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Old September 7th 06, 03:11 AM posted to rec.aviation.piloting
Jim Logajan
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Posts: 1,958
Default Why don't voice radio communications use FM?

Greg Copeland wrote:
On Sun, 03 Sep 2006 15:37:38 +0200, Mxsmanic wrote:
Do other aircraft hear the transmission when you make it, or when the
controller hears it? Granted, they are only supposed to listen to
the controller, but in practice they will be listening to other
aircraft as well.


Sorry. I forgot most people don't know how this stuff works. You are
queued when you activate your PTT but you don't actually get your
"beep" (think NexTel walkie-talkie sound) back until you're granted
your call. Only after you're granted your call do you speak.


Just tell people they would operate it like a telephone: the pilot would
direct her call to a particular listener (e.g. ATC) and ATC gets a signal
(like a phone ringing!) and can let it ring until they have time to answer
the call. But in a pinch, the system could also act like a party line system
and after hitting the emergency transmit button in her aircraft, the pilot's
distress call would automatically cut in over less-urgent calls to not only
ATC, but to any aircraft who have set their receivers to automatically accept
emergency calls.

In essence, digital systems provide multiple virtual private circuits if
needed, but still allow broadcast or "party" line equivalents for situations
where that communication mode is more useful.