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It's not so black and white. From what I've read, radio altimeters operate in the 4.2-4.4 GHz range, and the 5G spectrum in question is 3.7-3.98 GHz. RF systems

by kayson 5y ago

It's not so black and white. From what I've read, radio altimeters operate in the 4.2-4.4 GHz range, and the 5G spectrum in question is 3.7-3.98 GHz. RF systems generally work by choosing a center frequency (say 4.3 GHz for altimeters), and trying to filter off anything outside the band of interest (4.2-4.4 GHz). The junk that remains is noise as far as the system is concerned, and has to be budgeted for very carefully. The problem is that radio altimeters are pretty old, and their filtering technology is not great. So even though they are filtering the 3.7-3.98 GHz 5G band, it's not as effective as a modern RF system/filter would be. Previously, this wasn't the issue because nothing was broadcasting in that band at particularly high power. That's the second component of the problem: power. The radio altimeter system is capable of handling some amount of noise power caused by unwanted broadcasts, among other things. Previously, satellites were using this band, and they were so far away that between the distance-based attenuation of the signal and the limited filtering, radio altimeters could operate unaffected. 5G is a different story, though. The towers are much closer to the runway, so the leakage power is much higher. Whether this actually causes a problem for the altimeters is then dependent on a number of factors, including the distance to the 5G tower, the transmit power level, the direction of the 5G antennas, etc. Could it cause a problem? Sure. I bet if you pointed a directional 5G antenna straight at a particularly crappy radio altimeter, you could materially affect its operation. But there are many mitigation strategies that could be used, like requiring lower transmit power, further antenna distances, appropriate directional transmissions, etc. These strategies have been used successfully in other countries. The real concern should be the total lack of collaboration between the FAA and FCC. It's turning into a jurisdictional pissing contest.

zachberger 5y ago
> The problem is that radio altimeters are pretty old Interestingly the 737-200 is excluded from this airworthiness directive. 737-200s are the oldest, still-operating members of the 737 family.
- bastardoperator 5y ago
  Total noob question, why isn't Boeing building/buying new radio altimeters that can filter better? Is it a money thing, a technology thing, maybe both?
  - cyberpunk 5y ago
    Certification I guess. It probably takes 2 years (at least) and an insane amount of money to change any component on an already certified plane.
    - bastardoperator 5y ago
      Sounds like they better get to work.
      - tjohns 5y ago
        They already are. Are you implying otherwise? The entire aviation community has been aware of this issue and the manufacturers have been working on it. It's not an exaggeration to say these things take time. "Move fast and break things" doesn't work well with aircraft.
        jsjohnst 5y ago
        > "Move fast and break things" doesn't work well with aircraft. Sure, but that didn’t seem to stop faulty lithium batteries on the 787 or MCAS on the 737-MAX. If only Boeing would learn the lesson, maybe we wouldn’t have these messes.
        tjohns 5y ago
        As has been pointed out several times already, this is not a Boeing problem. It affects practically every major aircraft manufacturer. If anything, Boeing has been very proactive about identifying the affected equipment well before any accidents happen. This is precisely what they should be doing.
thrashh 5y ago
AFAIK it’s not so much a lack of collaboration. It’s more nuanced than that. The lower frequencies used by Europe for 5G are already allocated for amateur and other uses here in the US. I believe Europe has been more willing to take away amateur bands. However here in the US, the FCC was trying to maintain those existing allocations and thus they chose to reuse 3.7-3.98 GHz instead, which is currently allocated for fixed satellite and other links. Therefore what FCC did is potentially better for everyone, if we can find out that radio altimeters—which should be filtering out frequencies that never belonged to them—are still able to work correctly. The issue is that, as you may know, if you engineer for something but don’t actually regularly test for it, you never actually engineered for that case. Up until this point, this case was never tested in production… because it has never existed in production. At the end of the day, we are between a rock and a hard place.
paulmd 5y ago
> The problem is that radio altimeters are pretty old, and their filtering technology is not great. So even though they are filtering the 3.7-3.98 GHz 5G band, it's not as effective as a modern RF system/filter would be. and actually not only are they old but they are defective - they were designed to function with guard bands half the size that the FCC ended up giving them, and they still are running into interference. They are basically picking up way way way more than they were originally specced to, and that's suddenly a problem because somebody moved in next door. also to my understanding it's specifically a problem with Boeing and other aircraft are much less affected. So we are also running into the "boeing has become a quasi-state enterprise that is too-big-to-fail and will be protected under all circumstances even when they do something obviously dangerous", like the MCAS system on the 737 MAX. honestly I kind of feel like the real situation here is that Boeing needs to issue an airworthiness directive and have those altimeters replaced. If a wing broke at half the loading spec, we wouldn't hesitate to say that's faulty and should be replaced. But with guard bands twice as wide as initially specced these altimeters are still having problems? Get them out of the air until Boeing fixes them. > The real concern should be the total lack of collaboration between the FAA and FCC. It's turning into a jurisdictional pissing contest. they were collaborating a long time ago - but again, there is only so much you can do when it suddenly turns out that a 2x margin of safety from the design spec (which itself incorporated a margin of safety) is not nearly enough. the spectrum was sold a long time ago. the airworthiness certificates were issued decades ago. At this point there is no solution possible that doesn't make somebody unhappy. I lean towards the party that needs to be made unhappy being Boeing here - but that will make airlines very unhappy as well. Saying "no you can't use that spectrum" is an unconstitutional taking at this point, you auctioned it off for cash monies and that absolutely puts a legally enforceable price tag as a minimum bound - in practice the value is higher since business plans have been built on it. But there is no constitutional right to fly a defective aircraft, if the FAA grounds your plane because it's unsafe that's not a taking. But that will have a massive impact on boeing, airlines, and the economy as a whole.

by kayson 5y ago

zachberger 5y ago

> The problem is that radio altimeters are pretty old Interestingly the 737-200 is excluded from this airworthiness directive. 737-200s are the oldest, still-operating members of the 737 family.

bastardoperator 5y ago
Total noob question, why isn't Boeing building/buying new radio altimeters that can filter better? Is it a money thing, a technology thing, maybe both?
- cyberpunk 5y ago
  Certification I guess. It probably takes 2 years (at least) and an insane amount of money to change any component on an already certified plane.
  - bastardoperator 5y ago
    Sounds like they better get to work.
    - tjohns 5y ago
      They already are. Are you implying otherwise? The entire aviation community has been aware of this issue and the manufacturers have been working on it. It's not an exaggeration to say these things take time. "Move fast and break things" doesn't work well with aircraft.
      - jsjohnst 5y ago
        > "Move fast and break things" doesn't work well with aircraft. Sure, but that didn’t seem to stop faulty lithium batteries on the 787 or MCAS on the 737-MAX. If only Boeing would learn the lesson, maybe we wouldn’t have these messes.
        tjohns 5y ago
        As has been pointed out several times already, this is not a Boeing problem. It affects practically every major aircraft manufacturer. If anything, Boeing has been very proactive about identifying the affected equipment well before any accidents happen. This is precisely what they should be doing.

thrashh 5y ago

AFAIK it’s not so much a lack of collaboration. It’s more nuanced than that. The lower frequencies used by Europe for 5G are already allocated for amateur and other uses here in the US. I believe Europe has been more willing to take away amateur bands. However here in the US, the FCC was trying to maintain those existing allocations and thus they chose to reuse 3.7-3.98 GHz instead, which is currently allocated for fixed satellite and other links. Therefore what FCC did is potentially better for everyone, if we can find out that radio altimeters—which should be filtering out frequencies that never belonged to them—are still able to work correctly. The issue is that, as you may know, if you engineer for something but don’t actually regularly test for it, you never actually engineered for that case. Up until this point, this case was never tested in production… because it has never existed in production. At the end of the day, we are between a rock and a hard place.

paulmd 5y ago

> The problem is that radio altimeters are pretty old, and their filtering technology is not great. So even though they are filtering the 3.7-3.98 GHz 5G band, it's not as effective as a modern RF system/filter would be. and actually not only are they old but they are defective - they were designed to function with guard bands half the size that the FCC ended up giving them, and they still are running into interference. They are basically picking up way way way more than they were originally specced to, and that's suddenly a problem because somebody moved in next door. also to my understanding it's specifically a problem with Boeing and other aircraft are much less affected. So we are also running into the "boeing has become a quasi-state enterprise that is too-big-to-fail and will be protected under all circumstances even when they do something obviously dangerous", like the MCAS system on the 737 MAX. honestly I kind of feel like the real situation here is that Boeing needs to issue an airworthiness directive and have those altimeters replaced. If a wing broke at half the loading spec, we wouldn't hesitate to say that's faulty and should be replaced. But with guard bands twice as wide as initially specced these altimeters are still having problems? Get them out of the air until Boeing fixes them. > The real concern should be the total lack of collaboration between the FAA and FCC. It's turning into a jurisdictional pissing contest. they were collaborating a long time ago - but again, there is only so much you can do when it suddenly turns out that a 2x margin of safety from the design spec (which itself incorporated a margin of safety) is not nearly enough. the spectrum was sold a long time ago. the airworthiness certificates were issued decades ago. At this point there is no solution possible that doesn't make somebody unhappy. I lean towards the party that needs to be made unhappy being Boeing here - but that will make airlines very unhappy as well. Saying "no you can't use that spectrum" is an unconstitutional taking at this point, you auctioned it off for cash monies and that absolutely puts a legally enforceable price tag as a minimum bound - in practice the value is higher since business plans have been built on it. But there is no constitutional right to fly a defective aircraft, if the FAA grounds your plane because it's unsafe that's not a taking. But that will have a massive impact on boeing, airlines, and the economy as a whole.