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From How does an airborne DME unit differentiate itself?

The higher rate in SEARCH [150 PP/S] gives it more opportunity to detect the replies as it has no knowledge of range in search and thus has to scan for the entire range. Once it has detected the ground station replies by finding a correlated sequence of pulse pairs, it can switch to TRACK mode [30 PP/S].

How does increasing the pulse pairs per second (PP/S) here matter at all, since there will be replies anyway at the slower rate?

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A single pulse pair is not enough of an identification. As Gerry explained in the linked post, a pseudorandom number generator varies the pulse pairs. If you vary and repeat one pulse pair, the timing difference will be easily repeatable by other units and will not be unique.

So what the unit does is generate a train of pulse pairs, in which each pair is randomly shifted. And the unit listens for the same train back, but how?

  • In an analog circuit, the unit will interrogate then listen at a specific time slot (range gate), in steps of e.g. 12 µs, but given the range of 2.5 ms Gerry mentioned, those are ~200 steps. To go through them quicker, the unit increases the rate of interrogating/listening. Otherwise it can be upwards of 10 seconds to get a DME reading.

  • In digital circuits, the replies are stored and processed (after each cycle), and to avoid errors, consecutive valid replies are needed, e.g. ten in a row, not just one, so again a faster rate helps get those valid replies more quickly.

Once locked on and the scan range is narrowed down, the unit uses the slower-rate track mode.

Also note that the average allowed is 30 PP/S (otherwise a single DME ground station can't service many airplanes), and since the search mode is faster than 30, the track mode will typically be less than 30 (that's why Gerry wrote "up to"), so in the end the average will be around 30.


Ref. and further reading: Miller, Wayne L., and Ernest J. Carnicelli. "DME apparatus and method." U.S. Patent No. 4,028,698. 7 Jun. 1977.

(The above Cessna/Sperry/Honeywell patent explains the basics before explaining their own method.)

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  • $\begingroup$ So consecutive replies are needed in search mode ( eg 10 in row ) , so for that reason higher pps used ? and because consecutive replies are not needed once the relation is established, lower pps can be used right? also i want to know in search mode - the signals are radiated in all directions from dme unit? and once its established, the signals emit only in the direction of dme ground station? excuse me if i am wrong, if that's not the case, then why are there even two modes. $\endgroup$ Mar 31, 2022 at 20:47
  • $\begingroup$ @Sachin: Would you rather A) have the DME find the slant range in a long time or keep missing because the plane is moving B) overuse the DME ground station making fewer users benefit from it C) none of the above and utilize two modes to fix those issues? (Both are explained in the answer.) // Regarding the directional antenna, that's a third mode called scan (not search or track); "scanning DME" is where multiple stations are interrogated at the same time, allowing e.g. four distances/stations using one antenna. Does that help? $\endgroup$
    – user14897
    Mar 31, 2022 at 21:12
  • $\begingroup$ i'm just saying that the conclusion can be said that in search mode the reason for higher pps in search mode is because initially higher replies are needed eg (10 in row) as you mentioned in example? $\endgroup$ Mar 31, 2022 at 21:24
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    $\begingroup$ Good description. One point I like to make is that when you're searching, you're looking for the pulse pairs in a sea of RF noise. It's a needle in a haystack. But once you find it, it will only move temporally based on your a/c speed and time between interrogations which is all predictable. I liken it to looking at a MagicEye image. You have to stare at it a while to see the image, but once you do, it's easy to look away and then see it when you look back. $\endgroup$
    – Gerry
    Mar 31, 2022 at 22:07
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    $\begingroup$ @Sachin: Think of it in terms of distance. The unit sends an interrogation, and then checks each distance (1, 2, 3, ... 199, 200) for the reply. That's the search mode. If the search mode revealed the distance to be 50, the track mode will simply interrogate and check a much narrower window (49, 50, 51) instead of checking the whole range every time. Using one mode only will cause the problems explained in the answer and in my first comment. $\endgroup$
    – user14897
    Apr 1, 2022 at 8:46

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