Why don't voice radio communications use FM?

On Sun, 03 Sep 2006 15:37:38 +0200, Mxsmanic wrote:

Greg Copeland writes:

New systems (P25) already do this type of thing. I develop digital radio
systems. Police, fire, FBI, CIA, DoD, DoE, various municipal
utilities, and various branches of the military are all taking advantage
of this technology. In many cases, the old analog systems must co-exist
with the newer P25 systems. In some cases, more rural analog systems
actually connect with a P25 network via a specialized repeated.
Integration is not a problem.

So why wouldn't it extend to aviation?


Great question. I don't have an answer. I've been planting a seed to
have the federal marketing types start sniffing around for FAA/political
upstarts...but so far, for my company, it seems to fall on deaf ears.

Last I read, an FAA study indicated they need lots and lots of money
(sorry, don't remember the amount) to upgrade their infrastructure from
analog to digital.

They need not upgrade it all at once.


I agree with that. I didn't read the whole report and it was a couple of
years old. I believe the plan was to upgrade over a number of years...I
don't recall the window.

[snip]
Advantages of this technology include:
o call queuing - meaning, PTT places you in a queue so you can get a word
in, even when the controllers are very busy. BTW, this also means no more
"walked on" transmissions.

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. Otherwise, no one hears
you because your radio doesn't xmit. Thusly, no more "stomped on" radio
calls.

Example:
Pilot 1 Pilot 2
PTT PTT
"beep"
Pilot speaks
Hears pilot 1
Release PTT
"beep"
Pilot Speaks
Hears pilot 2


How do you make this work in parallel with analog systems that cannot
queue?


The repeater initiates the call on your behalf. The repeater is queued
rather than the analog radio. Likewise, the reply goes to the repeater,
which then re-RXs ("repeats") as analog. For this to work, the analog and
digitial systems must have their own frequencies.

o call prioritization - All sorts of cool things can be done here -
including, most recent exchange receives priority. Also, should IFR
traffic receive higher priority over that of VFR? What about commercial
traffic? Priority could be adjusted dynamically too. This means
planes in distress could be assigned higher priority. So on and so on...

It's best not to jump off the deep end with gadgets. Just because
something can be done doesn't mean that it should be done.

Agreed. I was just tossing stuff out to show the types of things that can
be done. A more likely scenario is to give priority to controllers. This
allows controllers to pre-empt pilots when the talk group is busy. Which
is, more than likely the prefered solution.

Also, the concept of "emergency" call is also very useful. For example,
it places you at the top of the queue. Combine "emergency" with a GPS
source, plus data services, and now your squawking 7700, your GPS position
is sent with your PTT, and you now have priority with the controller.


o caller id - imagine your tail number, altimeter, heading, and aircraft
type provided to the controller on every PTT.

Where does this leave people with analog equipment?

An anachronism? No worse off than they are today. Until everyone
is converted, such features would simply be a perk to controllers; with
the potential to increase QoS for those that digitally participate.


o Limited data services

What kind of data services do pilots need? Are they going to be
surfing the Web?

Oh, most definiately not web browsing. TAFs, METARS, in route weather,
PIREPs, TFRs, ATIS, ASOS, TWEB, NAV IDs, etc...

[snip]
The only con of digitial compared to analog is reception. With analog,
you can hear a weak signal. It may sound like absoluete crap, but you
can still hear it. With digitial, either you have a strong enough
signal to hear it...and it sounds awesome...or you hear absoluetely
nothing at all.

If the digital threshold is set where the threshold of intelligibility
would be in analog, there's no net loss.

Doesn't work that way. Nor, would you want it to. One of the key points
of digitial radio is that everything is crystal clear. This means lots
and lots of filtering takes place to pull out voice from the background.
If it's intelligible, chances are, it will be considered background noise
and filtered out.


Greg