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Do DME's transmit at all?

I'm being asked to test DME's as a possible "source" of electro magnetic interference (EMI), and state the tested frequency. This request suggests that DME's are capable of transmitting. I always thought that they are a receiver only (though I am certainly not an avionics expert by any means).

Can anyone tell me if a DME can transmit, or otherwise be a source of EMI. If a DME can transmit, how do I as a pilot make it do that on command? Can I select a transmitting frequency for a DME?

Thanks...

Home runway, in central Ontario, Canada

Yes, DME is a bit like an SSR transponder, but in reverse: an onboard DME interrogates the ground station and receives a signal back on a shifted frequency, then measures round-trip time (less circuit delays). The carrier frequencies involved are around 1 GHz.

LKBU near Prague, Czech Republic

Yes. Look at the radio license for the aircraft. One of the frequencies mentioned should be the DME transmit frequency.

DME is frequency paired with VOR, just like the ILS (LOC and GS) is. So if you connect the DME to the VOR (the RMT setting), it uses the DME frequency that's paired with the VOR frequency to transmit and receive. But you can also switch the DME to FREQ or whatever it's called, and then you can set the frequency on the box directly. Again, what you then select is the VOR frequency, but what really happens is that it sends and receives at the corresponding frequency paired DME frequencies.

Thanks guys, I also got some good answers on the "other place" and I have it figured out for my testing. I learned something!

Home runway, in central Ontario, Canada

Obviously all the gory details are on google but in essence when you tune a VHF VOR frequency, the VHF setting (e.g. 117.00MHz) is translated (via a lookup table; I don't believe it is a simple arithmetic relationship, but it may be) to a corresponding DME frequency in the L-band range, in the region of 1.1GHz. The ground station responds on a matching (lower) frequency.

I have a funny problem in my plane: setting a specific VOR/DME frequency wipes out the panel mounted GPS. I only ever saw this once, on a flight to Sardinia, and cannot replicate it by setting the same frequency in the absence of the DME station (which was in S. France) so presumably it isn't the outgoing frequency that is doing it. I don't have the figure handy though - it's written down in the plane. I did a lot of googling etc and no imaginable harmonic of anything is anywhere near the 1575MHz GPS carrier, so this remains a mystery. It wiped out the panel mounted unit but not any handheld unit we had with us.

When I had my TCAS installed, I gave the setting to the shop doing it and they said they would test it because, they said, they have a DME emulator. But they didn't do it... and when the TCAS was done they said they could not even do a normal ILS radio check. So I am none the wiser...

Administrator
Shoreham EGKA, United Kingdom

setting a specific VOR/DME frequency wipes out the panel mounted GPS

Yes, this seems to be the kind of gremlin they are sending me to search for. FWIW, it was explained to me as follows: You have the wire which connects the transmitter to it's fuselage mounted antenna - so far so good, it works. Your GPS is installed, and it has antenna wire too, If the installer did it right, that wire is not at all influenced by the other transmitter wires (presumably they are shielded, and suitably distant).

Now some horrible aircraft modifier person comes along, installs some bothersome equipment in the fuselage, and wires to connect it. These wires have no relationship whatever to the avionics - except proximity. The horrible wire crosses, or worse parallels the avionic antenna wire, and also the GPS antenna wire. Now when a transmission of any power is made (or received?), that signal traveling along the antenna wire is picked up by the horrible wire, and carried to the GPS antenna wire, and obliterates the very weak signal of the GPS reception. I'm told I have to xmit on specified harmonic comm frequencies for 35 seconds to assure that there is time to detect the fault on the GPS satellite page.

This was an observed fault in a modified Dash 8, and explained to me by the observer. Perhaps it is affecting Peter's plane too. But, I am by no means an avionics expert!

Home runway, in central Ontario, Canada

I have always found DME technology to be interesting. It transmits a pulse pair that the ground DME station or VORTAC repeats back to the DME receiver after a short delay and on a different frequency. The receiver detects the repeated pulses, subtracts the fixed delay and determines the distance by using the round trip time. Since the ground transponder repeats all pulses it receives, both from your aircraft and from all the other aircraft in its airspace who are DME equipped, how does the DME recognize its own pulses. It does this by randomizing the time between the pulse pairs it sends out and looking for sequences of pulse pairs with the same timing, only delayed by a similar time window. When the DME is attempting to lock on to the ground transponder, it is permitted to transmit a larger number of pulse pairs during acquisition, but once it recognizes its own pulses by a consistent delay, it settles down to a lower permitted pulse repetition rate used for tracking, Most DME ground stations can handle up to 100 users at the same time, some can handle up to 200. Whenever it gets saturated, it lowers its receiver sensitivity to ignore the DME's further away or with low power units.

KUZA

Yes; it's really clever, and all the more so given that the implementation used to be done with masses of TTL logic in the early days.

There were some clever engineers around in those days...

Those "hard wired logic" DMEs are still in use here in the UK. They are many years past any manufacturer support, and there is a group of blokes (with beards, presumably) who go around, making money out of swapping boards over from one 1970s DME to another 1970s DME...

PilotDAR - here I have some background to interference. Those tests should be standard for any IFR aircraft.

Administrator
Shoreham EGKA, United Kingdom

Peter,

I suspect that the spurious signal is more likely to come from a VOR or COM frequency. Occasionally, it can result from harmonics of signal leaking thru the front of a Bendix King VOR/COM unit.

KUZA

Now that we're on to interesting stories... DME was developed in WWII for accurate bombings of Germany. But it worked the other way around, with the airplane reflecting the pulses back to the operator on the ground.

In the UK there were several ground stations, spread from north to south. For each bombing run, the distance from the target to two specific ground stations was calculated. The pilots flew an arc defined by the distance to one ground station, and the ground operator would use morse signals to tell them they were on the arc, or too near, or too far. Simultaneously the ground operator was also watching the "DME readout" from the other ground station, and when that DME showed the right value, another morse signal was transmitted which prompted the bomb release.

The Germans were baffled by the accuracy (< 30m) of this type of bombing, because the Allied forces were able to do accurate night bombings from over an overcast. They eventually caught on and began jamming the signals. So the Allied forces switched to a higher frequency band, but still kept using the "old" frequencies as decoys. This worked fine until one ground operator working the "old" frequency forgot to send the "bomb release" signals, while the "new" frequency operator did.

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