Why don't voice radio communications use FM?

Mxsmanic wrote:
Perhaps this is a naive question, but: Why don't voice radio
communications for aviation use FM radio instead of AM radio? I
realize there's substantial inertia in the installed base of AM
equipment, but surely one could allocate some new frequencies to FM
and use them in parallel for some years to ease the transition.

If one were to mandate a replacement technology, it would be far far more
effective to use the packet-based mechanisms that digital cellular phone
technology and 802.11 wireless Ethernet (aka WiFi) rely on. Both these
technologies turn over the job of transmission collision resolution to
chip logic and take humans out of the loop. And it is possible to put
audio over WiFi using Voice over IP (VoIP) technology.

Such a system would be incredibly flexible. If one had, say, ten planes
in the air and they all started to talk to ATC at once, a packet-based
system would make it possible to do any of the following:

1) Clearly deliver only one of the voice signals to the controller and
provide a visual display that indicated 9 other planes had attempted to
speak also. It could even provide audio or visual feedback to the other 9
pilots that their transmissions were not delivered - or it could
automatically sequence the delivery of the transmissions to the
controller if the transmissions were not too lengthy.

2) If multiple controllers were available, the audio from several of the
planes could be routed to multiple controllers with no impact on audio
fidelity as far as the controllers or pilots are concerned.

3) Once you go packetized audio, you can put all sorts of useful stuff in
the packets for presentation to the other end - such as aircraft number,
the location and velocity vector from the aircraft's GPS or
altimeter/DG/airspeed indicator, and so on. A pilot could key the mike
and make a request without needing to ID themselves or their position -
that information would be extracted from the audio packet's header and
automatically presented on either a simple display to the controller or
mapped to a fancy map display.

The technical issues have been pretty much solved and commoditized in
both the WiFi VoIP and digital cellular realms. It is my humble opinion
that the radio technology currently being used for aviation
communications is now less reliable and useful than even that used in
home WiFi networks.

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

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.

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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.

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

Analog cell phones are being replaced with digital cell phones, so I fully
expect the same co-existence can be done with a changeover from analog
aviation radio to digital radio. 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.

On Sat, 02 Sep 2006 21:06:47 -0000, Jim Logajan
wrote in :

Analog cell phones are being replaced with digital cell phones, so I fully
expect the same co-existence can be done with a changeover from analog
aviation radio to digital radio.

They operate on different frequency bands, so that is not a good
analogy unless you can get the FCC to commit to allocating frequency
spectrum for aviation use.

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.

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

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.

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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.

Jim Logajan writes:

...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.

The second instance involves additional or different information being
transmitted over one channel, but not the other. The first instance
involves only a reduction of noise; the information content is the
same in both channels.

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.

I don't know if it's better than any other transition; I just think
that something should be done to improve the archaic system that
exists now.

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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.

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