Why don't voice radio communications use FM?

On Thu, 07 Sep 2006 02:11:51 +0000, Jim Logajan wrote:

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.

That would be very frowned upon. A unit to unit call (like a non-party
line telephone) would tie up resources which should otherwise be shared.
That's not to say they don't have their place, but it's not something you
would want happening all the time.

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.

Emergency calls can be group calls just as they are today with their
analog equivalent. The point of an emergency call is to give priority to
your conversation with someone on the other end. Anything beyond that is
a perk.


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.

I would emphasize it the other way around. A unit to unit call ties up an
entire channel for its duration. So you would not want that to be the
common case.

Greg