The Instrument Landing System (ILS) is an instrument approach system for landing in instrument meteorological conditions. It serves the terminal segment of the air navigation system.

Components:

1. Localiser (LOC)

Informs the pilot with the airplane’s lateral position relative to runway centreline. Localiser signals are transmitted from beyond the departure end of the runway with a horizontal antenna array. Course width is adjusted to provide full scale deflection left or right at 350 feet off centerline when over the approach end of the runway. Course width thus varies with runway length, but nominally is 2.5 degrees each side. It operates on frequences 108.0 to 111.95 MHz with the channel width of 50kHz.

2. Glideslope (GS)

Informs the pilot with the airplane’s vertical position relative to the ideal approach path. Glideslope antenna is located off one side of the runway, approximately abeam the touchdown point (typically 1000 feet past the approach end of the runway.) Typical glideslope signal has a 3 degree slope. The operating frequences reach from 329.15 to 335.0 MHz with the channel width being 150 kHz.

3. Outer marker 

Situated on the same line with the localiser and the runway centerline, four to seven miles before the runway. Typically located about 1 nautical mile inside the point where the glideslope intercepts the intermediate altitude. Transmit low powered (3 watt), 400 Hz tone signal on a 75 MHz carrier frequency. The antenna is highly directional, and is pointed straight up. The valid signal area is 2,400 ft. by 4,200 ft. ellipse (as measured 1000 ft. above the antenna.) When the airplane passes over the outer marker antenna, its marker beacon receiver detects the signal. This is shown for the pilot by blinking the blue outer marker light of the receiver, and by a series of audio tone morse-code 'dashes' in his headset. Some countries, such as Canada, have abandoned marker beacons completely, replacing the outer marker with an NDB. In the United States, the outer marker will often be combined with an NDB to make a Locator Outer Marker (LOM). Some ILS approaches have no navigation aid at all situated at the outer marker, but use other means, such as radial intersections or distance measuring equipment (DME) fixes, to identify the position.

4. Middle marker 

Functions as the outer marker, positioned 0.5 to 0.8 miles before the runway. When the airplane is above the middle marker, the receiver’s yellow middle marker light starts blinking, and a repeating pattern of audio morse code dot-dashes in the headset. This is intended as a 'wake up call' for the pilot, to alert him that he has already passed the missed approach point (typically 200 feet altitude on the glideslope) and should have already initiated the missed approach if the runway is not in sight. Most ILS approaches do not have a middle marker.

5. Inner marker 

Similar to the outer and middle markers; located at the beginning of the runway on some ILS approach systems (Category II and III) having decision heights of less than 200 feet AGL. Triggers a flashing white light on the same marker beacon receiver used for the outer and middle markers; also a series of audio tone 'dots' sound in the headset. These are common only at the largest airports, and not in all countries.

6. Approach lights 

Most installations include medium- or high intensity approach light systems. The approach light system (abbreviated ALS) assists the pilot in transitioning from instrument to visual flight, and to align the aircraft visually with the runway centerline.

A standard ILS is termed "Category I", allowing landings for suitably equipped aircraft in weather as low as 550 meter visibility and 200-ft ceiling (cloud base or vertical visibility). More advanced Category II and III systems allow operations in near-zero visibility, but require special additional certification of the aircraft and of the pilot. Category II approaches permit landing with a 100 foot decision height and visibility as low as 300 meter. Category III is flown by an autoland system on board the landing aircraft, and permit operations even with no decision heights and visibility of 75 meter or less. Each operator certified for Cat III operations will have specific decision heights and visibility minima established which are unique to their certification. Some operators are authorized to land in zero/zero conditions (Cat IIIc). Category II/III installations include in-runway centreline and touchdown zone lighting, as well as other aids and enhancements.

A limitation of ILS is its sensitivity to obstructions in the signal broadcast area. In the 1970s there was a major US & European effort to establish MLS Microwave Landing Systems , which are not similarly limited and which allow curved approaches. However, a combination of slow development and airline reluctance to invest in MLS has resulted in its failure to be widely adopted.

Procedures based on the extreme three-dimensional accuracy of the GPS Global Positioning System are now being developed and implemented. It is widely expected that eventually these procedures, with related ground-based enhancements, will replace ILS, though it will still be necessary to keep some ILS facilities as backups in case the GPS constellation is shut down by the U.S. military.