Why don't voice radio communications use FM?

Larry Dighera wrote:
Because it is unlikely the FCC will agree to allocate additional
frequency spectrum for the proposed new communications system.

The frequency allocation would need to be changed or added to on an
international scope, so I believe the operative organizations would be the
ITU and the ICAO or IATA - the FCC would simply enforce the change within
the U.S. Like you, I would have thought new allocations or changed
allocations would be hard, but changes are made every four years and in the
GHz range they seem to been more readily done; e.g.:

http://www.boeing.com/connexion/news...r_030707j.html

Jim Logajan writes:

Analog AM and FM are fundamentally incompatible with each other.

Not if they are on different frequencies.

Analog AM and digital encoding over spread-spectrum are fundamentally
incompatible with each other.

Analog AM is used for digital spread-spectrum encoding. AM is the
modulation. Digital is the encoding. Spread-spectrum is just a
frequency and bandwidth assignment.

You asked why AM is being used and not FM and all I'm
pointing out is that if you are willing to consider any new system that is
incompatible with an older system (like FM replacing AM), you may as well
do it with something more advanced and capable, like digital packets over
spread spectrum (which could be considered a relative to FM).

Switching from AM to FM doesn't involve incompatibilities. You can
run both in parallel indefinitely, providing identical services (just
as some commercial radio stations have broadcasts on both AM and FM
simultaneously). Introducing FM doesn't obsolete any of the AM
equipment.

Adding all sorts of digital gadgets is quite a different matter. Now
you are adding functionality that will be available only to the
FM/digital community. This introduces potential safety and usability
issues. Stacking transmissions digitally isn't going to work when the
same transmissions must be mirrored on analog AM--and they have to be
if you want to maintain safety and keep controller workload
reasonable.

One does _not_ need to implement any of the fancier capabilities that
I mentioned. I stated them only as what could be easily done once the
capability is in place.

A change from AM analog to anything else would be glacially slow, and
small steps are safest. I see a direct safety benefit in having the
clarity of FM transmission. I don't see a direct safety benefit in
having other unnecessary features, and I do see potential risks.

Analog cell phones are being replaced with digital cell phones ...

Analog cell phones were replaced with digital well over a decade ago
throughout the world, except for a couple of countries.

There would be no need to replace
everything at once and I'm not sure why you think that would need to be the
case.

The need arises as soon as you add new functionality.

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

Mxsmanic wrote:
Switching from AM to FM doesn't involve incompatibilities. You can
run both in parallel indefinitely, providing identical services (just
as some commercial radio stations have broadcasts on both AM and FM
simultaneously). Introducing FM doesn't obsolete any of the AM
equipment.

Regarding your argument in the paragraph above and the one below...

Adding all sorts of digital gadgets is quite a different matter. Now
you are adding functionality that will be available only to the
FM/digital community. This introduces potential safety and usability
issues. Stacking transmissions digitally isn't going to work when the
same transmissions must be mirrored on analog AM--and they have to be
if you want to maintain safety and keep controller workload
reasonable.

....honestly don't make any sense to me. In the first paragraph you see no
problem with two transmitters being used to transmit the same thing using
different frequencies and different modulation techniques, and in the
second paragraph you do. I think you could turn the first paragraph into
the second or vice-versa with appropriate special pleading - which is why
I'm confused about why you find a switch from AM to FM a better transition
than any other transition. I guess I just don't see what you see.

On Sat, 02 Sep 2006 20:19:15 +0200, Mxsmanic wrote:

Jim Logajan writes:

details snipped

Maybe someday the FAA and/or ICAO will consider replacing analog radios
with a more capable digital system....

All very interesting, but one of the criteria that any new system
would have to satisfy is that it would have to work in parallel with
the existing system. Adding features to the new system that are not
available in the old system would create dangerous differences between
the two. Seeing fancy displays in the ATC or tower for the lucky
digital users won't help deal with traffic from old AM users, and it
might even confuse things enough to cause problems.

A highly advanced solution would require replacing everything at once,
which isn't going to happen. A simpler solution that just provides
better quality audio could coexist with older systems without a
problem.

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.

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. The sad thing is, it does not appear Congress is going
to give it to them. Worse, the same report indicates, over the next
10-years, the FAA will exceed their required conversion dollars by simply
maintaining and repair their existing, archaic, analog infrastructure. In
other words, the FAA needs to do something...even if they are simply
updating their existing analog infrastructure. Regardless, the money does
not appear to be available.

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.

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...

o hang timer detection - a stuck PTT is not going to lock everyone out

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

o MUCHO better frequency utilization

o Limited data services

The list could go on and on...needless to say, digital has some neat
features.

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.

Greg

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?

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.

The sad thing is, it does not appear Congress is going
to give it to them.

Congress, like most of America, is hysterical about imaginary human
threats these days, and has probably lost track of the much more
mundane but much more serious safety risks associated with
infrastructure, aircraft, and crews.

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.

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

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.

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?

o Limited data services

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

The list could go on and on...needless to say, digital has some neat
features.

Neat features aren't necessarily desirable features. There is too
much of a tendency to bloat digital systems with features that have
been hastily designed, inadequately analyzed, and barely tested at
all.

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.

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

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

On 2006-09-02, Mxsmanic wrote:
The reason I ask is that improper and misunderstood radio
communication is a leading cause of accidents, and so it seems that

If you look at the NTSB reports, you'll find that this is not so - in
fact, accidents caused by bad radio communications are so rare, they
barely register as statistical noise! For each accident caused by poor
communication, there are probably thousands of accidents caused by a
pilot flying into weather they cannot handle.

Aircraft fly on the principles of Bernoulli and Newton, not Marconi, and
will fly quite happily with no radio at all, so long as the pilot
remembers to look out of the window and not bang into anything.

--
Yes, the Reply-To email address is valid.
Oolite-Linux: an Elite tribute: http://oolite-linux.berlios.de

Dylan,

accidents caused by bad radio communications are so rare, they
barely register as statistical noise!


Ah, but if it weren't for AM, that noise would be so much less ;-)

--
Thomas Borchert (EDDH)

FM receivers typically use a PLL (phase lock loop) circuit for
demodulation. In the presence of multiple received signals the largest
amplitude one will be locked to and thus demodulated. In AM receivers
all signals present at the detector above a certain noise threshhold
are detected. This feature, while often noiser, allows your friendly
controller to hear two guys calling simultaneously. For voice
transmissions, narrow band FM can be used and equivalent bandwidth
results.

Sorry if this is addressed somewhere in this thread already. I didn't
wade through all of it.

Mxsmanic wrote:
Perhaps this is a naive question, but: Why don't voice radio
communications for aviation use FM radio instead of AM radio?

FM offers better quality that AM when signal is relatively strong (or
signal to noise ratio is high). As the signal strength decreases, there
is a point when the quality is identical in both cases and then the
quality of FM deteriorates _rapidly_, while AM is still usable.
So, it might be a safety issue - using FM would effectively filter out
the weakest stations that could be heard if AM was used. Just a theory
;-)

Bartek