Editing Instrument landing system (section)

===Marker beacons===
{{Main article|Marker beacon}}
On some legacy installations, [[marker beacon]]s operating at a [[carrier frequency]] of 75&nbsp;MHz are provided. When the transmission from a marker beacon is received it activates an indicator on the pilot's instrument panel and the identity code and tone of the beacon is audible to the pilot. The distance from the runway at which this indication should be received is published 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 glide slope.  Instead of marker beacons, modern ILS installations use [[distance measuring equipment|DME]]. Co-located with the ILS glidepath transmitter near the touchdown point, the DME provides a display of aircraft distance to the runway.
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===Outer marker===
[[Image:Outer Marker Indicator.gif|frame|left|blue outer marker]]
The outer marker is normally located {{convert|7.2|km|nmi mi}} from the threshold, except that where this distance is not practical, the outer marker may be located between {{convert|6.5|and|11.1|km|nmi mi}} from the threshold. The modulation is repeated ''Morse-style dashes of a 400&nbsp;Hz tone'' (--) ("M"). 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. In the United States, a [[non-directional beacon|NDB]] is often combined with the outer marker beacon in the ILS approach (called a [[Locator Outer Marker]], or LOM). In Canada, low-powered NDBs have replaced marker beacons entirely.
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===Middle marker===
[[Image:Middle Marker Indicator.gif|frame|left|amber 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 {{convert|3500|ft|m|abbr=on}} from the threshold. The modulation is repeated ''alternating Morse-style dots and dashes of a 1.3&nbsp;kHz tone at the rate of two per second'' (·-·-) ("Ä" or "AA"). The cockpit indicator is an [[Amber (color)|amber]] lamp that flashes in unison with the received audio code. In the United States, middle markers are not required so many of them have been decommissioned.{{Citation needed|date=January 2010}}
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===Inner marker===
[[Image:Inner Marker Indicator.gif|frame|left|white 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 minimums, ideally at a distance of approximately {{convert|1000|ft|m|abbr=on}} from the threshold. The modulation is repeated ''Morse-style dots at 3&nbsp;kHz'' (····) ("H"). The cockpit indicator is a white lamp that flashes in unison with the received audio code.
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