Why are airlines predicting doom and gloom about 5G?

[Bill Horne]

Tom,

Please listen to the podcast available here, and pass this along to any of the heavyweight techs in your ham club.

Long story short: the aviation industry is hyping the possibility of dead bodies all over the place if the cellular carriers (which have paid big bucks to put 5G equipment in "C Band") don't stop and change their plans and go someplace else.

Here's the point that I'm confused about: according to the podcast, the cellular authorizations go up to 3.98 GHz, and the aircraft altimeters that we're hearing all these dire warnings about are assigned to a range which starts at 4.2 GHz. I'm old-school, admittedly, but having 220 MHz of "guard" space between those two services seems adequate to me.

Ergo, why the fuss?

• Are the avionics salesmen trying to create a firestorm of fear that motivates airlines to buy brand new radar altimeters?
  
• Do the old altimeters have substandard design?
  
• Is the cellular industry choosing to ignore known risks?
  
• Is it all a ploy by the cellular carriers to grab more spectrum for cheaper prices?
  

I'm just waiting for the other shoe to drop, because there's something unsaid in this debate.

Replies intended for publication should be sent to telecomdiges...@telecom-digest.org.

Bill Horne
Moderator, The Telecom Digest
http://telecom-digest.org

-- 
I don't want to say that I'm old and worn out, but I'm never anywhere near the curb on trash day

[Dave Platt]

>Here's the point that I'm confused about: according to the podcast, the
>cellular authorizations go up to 3.98 GHz, and the aircraft altimeters
>that we're hearing all these dire warnings about are assigned to a range
>which starts at 4.2 GHz. I'm old-school, admittedly, but having 220 MHz
>of "guard" space between those two services seems adequate to me.

>Interim moderator's response:
>
>The problem is that the FAA standards for altimeters required them to
>exclude signals from "more than 10%" away. That's 420 MHz of allowable
>sloop in the receiver. They made that standard in 1983 and never updated
>it. Many altimeters are better than that, but some apparently aren't, or
>aren't much better, so they can pick up signals from 3.8-3.98 GHz.

There's a graphic showing that portion of the spectrum in the article
at:

https://www.aviationtoday.com/2021/12/23/faa-issues-new-radar-altimeter-5g-c-band-risk-assessment-request-aviation-industry/

The "Typical RA filter tolerance mask" as shown there (without a dB
scale, alas) is quite broad, and the 5G emission strength shown is
quite high (higher than the radar-altimeter emissions, and far
higher than the satellite signals previously used in that part of
C band). I haven't found a document yet which shows the actual
emissions strengths and masks with an accurate scale.

My guess is that the problem with some altimeters may be one of
"desense" (de-sensitization). If the altimeter receiver doesn't have
sharp filters before its first active gain/detector stage, the
out-of-band signal can saturate the gain stage, and this reduces the
gain for the desired in-band signals. In effect, the radar
reflections "go away" when the altimeter enters the area on which 5G
emissions are strong. To use an audio analogy: you can't hear the
piccolo, when the bass guitarist has cranked the volume up to 20 and
the amplifier is clipping and the speaker cones is being driven to its
limit. Filtering at a later point in the chain, after the point of
amplifier clipping, doesn't help.

It's also possible in principle for multiple, strong out-of-band
signals to mix (heterodyne) inside the receiver, creating spurious
signal products inside the altimeter bandwidth and "confusing" the
altimeter.

It's surely possible to build altimeters with the necessary filtering,
and to replace existing altimeters with better ones, but that will
take a long time and cost a pretty penny.

There seem to be two issues here in the US which are making this
problem more severe than in Europe: the authorized 5G C-band emission
limits are higher, and the C-band antennas here aren't required to
have a "down-tilt" pattern to limit their emissions to mostly "below
the horizon". So, when a plane is in its landing pattern (which is
where you'd most want the altimeter to be working correctly) it will
enter the area where the altimeter is most likely to be overloaded
by strong C-band 5G signals.

[John Levine]

It appears that Bill Horne <malQRMass...@gmail.com> said:
>Ergo, why the fuss?
>
> * Are the avionics salesmen trying to create a firestorm of fear that

> motivates airlines to buy brand new radar altimeters?

To some degree, but a lot of it is "everything was fine until you guys
showed up" and the FAA doesn't understand radios very well. The first
study of interference with identified equipment only showed up a
couple of weeks ago, after years of duelling press releases.

> * Do the old altimeters have substandard design?

Definitely.

> * Is the cellular industry choosing to ignore known risks?

It depends on your definition of known risks. There are duelling studies,
the risks look pretty low to me, particularly if the telcos limit power
in cells near runways which would not be a big deal.

> * Is it all a ploy by the cellular carriers to grab more spectrum for
> cheaper prices?

Check how much they paid for C-band in the spectrum auction and you'll know
the answer to that one. Heck, no. But they may end up paying a lot more
if they're stuck with the cost of replacing every 30 year old altimiter
in the country.

R's,
John