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How can the A320 receive simultaneously 3 DME signals (VOR1 + VOR2 + ILS) if it has only 2 DME receivers (and antennas)?

Does it multiplex them in some way? If yes, does it mean if one DME receiver fails I can still get simultaneously 3 DME values?

There is a video that shows 3 DMEs tuned simultaneously: Airbus Cockpit Views | Episode #6 | PFD View Landing STN RWY22

enter image description here
VOR1 (DME1) being automatically re-tuned, followed shortly by VOR2 (DME2), plus the already tuned ILS

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Mighty interesting! There is a clue in the FCOM:

Four frequencies per FMGC

If you check FCOM Chapter 22 § 20-20-30 you'll note it says:

Each FMGC [Flight Management and Guidance Computer] automatically uses its four DME frequencies as follows:

  • One DME frequency for display. It is possible to tune it manually or automatically. This DME frequency is also used for VOR/DME position computation.

  • Two DME frequencies in DME/DME mode for calculating the radio position of the aircraft. The FMGC automatically tunes these as a function of their best accuracy. The flight crew does not receive any indication that this process is happening.

  • One DME frequency is connected to the ILS/DME.

Note my bold emphasis above. Similarly for the A380, in clearer language:

In normal operation, each FMS can tune four DME s at the same time. (...)

Scanning DME

Wikipedia says:

Scanning DME (distance measuring equipment) that check the distances from five different DME stations simultaneously in order to determine one position every 10 seconds [emphasis mine]

—Wikipedia: Flight management system § Position determination

The patent for this scanning-type DME was filed in 1982:

(...) a scanning type DME unit which is rapidly sequenced to interrogate said multiple ground stations successively and determine distance thereto for all stations within range, and a computer to validate and process these distance measurements in sequence to provide an accurate computation and display of aircraft position and wind information. [emphasis mine]

—Meyer, Donald H. "Accurate DME-based airborne navigation system." U.S. Patent No. 4,583,177. 15 Apr. 1986.

Failure scenario

Regarding the failure scenario, each FMGC can tune both its interrogator, and the other interrogator if the other FMGC failed.[1] Each FMGC is still limited to four frequencies though. So, if one DME fails (antenna or its interrogator), you'll have two frequencies which you can control (one for display, and one for ILS), and two the FMGC will use for DME/DME position, but you can't have two different DMEs for display.

1: Airbus Single Aisle Training Manual ATA 34

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  • $\begingroup$ Thanks! Excellent answer. Just a note, the Wikipedia's sentence "one position every 10 seconds" seems too slow (also compared to the video), probably it's legacy DMEs. $\endgroup$
    – ocirocir
    Mar 31 at 8:50
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    $\begingroup$ @ocirocir The 10 seconds here mean that one position update for DME/DME can be performed every 10 seconds, not that the displayed DME is only updated once every 10 seconds. The DME/DME update of the MIX IRS position bias is inhibited anyway, if GPS PRIMARY is active. $\endgroup$
    – Bianfable
    Mar 31 at 11:06
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    $\begingroup$ @ocirocir: I suppose the Wikipedia text is the generic implementation, it still makes sense. In the video you have DME1 and DME2 from FMGC1 and 2, plus the ILS. The update (however frequent) as the FCOM notes is behind the scenes, and relates to two other frequencies per FMGC (not DME1 and 2). / Thanks for the interesting question ;) $\endgroup$
    – ymb1
    Mar 31 at 14:47

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