This question is in the context of pilot-activated runway lighting, which involves clicking the microphone e.g. 7 times to turn on the lights at night. I was wondering what happens when you are clicking the radio button, which typically takes several seconds, and suddenly someone else starts transmitting on the same frequency while you are still clicking? Does that mean your attempt has failed, since the other person's transmission is often continuous? To what extent is the receiver on the ground able to discern between the two signals? Does it depend on our relative proximity to the airfield? Or is this nothing I should worry about?


1 Answer 1


From a technical standpoint, it's extremely difficult to tell two different signals on the same frequency apart. So, if someone steps on you while you're trying to turn the lights on, then the ground equipment can't tell the difference between their radio and yours. So, unless the other radio is far enough away that it doesn't register on the PCL receiver at all*, it won't activate. You'll just have to try again when the other pilot's done talking.

In general, it's usually nothing to worry about, because the other pilot(s) nearby will hear you clicking, and know to avoid using their radio until you're done.

* Many airports turn the sensitivity on their PCL down so that distant planes on the same frequency won't interfere with a local pilot trying to land.

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    $\begingroup$ Why wouldn't the mic clicks still register while the other pilot is talking? $\endgroup$ Mar 15, 2020 at 17:13
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    $\begingroup$ @MichaelHall Because the signal level never falls below the detection threshold, so the receiver sees it as one continuous signal. That's what I was talking about when I said it was difficult to tell the signals apart, they get mixed together at the antenna. $\endgroup$ Mar 15, 2020 at 18:17
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    $\begingroup$ Gotcha, thanks! $\endgroup$ Mar 15, 2020 at 18:19

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