DME above 199 users


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DME above 199 (or MAX) users

What happens if a DME has more than 199 interrogations - if 199 is the max number of users? I've been talking about this with some people at the FBO, and no one is ever able to really answer what would happen.

1) How does the computer inside the DME station "know" who are the first 199 users? Does it assign a code to each receiver in the same way a DNS (computer) functions by assigning a code to each person using it?

2) What happens to the 200th DME interrogation?


Federal Radionavigation Systems

The interrogator in the aircraft generates a pulsed signal (interrogation) which, when of the correct frequency and pulse spacings, is accepted by the [DME's] transponder. In turn, the [DME] transponder generates pulsed signals (replies) that are sent back and accepted by the interrogator’s tracking circuitry. Distance is then computed by measuring the total round trip time of the interrogation and its reply. The operation of DME is thus accomplished by paired pulse signals and the recognition of desired pulse spacings accomplished by the use of a decoder.
1) The DME station probably has a maximum limit for interrogation activity.

How do all the interrogators guarantee they're transmitting a distinct pulse train? Well, its not guaranteed, but the interrogator transmits a random series of pulses that the DME transponder parrots. The value for the number of distinct pulse train possibly divided by the maximum number of users must have been found to be acceptably high.

How does the DME know which pulses are from which pulse train? Beats me. If I had to guess, the DME just parrots back everything it receives with a built-in delay, and the interrogator looks for its exact pulse train in the whole mess. A DME transponder probably becomes saturated when it is pulsing more than some threshold level (or duty cycle) and the superposition of all the repeated pulse trains could lead to indistinct returns. :crazy:

2) When the DME station nears saturation I believe it stops responding to weaker interrogations (whom are presumed to be further away).


Well-Known Member
if more than 199 operators are there, only the closest ones will get the signal.

at least tahts what I was told, i have no source for that


This is my Custom Title
Re: DME above 199 (or MAX) users

Okay, don't believe everything you read online, but I think I might have found a great resource.....

Check this out and see what you all think. This also blows apart the whole time between send and receive (interrogation) divided by two. I have to think this is pretty on track.....


How It Works

Once you've channeled your DME, it starts transmitting a stream of interrogations to the ground station. Each interrogation is made up of a pair of RF pulses. When the ground station receives the interrogation, it waits for 50 microseconds and then sends a pair of reply pulses back to the aircraft. Incidentally, the purpose of the 50 microsecond delay is to eliminate the possibility of uncoordinated operation when the aircraft is very close to the ground station

Your airborne DME equipment receives the reply and measures the elapsed time from when it sent the interrogation until it received the reply. It subtracts the 50 microsecond delay that the ground station introduced to come up with the round-trip time. From this, it can figure out its exact distance from the ground station using simple arithmetic, given the fact that it takes 12.359 microseconds for a signal to go out and return one nautical mile. It then displays the computed distance on your DME readout.

If this sounds a lot like how secondary radar works, it is. Only backwards. Your DME unit is essentially a little radar interrogator, and the DME ground station is just a fancy radar transponder.

There's one little complication, however. The ground station may be replying to hundreds of other airborne DMEs in addition to yours. How can your DME sort out replies to its interrogations from replies to other aircraft? Here's where some clever magic comes in. Your DME doesn't send an equally-spaced stream of interrogations, but randomly jitters the spacing of its interrogations to create a unique "signature". It then examines the ground station replies looking for a sequence with the same randomly jittered signature. When it finds that, it knows they're replies to its interrogations.


Well-Known Member
Re: DME above 199 (or MAX) users

My understanding is that when the DME approaches its service limit, it reduces its receiver gain so that it can no longer "hear" more distant aircraft.

I picked up on that little tidbit on the website of a manufacturer of various navaid equipment.