Why are airlines predicting doom and gloom about 5G?

Tom,

Please listen to the podcast available here

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, 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.

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Bill Horne Moderator, The Telecom Digest

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***** 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.

So both sides are at least partly to blame. The FAA allowed crap altimeters to stay around too long, simply because there was no immediate need to do better. And the FCC discounted their concerns, because the actual risk is pretty small. Also, the FCC allowed the mobile base stations to operate at higher power levels than European ones can, and if they actually do run full power

Reply to
Bill Horne
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There's a graphic showing that portion of the spectrum in the article at:

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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.

Reply to
Dave Platt

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.

Definitely.

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.

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

Reply to
John Levine

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