COMMERCIAL AIRCRAFT GOING DIGITAL???

BNN123 · Post by **BNN123** » Fri Oct 20, 2006 4:11 am

does anyone here know if the commercial airline freq from 118.00-136.00 plan on going from AM mode to digital voice???

thanks

spectragod · Post by **spectragod** » Fri Oct 20, 2006 4:14 am

Doppler effect says AM will be the choice of aircraft everywhere.

SG

Post by **Wowbagger** » Fri Oct 20, 2006 5:43 am

spectragod wrote:Doppler effect says AM will be the choice of aircraft everywhere.

SG

I keep seeing people say this, and I keep saying Male Bovine Scat.

All Doppler does is cause a carrier frequency error - which ANY FM or digital system has to deal with anyway.

Let's do the math. Assume an APCO-25 source at 800 MHz moving at Mach 1 toward the repeater - what is the carrier frequency error?

Mach 1 is roughly 1200 km/h or 333 m/s. The observed frequency is:

Fo = SQRT( (1-V/c)/(1+V/c))Fc

Fc = 800 MHz, v = 333 m/s, c = 3E8 m/s

Fo = 799999112.000

Frequency error is therefor 888 Hz. While that is more than the minimum symbol deviation of 600 Hz, it is perfectly possible for a decoder to lock onto that, as I well know because the decoder I wrote for the 2975 will lock onto a signal up to 2 kHz away from nominal carrier frequency.

For a normal NBFM voice, that error will appear as a DC component of the recovered audio - and will be dropped by the first DC blocking cap in the system (or equivalent signal processing element).

The only real advantage of AM vs. FM is the absence of capture effect - the tower can hear if another aircraft was transmitting and can thus notify the aircraft that they've doubled.

There *is* a move to go to a more trunking like system to prevent aircraft from doubling and covering up each other's conversation.

The biggest single problem it that in the aviation business, *any* change is seen as bad due to the potential for litigation:

If the new system fails you get sued (even if the old system would have failed WORSE) .

If the new system DOESN'T fail, you get sued for all the times the old system failed because you didn't deploy the new system.

Why do you think so many general aviation craft, even ones being built RIGHT NOW, use magneto driven ignition rather than electronic ignition? - Fear of change and the lawsuits caused thereby.

bellersley · Post by **bellersley** » Fri Oct 20, 2006 12:46 pm

If it isn't broken, don't mess with it. AM has been around on the air band for ages, and isn't going anywhere anytime soon.

Doesn't IMBE have a "speed limit" of around 100mph anyway? I know when I'm zipping down the highway faster than I should be, I start to get packet loss, and its not due to a weak signal.

Post by **Wowbagger** » Fri Oct 20, 2006 1:52 pm

bellersley wrote:If it isn't broken, don't mess with it. AM has been around on the air band for ages, and isn't going anywhere anytime soon.

Precisely the attitude the aviation crowd have, as I had said.

bellersley wrote: Doesn't IMBE have a "speed limit" of around 100mph anyway?

IMBE is a vocoder - it knows nothing of how the data is formatted on the air.

If you mean APCO-25, please say so.

bellersley wrote: I know when I'm zipping down the highway faster than I should be, I start to get packet loss, and its not due to a weak signal.

Not knowing any other details I cannot say - it could be picket fencing for all I know. I know that the Doppler shift at those speeds is going to be about 80 Hz (assuming 800 MHz band operation), or about .1 ppm - within APCO spec. Assuming your radio is properly aligned that in and of itself wouldn't put you out of spec. Also, since Phase 1 is not a TDMA system, there wouldn't be any issues with time slot drift like there would be for GSM.

airman1952 · Post by **airman1952** » Fri Oct 20, 2006 3:23 pm

Being a pilot myself, the biggest necessity in aviation communications is everyone NEEDS to be able to hear EVERONE that is assigned to any specific frequency during a flight in the air traffic control system at all times, especially when flying under instrument flight rules.. AM, to me, is the simplest, most reliable way at the moment. Other than dealing with an occasional double and/or crappy functioning radio gear, if you add any other "oddities of complication" in the mix (such as digital or trunk), it would make me very nervous about myself, or someone else missing an assigned altitude or clearance because I, or they did not hear it. Talk about litigation!!! I take a lot of comfort in being able to hear the other guy repeat back his clearence or altitude assignment correctly. He probably feels the same about me, especially when we both know we are close to each other.

Gary

MTS2000des · Post by **MTS2000des** » Fri Oct 20, 2006 4:59 pm

I've said it before and I'll say it again: digital radio isn't ready for primetime in mission critical hot zone enviornments, such as hot zone firefighting and especially aircraft communication. Digital hasn't been around long enough to mature and is still a work in progress. Yes, I am basing this most on my personal experience on APCO P25 Phase I, which IMO was pushed into the marketplace without much forethought.

When it comes to life and death mission critical radio, the tried, tested, predictable and solid AM and NBFM modes still shine. No worries of bad code in vocoders or DSP, BER, etc. And the most important part of the chain is the radio operator who himself can be just as important as the equipment. Digital radio takes the element of being able to discriminate intelligence from the noise by human ear. The DSP and vocoders do all that for you, and they are only as good as the human who designed and programmed them.

BNN123 · Post by **BNN123** » Fri Oct 20, 2006 7:16 pm

I seen that NEXCOM is working on a multimode radio, analog and digital for commercial airlines...projection for this to be fully in place is 2015 rumor.

anyone hear the same????

N9LLO · Post by **N9LLO** » Sat Oct 21, 2006 12:41 am

As stated, doppler is not the problem, its' FM multipath interference the avoid by using AM. That is what I seem to remember from the old 1st class radiotelephone license study guide.

Chris
N9LLO

Hightower · Post by **Hightower** » Sat Oct 21, 2006 1:22 am

The reason for continued AM use is that AM doesn't suffer from the "capture" effect FM exhibits. If two pilots key at the same time, control might not be able to decypher what the distant pilot says, but hears that someone was keying at the same time.

FM tends to capture the strongest signal, and you can't even hear a second (even slightly lower power) guy keying.

Hightower · Post by **Hightower** » Sat Oct 21, 2006 1:26 am

N9LLO wrote:As stated, doppler is not the problem, its' FM multipath interference the avoid by using AM. That is what I seem to remember from the old 1st class radiotelephone license study guide.

Chris
N9LLO

I would not think that multipath would even come into play, seeing as your transmitting to an object line of sight, with nothing in the path of the signal transmitting/receiving. Unless the plane is below the horizon from the transmitting source, how could multipath come into play?

bellersley · Post by **bellersley** » Sat Oct 21, 2006 5:17 am

As well, AM isn't as likely to fade or whatever with different polarizations. Hopefully that doesn't become a problem for aircraft, but.

N4DES · Post by **N4DES** » Sat Oct 21, 2006 5:35 am

Who say's it has to go FM, it could go digital and still be AM.

Post by **Wowbagger** » Sat Oct 21, 2006 5:53 am

bellersley wrote:As well, AM isn't as likely to fade or whatever with different polarizations. Hopefully that doesn't become a problem for aircraft, but.

Polarization will affect FM just as much as AM - modulation has nothing to do with polarization (unless you are "modulating" the signal by changing the polarization, but that would not be the best way to encode a signal).

BNN123 · Post by **BNN123** » Sat Oct 21, 2006 6:44 am

word/rumor is that by 2015 all commercial aircraft radios will be digital, as per nexcom who is promoting this type of mode for the entire airband.

they just made the 8.33 spacing, is this change made instead of digital??

different people give conflicting information, last person said by 2015 it will be done, it is on hold for know!!!!!!!!!!

confused!!!!!!!!!!!

tvsjr · Post by **tvsjr** » Sat Oct 21, 2006 7:02 am

You can file this in the same location as:
All public safety narrowband by 2006... err, 2009.
All public safety being forced P25 by 20xx...
All public safety moving to the gov't-owned iDEN network by 2010...
700MHz being completely cleared by 2006... err, not so much.
All NTSC TV being switched off by 2005... err, 2009.

Get the picture? It's going to be a *long* time. I'll probably be old and gray before it actually happens.

wa2zdy · Post by **wa2zdy** » Sat Oct 21, 2006 5:19 pm

Economics will also play into such a move in the aviation system.

To make a change like this, every civil aircraft and ground station in the world would have to make the change at the same time of the same day.

With the airlines claiming such financial difficulties as they are, they'd never be forced into such a move. And even if they were, the logistical nightmare would be nearly insurmountable.

mike m · Post by **mike m** » Sun Oct 22, 2006 7:26 pm

Its called VDL mode 2, data and VDL mode 3 voice and data . It uses TDMA modulation and there are already radios on the market that are mixed mode, AM and digital capable but I don't see the digital voice portion being turned on for some years.

The Wulfsberg CVC-151 xcvr which just received FCC approval is a VDL mode 2, 3 capable radio but since no one on the ground is transmitting digital voice yet the digital comm part has not been enabled.

It looks like Europe will be turned on before the US does but even when is not known for sure.

Mike

BNN123 · Post by **BNN123** » Mon Oct 23, 2006 5:53 am

where do they sell this radio, curious what the price is

mobile/portables etc

thanks

k2hz · Post by **k2hz** » Mon Oct 23, 2006 6:25 am

My understanding of the VDL modes is that they are an enhanced data mode that is proposed to replace ACARS. The idea is that VDL will handle both ATC and operational messages vs ACARS which is strictly for operational traffic.

I have not seen anything about digital voice for aircraft but the idea is that much of the present ATC voice traffic such as clearences will be sent as digital data to a cockpit terminal and aircraft status messages will be sent automatically to ATC via VDL.

Here is a link to a description of VDL:

http://spacecom.grc.nasa.gov/icnsconf/d ... -Burks.pdf

airman1952 · Post by **airman1952** » Mon Oct 23, 2006 7:10 am

I'm just a simple minded general aviation pilot that occasionally uses ATC system for a IFR flight. So, where some may see aviation communications possibly going, in reality, it will be long time coming. The airlines would love to see it happen very soon just to force GA out of the skies through pure cost prohibition. The problem there is, the GA lobby is very strong. However, this is for another thread in another forum.

My stance is: When I'm using the ATC system using IFR, in my very simple, instrument capable, GA aircraft, I want to be able to hear what all is being said on whatever frequency I'm assigned to during my flight progression. In rough weather, I do not have the time to be looking at some digital display or whatever for any in-flight instructions. Amplitude Modulation works very well, IMHO. Even the squealing hetrodyne of a double wakes me up to the fact that someone else is out there and someone eventually is going to say, "Say again, please!" If I have to ask a controller fifteen times to repeat an altitude or a clearance (dispite his or her resulting agitation), it is my right as the PIC. It is very simple to push the TX button on the control yoke and talk away. It has to be KISS in aviation as much as possible!

Off my soapbox, now.

Thanks,
Gary

COMMERCIAL AIRCRAFT GOING DIGITAL???

COMMERCIAL AIRCRAFT GOING DIGITAL???

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