relationship to the glideslope, and the VOR receiver needle rises or falls to display vertical guidance. The FMA shows LOC when the aircraft receives the localizer signal. Short-range guidance allows aircraft to approach the runway in bad weather. ILS approaches allow most general aviation pilots to land in as low as 200-foot (60.96 meters) cloud ceilings and as little as 1/2 statute mile (0.8 kilometers) visibility. The Instrument Landing System can take commercial aircraft right down to the runway provided the pilot observes the required visual cues before descending below 200 ft (60.96 meters) AGL.
The Instrument Landing System broadcasts two narrow radio beams: one gives lateral guidance, the other gives vertical guidance. An aircraft receiver compares the depth of modulation in each beam. Deviation needles in the cockpit show the pilot exactly where to adjust left, right, up, or down. By keeping the needles centered, the crew follows an invisible three-degree glide-path that ends at the runway threshold. Because the system works in fog, rain, or darkness, the dependability of the ILS is a foundation of all-weather operations.
What are ILS errors in aviation?
ILS errors include false glide slope, incorrect course, wrong frequency, and poor instrument scan. False glide slope is an error type that appears at odd multiples of the true glide-slope angle; it exists in the area of the localizer back course approach and causes the glide slope flag alarm to disappear, presenting unreliable glide slope information. Pilots encounter this reading in mountainous areas or when approaching more than 10 NM from the airport, and an erroneous glide slope capture causes the aircraft to get low on approach. Sudden glideslope fluctuation sets up loss of control at a point where there is not much room for recovery.
ILS signals are susceptible to interference by moving objects on the ground: vehicular traffic not subject to ATC causes momentary deviation to ILS course or glide slope signals, and aircraft or vehicles taxiing up to the hold-short line while another aircraft is on approach disrupt the signal's integrity, causing unreliable approach guidance. Disruption is caused by terrain, buildings, power lines, dense vegetation, or water. When an anomaly is detected, the installation is automatically switched off and inoperative flags are displayed on aircraft ILS displays. ILS critical areas are established to mitigate interference, yet these areas are only protected when a control tower is in operation.
