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The image below shows a low IFR chart with a VORTAC on the right and a TACAN on the left. I understand the VORTAC is labelled with both VOR frequency and TACAN channel so that a civilian user could tune the VOR and a military user could tune the TACAN.

Why does the TACAN on the left show the frequency 112.3? I understand it's a paired VOR frequency, but if there's no VOR then why show the freq? My best guess is that perhaps a civilian user could tune 112.3 to receive DME information from the TACAN...

Low IFR Chart Example

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Your supposition is correct. Tuning the paired frequency will allow you to receive DME information from the TACAN.

If your DME tuning is through your VOR/ILS receiver, the VOR output will be flagged no computed data (NCD) as there's no VOR signal to receive.

The DME will tune the appropriate paired frequency and the DME should display normal data.

Having DME data alone is not all that helpful. However, if you have an FMS and a multi-channel DME, the FMS will tune the DMEs independent of (or in conjunction with) the VORs. This will allow the FMS to perform DME/DME navigation. That was the primary FMS navigation method until GPS came along. These days, DME/DME is a backup to GPS or GPS/IRS navigation.

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  • $\begingroup$ Depending on the procedures around, a DME data alone (well, in conjunction with other navaids not colocated with that DME) might be helpful. Some procedures use DME arcs, DME in conjunction with NDB or ILS with non-colocated NDB. DME may also be used in defining area where circling or holding is safe and corresponding safe altitudes. $\endgroup$ – Jan Hudec Nov 10 '20 at 21:24
  • $\begingroup$ @JanHudec You are correct, but those procedures are not common. When I worked in FMS development we tested procedures that used all different combinations of navaids. Procedures that use non-colocated navaids are very rare. And those are slowly going away as they are replaced with RNP procedures. $\endgroup$ – Gerry Nov 11 '20 at 4:31

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