Can a DME transmit a signal?

[Pilot DAR]

As a part of a witnessed EMI test, 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...

[Cough]

A DME works by the aircraft transmitting a pulse, the ground station replying. The distance is worked out by the time taken for the signal to go out and back..

[HEATHROW DIRECTOR]

Check this: Distance measuring equipment - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/Distance_measuring_equipment)

[Pilot DAR]

Excellent, thanks! So as a pilot, to "transmit" with a DME, I just turn it on, and it is transmitting?

[QA1]

Google - dme principals.

Yes DME does transmit, as does the ground station when replying to the aircrafts interrogation.

The DME frequency is usually tuned by selecting a VOR or ILS frequency which has a co-located DME ground station. DME frequencies are paired with VOR / ILS frequencies.

Any electronic device can be a cause of EMI given the right environment - not just things designed to transmit.

[Cough]

...yup... :E

[mad_jock]

Yep DAR think of it as the oppersite end ie none aircraft end to your transponder.

As soon as you turn it on it starts pulsing a very weak UHF pulses to anything that will listen. When they reply as your transponder does to ATC it works out the distance.

So ATC sends out a pulse and your transponder replys, and DME the aircraft sends out a pulse and the DME transponder on the ground replies.

[Pilot DAR]

I'm learning, thanks. So in the test plan I'm writing, I state that the DME test frequency I used was the VOR or ILS frequency I tuned, and some super smart avionics person could work out the pairing if they needed to?

And, I could tune the VOR or ILS frequency, get the DME going, and then select DME "hold" and it stays on that frequency, when I reselect that VOR/ILS?

I think this is all coming back to me from my copilot days in the Cheyenne of 30 years ago! (I honestly have not used DME since!)

[mono]

As has been stated DME does indeed transmit.

To proprly test for EMI you'll need a licenced radio engineer. The DME PRF (pulse repetition frequency) is different when a DME is searching for a ground station and when its locked on. The frequency will vary according to the co-located VOR frequency selected but its in the L-band region which is a little above 1000Mhz.

Transponder is similar and there are different pulse characteristics for the different modes. A test using the approprate ground test equipment should be carried out. (unless of course you'll do these tests while flying, which I doubt)

IMHO unless you do these checks simulating an operating situation the EMI checks would not be valid.

[mad_jock]

DAr there is a list of coupled frequencys which you will be able to get from Transport Canada so I don't think you need to be super smart just know where to look.

So when you select say 109.20 that is coupled to a UHF frequency the DME uses.

When you selct DME hold it then holds the UHF frequency and allows you to select a VHF freqency without dragging the UHF coupled freqency into the DME unit.

Just like mobile phones they have search modes and power levesl which are then adjusted as required once they have aquired something usefull I presume down the way. So you should get your max emmission when it can't see anything.

[Pilot DAR]

Some great information, thanks! My EMI test plan will be much the better for it! It's nice to get it right, but even nicer to know why you're getting it right! BTW, this started out as a very simple EMI test, for cabin installed survey equipment. It's kind of snowballed from there. It is not proposed as a highly qualified examination. This would be done, if I observed any EMI during the basic test. My test is only eyes and ears, no special equipment.

This will be my 13th time doing this test on the aircraft. Interestingly, last September I was witnessing this test in flight, while riding jump seat. I could hear a faint transponder signal over the DC noise cancelling headset I was wearing. When I unplugged the headset, the noise continued! I sat, looking at the two plugs in my hand, and hearing the signal. When I moved away, it stopped. I can only presume that some form of induction was affecting the headset, while in close proximity of the avionics compartment.

[mad_jock]

Best way to test is have a portable radio on and see if you get changes in static on VHF and MW/LW/SW bands.

I am sure there is fancy equipment you can get though.