ILS Markers RNAV

ILSILS —Instrument Landing System markers are an older system used to tell pilots how far from touchdown they were. There are several exam facts that need to be known about these beacons, including:

  • The approximate range from touchdown.
  • The light colour.
  • The audio code and frequency that illuminate and sound as you pass over them.
  • The frequency of modulation.

This data is summarised in the table below.

Outer MarkerMiddle MarkerInner Marker
Audio codeDashes onlyDots and dashesDots only
Audible frequency400 Hz1300 Hz3000 Hz
Modulating frequency75 MHz75 MHz75 MHz
Range to touchdown6.5-11km (often represented as 4nm)1050 m75-450 m

Do not get the audio frequency for each marker confused with this modulation frequency. The audio frequency changes (becoming higher in pitch each time), but all the markers are modulated at 75MHz.

Remember that audio frequencies must be audible, so they are measured in Hz, not MHz.

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What is the light colour of the inner ILSILS —Instrument Landing System marker?
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