IVAO Belgium

An instrument landing system (ILS) is a ground-based instrument approach system that provides precision guidance to an aircraft approaching and landing on a runway, using a combination of radio signals and, in many cases, high-intensity lighting arrays to enable a safe landing during instrument meteorological conditions (IMC), such as low ceilings or reduced visibility due to fog, rain, or blowing snow.

Instrument approach procedure charts are published for each ILS approach, providing pilots with the needed information to fly an ILS approach during instrument flight rule (IFR) operations, including the radio frequencies used by the ILS components or navaids and the minimum visibility requirements prescribed for the specific approach.

Radio-navigation aids must keep a certain degree of accuracy (set by international standards of CAST/ICAO); to assure this is the case, flight inspection organizations periodically check critical parameters with properly equipped aircraft to calibrate and certify ILS precision.

Principle of operation

An ILS consists of two independent sub-systems, one providing lateral guidance (localizer), the other vertical guidance (glide slope or glide path) to aircraft approaching a runway. Aircraft guidance is provided by the ILS receivers in the aircraft by performing a modulation depth comparison.

A localizer (LOC, or LLZ until ICAO designated LOC as the official acronym) antenna array is normally located beyond the departure end of the runway and generally consists of several pairs of directional antennas. Two signals are transmitted on one out of 40 ILS channels between the carrier frequency range 108.10 MHz and 111.95 MHz (with the 100 kHz digit always odd, so 108.10, 108.15, 108.30, and so on are LOC frequencies but 108.20, 108.25, 108.40, and so on are not). One is modulated at 90 Hz, the other at 150 Hz and these are transmitted from separate but co-located antennas. Each antenna transmits a narrow beam, one slightly to the left of the runway centerline, the other to the right.

The localizer receiver on the aircraft measures the difference in the depth of modulation (DDM) of the 90 Hz and 150 Hz signals. For the localizer, the depth of modulation for each of the modulating frequencies is 20 percent. The difference between the two signals varies depending on the position of the approaching aircraft from the centerline.

If there is a predominance of either 90 Hz or 150 Hz modulation, the aircraft is off the centerline. In the cockpit, the needle on the horizontal situation indicator (HSI, the instrument part of the ILS), or course deviation indicator (CDI), will show that the aircraft needs to fly left or right to correct the error to fly down the center of the runway. If the DDM is zero, the aircraft is on the centerline of the localizer coinciding with the physical runway centerline.

A glide slope (GS) or glide path (GP) antenna array is sited to one side of the runway touchdown zone. The GP signal is transmitted on a carrier frequency between 329.15 and 335 MHz using a technique similar to that of the localizer. The centerline of the glide slope signal is arranged to define a glide slope of approximately 3° above horizontal (ground level). The beam is 1.4° deep; 0.7° below the glideslope centerline and 0.7° above the glideslope centerline.

These signals are displayed on an indicator in the instrument panel. This instrument is generally called the omni-bearing indicator or nav indicator. The pilot controls the aircraft so that the indications on the instrument (i.e., the course deviation indicator) remain centered on the display. This ensures the aircraft is following the ILS centreline (i.e., it provides lateral guidance). Vertical guidance, shown on the instrument by the glideslope indicator, aids the pilot in reaching the runway at the proper touchdown point. Many aircraft possess the ability to route signals into the autopilot, allowing the approach to be flown automatically by the autopilot.

Marker beacons

On some installations, marker beacons operating at a carrier frequency of 75 MHz are provided. When the transmission from a marker beacon is received it activates an indicator on the pilot's instrument panel and the tone of the beacon is audible to the pilot. The distance from the runway at which this indication should be received is promulgated in the documentation for that approach, together with the height at which the aircraft should be if correctly established on the ILS. This provides a check on the correct function of the glideslope. In modern ILS installations, a DME is installed, co-located with the ILS, to augment or replace marker beacons. A DME continuously displays the aircraft's distance to the runway.

The outher marker

The outer marker is normally located 7.2 kilometres from the threshold except that, where this distance is not practical, the outer marker may be located between 6.5 to 11.1 kilometres from the threshold. The cockpit indicator is a blue lamp that flashes in unison with the received audio code. The purpose of this beacon is to provide height, distance and equipment functioning checks to aircraft on intermediate and final approach.

The middle marker

The middle marker should be located so as to indicate, in low visibility conditions, the missed approach point, and the point that visual contact with the runway is imminent, ideally at a distance of approximately 3,500 ft (1,100 m) from the threshold. It is modulated with a 1.3 kHz tone as alternating Morse-style dots and dashes at the rate of two per second. The cockpit indicator is an amber lamp that flashes in unison with the received audio code.

The inner marker

The inner marker, when installed, shall be located so as to indicate in low visibility conditions the imminence of arrival at the runway threshold. This is typically the position of an aircraft on the ILS as it reaches Category II minima. Ideally at a distance of approximately 1,000 ft (300 m) from the threshold. The modulation is Morse-style dots at 3 kHz. The cockpit indicator is a white lamp that flashes in unison with the received audio code.