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

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.

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!)

"Your call is important to us. Our operators are busy right now giving
their full attention to other airlines. We will be with you shortly.
Did you know that you can find most of the information you seek on our
website? Please log on to www.getlostspamcan.com. In the mean time, we
hope you enjoy our new rap hold music."

after hitting the emergency transmit button in her aircraft

Yanno, this reminds me of what was promised in our health care system
about twenty or thirty years ago.

Jose
--
There are more ways to skin a cat than there are cats.
for Email, make the obvious change in the address.

Jim Logajan writes:

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.

What about analog users? What if an analog user transmits while a
queued message is being transmitted?

--
Transpose mxsmanic and gmail to reach me by e-mail.

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

Greg Copeland writes:

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.

Is there a guarantee that transmissions will occur within a certain
period? Are these systems verified for safety-of-life applications?

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.

Interesting.

This does bring to mind something else, though: If your channels are
so crowded that you need a system to queue messages and give priority
for emergencies, you need more channels. It's much safer to have
multiple channels that don't require queuing than it is to queue on a
single channel.

Also, how do you deal with analog users who have no queuing? They
will still walk over the simultaneous transmissions in digital and
analog.

An anachronism? No worse off than they are today.

Actually they would be, since practices extended to digital users
would naturally tend to affect analog users, even though they don't
have the same advantages. This would put them at a safety risk.

Until everyone
is converted, such features would simply be a perk to controllers; with
the potential to increase QoS for those that digitally participate.

Quality of service has to translate to increased safety in my book.
As I've said, if fancy queuing systems are required just to manage
traffic on the channel, then there are not enough channels, digital or
otherwise.

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

As long as someone is still actually flying the plane. A beautiful
digital display of weather 300 nm ahead doesn't help if it distracts
you from the mountainside looming just ahead through the cockpit
window.

--
Transpose mxsmanic and gmail to reach me by e-mail.

"Mxsmanic" wrote in message
...

SNIP


As long as someone is still actually flying the plane. A beautiful
digital display of weather 300 nm ahead doesn't help if it distracts
you from the mountainside looming just ahead through the cockpit
window.


If you're flying something that requires you to be concerned (near term)
with Wx that is 300nm ahead...and you are low enough to hit something that
is part of Earth...you've got larger issues to deal with than how you are
communicating.

;O)

Jay B

On Thu, 07 Sep 2006 07:41:03 +0200, Mxsmanic wrote:

Greg Copeland writes:

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.

Is there a guarantee that transmissions will occur within a certain
period? Are these systems verified for safety-of-life applications?


You consider DoD, DoE, CIA, FBI, military, fire, and police to be
"safety-of-life applications?" If so, yes. Do keep in mind, these are
existing systems but do not specifically target aviation. I would imagine
aviation would require some adaptation or P25 or a P25-like standard.

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.

Interesting.

This does bring to mind something else, though: If your channels are
so crowded that you need a system to queue messages and give priority
for emergencies, you need more channels. It's much safer to have
multiple channels that don't require queuing than it is to queue on a
single channel.


Yes. That's a function of scalability. It's up to the customer (FAA in
this case) to ensure enough talk groups exist to meet their often growing
needs. As it applies to aviation, talk groups would be ground, tower,
departure, arrival, etc...

Also, I don't want to be misleading. Queuing serves as a short term
solution for **peak** periods. In other words, queuing is only honored
for brief periods of time; usually in the 1-3 second range. The notion is
to allow for first-come, first-serve without forcing users to constantly
rekey their PTT if they didn't get their call grant. If a queue depth
greater than one becomes the norm, someone failed to properly scale the
system.

Also, how do you deal with analog users who have no queuing? They
will still walk over the simultaneous transmissions in digital and
analog.

Analog users would require an analog system to sit beside it or you would
require an analog/digital repeater. I must profess, I've never used the
analog P25 repeater. I'm in the P25 infrastructure development group at
my company so I don't use the analog stuff. As such, I must profess some
ignorance.


An anachronism? No worse off than they are today.

Actually they would be, since practices extended to digital users
would naturally tend to affect analog users, even though they don't
have the same advantages. This would put them at a safety risk.

How so? How is a current analog user "at risk"? It's not like it's
removing existing capabilities.


Until everyone
is converted, such features would simply be a perk to controllers; with
the potential to increase QoS for those that digitally participate.

Quality of service has to translate to increased safety in my book.
As I've said, if fancy queuing systems are required just to manage
traffic on the channel, then there are not enough channels, digital or
otherwise.

With analog, you don't have a queuing system...which translates directly
into walked on radio calls. That's a loss of service and wasted airtime.
"Fancy" queuing and resource allocation immediately translates into
increased QoS. In this case, that translates to increased safety. That's
just for starters, directly comparing analog to digital. By having an
analog to P25 repeater, on the digital side, you reap the same benefits.


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

As long as someone is still actually flying the plane. A beautiful
digital display of weather 300 nm ahead doesn't help if it distracts
you from the mountainside looming just ahead through the cockpit
window.

If you're trying to get weather while flying toward a mountain, you have
priority issues which conflict with safety. Somehow, I don't imagine data
services will be the probable cause of death.

Lastly, let me stress, I'm talking about existing services which does NOT
serve aviation; save only a few police helicopters and planes. And those
radios are used for unit to unit (person to person) and group calls
("party line")...not for aviation specific communication. As I said
above, I'm sure parts of P25 would need to be adapted to better serve the
needs of pilots. Nonetheless, the technology is both here and proven.


Greg