From: dst+@cs.cmu.edu (Dave Touretzky)
Subject: DME Answers
Organization: School of Computer Science, Carnegie Mellon
Date: Mon, 17 Aug 1992 22:43:44 GMT


Matthew Carruth asks:

> It is possible to id DME the same way a VOR or NDB can be handled.  I have
> several questions about this:
>
> 1) Why is it almost never done?

Most DMEs in small planes are King KN64s.  These have a remote channeling
capability, so that when you tune your nav radio, the DME is automatically
tuned to the associated DME channel.  Thus, as long as you ident the nav,
your DME is guaranteed to be tuned to the correct frequency.  Planes with
two nav radios sometimes have a channeling switch put in so you can channel
the DME from either Nav 1 or Nav 2; this adds one or two hundred bucks to
the installation cost.

It's also possible to override the remote channeling and tune the DME
manually.  You do this by inputting a VOR frequency; the DME automatically
selects the correct associated DME frequency, which is in the UHF band.

When I tune a DME manually, I *do* ident it.

> 2) Is the ID on the same carrier as the VOR signal?

The three letter ident is the same, but DMEs broadcast and receive in the
UHF band; VORs use the VHF band.  There is a frequency-pairing scheme,
published in the AIM, that ties each VOR frequency to a DME frequency.

> 3) Is the ID the same as the VOR signal, but at different rate?

The Morse code for the DME ident is at a higher pitch (I think 3000 Hz)
than the morse for the VOR.  Also, it's broadcast less frequently; about
once every 30 seconds.  This info is covered in detail in the AIM.

> 4) Is the lack of indication on a DME display a valid indication of
>    an unreliable signal and vice versa?

If the DME display is blank, it could be that you're too low to get a
line-of-sight signal to the ground station.  Or the ground station could be
out of service.  Or (rarely) there could be so many units interrogating the
ground station that it is temporarily overloaded and cannot reply to your
unit's interrogation pulses.

On the other hand, just because the box is displaying distance info doesn't
mean the info is correct.  Like VORs, DME frequencies are reused; several
transmitters share the same frequency, but they have to be far enough apart
that they don't (usually) interfere with each other.  On rare occasion,
though, you can see funny behavior.  I've gotten replies from a ground
station 100 miles away when I was trying to tune in a unit 70 miles away in
another direction.  This condition is transitory; as the plane moves closer
to the intended station and further from the incorrect one the problem
resolves itself.

Nonetheless, because this problem can occur, it makes sense to ident a
DME if it's very far away and you're concerned about possible conflicts
with other stations.

Finally, when VORs are in maintenance mode they broadcast T-E-S-T instead of
their normal ident, and their signal is not guaranteed to be usable for air
navigation.  I've actually heard the T-E-S-T ident once in 1900+ hours of
flying.  It's conceivable that the same could occur with a DME, although
I've never heard it personally.  I suppose this would be another reason to
ident the DME separately, but most people wouldn't bother.

-- Dave Touretzky,  CFI-AIM,  Seneca N8147E (``The Beast'')