What do you think about this airline vs 5G brouhaha?

Bryan Fields Bryan at bryanfields.net
Thu Jan 20 15:22:24 UTC 2022

On 1/19/22 10:33 PM, Jay Hennigan wrote:
> On 1/19/22 18:31, Bryan Fields wrote:
>> The narrower the filter is,
>> the higher the loss is. The greater the stopband attenuation is, the more
>> elements required and more ripple is present in the pass band.  Now granted
>> for avionics, this is doable in the thousands of dollars, but older radar
>> altimeters will not have this level of filtering, nor can you slap a filter on
>> avionics without manufacturer support.
> While the passbands are different in these as they're designed to pass 
> the C-band satellite signals and reject the radar, C-band filters with 
> insertion loss in the 1.4 dB range with 60dB rejection 20 MHz down have 
> been available for quite a while.
> https://cdn.shopify.com/s/files/1/0529/5806/8919/t/9/assets/eBPF-C-Spec-Sheet.pdf

My point was not they are not available, just that you can't take an existing
receiver and slap it in, cuz avionics.  It was really on the FAA to set some
basic receiver performance requirements as the FCC doesn't care about receivers :)

>> Further complicating this, radar altimeters in the 4200-4400 MHz band are
>> frequency modulating continuous wave transmitters.   In this configuration the
>> frequency is not closed loop controlled, it can be anywhere in the 200 MHz
>> band, as it's modulating a free running VCO nominally at 4300 MHz. This is a
>> non-issue as the transmitter is used for the receiver reference, so they are
>> locked to the same free-running oscillator.
> Fair enough, but C-band below 4200 has hardly been a desert all of these 
> years. TD-2 was on mountaintops all over the country pushing a couple of 
> watts into huge KS-15676 horns with something like 39dB of gain. 4400 > and above is also licensed for mobile use.

Those are narrow beams, typically 4 degrees or less half power beam-width in
both planes.

>> Only in recent avionics has the receiver been improved via DSP circuits and
>> FFT to do real time spectral analysis and pick out the right receive signal.
>> The older altimeters out there use simple zero crossing counting to determine
>> the frequency of the strongest signal.  This leaves them open to potential
>> interference by strong near band signals. Exasperating this is the poor
>> filtering on the RF receiver in 99% of altimeters when dealing with wide band
>> signals.
> If that's the case, how have they dealt with the signals from other 
> aircraft in busy airspace that are operating in the same band all of 
> these years?

Law of averages.  Every transmitter is offset from the others.  Since the
transmitter is phase locked to the receiver it forms an effective filter for
similar signals.

> The poor filtering on the receiver is obviously the issue. However, 
> suitable filters have been available for decades, adjacent frequencies 
> have been in use for decades, and it isn't the FCC's fault nor the 
> cellular carriers' fault that FAA has certified crappy receivers for use 
> in mission-critical applications.

Bingo.  This is something that the FAA/IACO should have been testing for and
set requirements for decades ago.

> Somebody using a crystal set to listen to a 1KW AM station 20 miles away 
> isn't going to get very far complaining to FCC about a new 5KW signal 
> 100 kHz below it and a couple of miles away. That the FAA would certify 
> radars with a front-end like a crystal set is the problem.

lol; that's what most pulse radars receivers are!

>> So can this LTE at C band work? Yes.
>> Will it require upgrades to avionics and standards? Yep.
> If the 5G allocation were shared spectrum with the radar altimeters, I'd 
> concede your point. However, it's at least 220 MHz away, over 5% of the 
> actual frequency in use.

5% is nothing when you have no front end filter other than the antenna.

> All of this talk so far is speculation about potential harmful 
> interference. Radar altimeters exist. Cell towers exist. Has anyone 
> gathered any real world data demonstrating actual interference?

Honeywell gave some proof of testing in their comments to the FCC.  There are
others too.

I think the potential for interference here is way over estimated, but the
issue is one of equipment that may be from the 1960's is not robust to
withstand interference in modern times.  This is an issue which must be
resolved prior to widespread deployment.  If it's a FAA certification process
or people having to replace altimeters with modern units, then that should
have been started years ago.

This could be a life safety issue that the FAA and industry didn't address,
but you can't just say "your receiver is worse than a baofeng, it's your
problem, screw your 220 passengers.  The Internet is serious business".  It's
a bit like a protester running in traffic and jumping on your car.  You should
be able to run them over, but at the end of the day it's property damage to a
car vs death to some idiot so it's not legal to run them over.

FAA dropped the ball and we all have to deal with it.
Bryan Fields

727-409-1194 - Voice

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