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ILS instrument approach chart

A lot of instrument aproach charts (VOR or ILS) have a circle around the navaid. It can be 10/20 NM or in km, like the 20km circle in the image. What does this circle mean?

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    $\begingroup$ You may want to ask what it's used for rather than what it means, as I think you know it shows the distance that is 10 NM from the centre. $\endgroup$ – Notts90 Sep 10 '17 at 21:33
  • $\begingroup$ Yeap. Because, as I understood from the answer below (as Annex 4 says), it shows only radial scale. I thought, it has some additional function. $\endgroup$ – dovganutsra Sep 11 '17 at 15:41
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It can be 20 km, or 10 NM.

Since you've shown an ICAO-style plate, ICAO Annex 4 Aeronautical Charts says:

11.3.3.1 Except where this is not practicable, a distance circle with a radius of 20 km (10 NM) centred on a DME located on or close to the aerodrome, or on the aerodrome reference point where no suitable DME is available, shall be shown; its radius shall be indicated on the circumference.

So whenever possible, this circle must be shown.

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    $\begingroup$ But why is it required? To quickly estimate the relative distance? $\endgroup$ – Nit Sep 10 '17 at 20:40

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