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Comment by kube-system | Hacker News Reader

As I understand (not a pilot but just interested in aviation), these altimeters are critical for use during low visibility landings, to determine when it may be safe to continue landing, or when they must abort landing (e.g. if they can't see the ground yet because of fog)

Being wholly inaccurate or no reading may be equally dangerous if it happens at the wrong time.

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Right, and I think the FAA requires airliners to be capable of landing in IFR ("low visibility") conditions, which makes sense—you don't want trans-oceanic flights to crash because the weather abruptly got worse at their destination and they can only land when it's clear skies.

So "this altimeter may be inaccurate near 5G" means "this airliner may not be airworthy near 5G" which means airlines must ground those planes. It sounds like there are enough of them that it'd seriously affect air travel.

Recommend https://youtu.be/I9QHvd2bOvU which even managed to work some 5G-related jokes in there.

> So "this altimeter may be inaccurate near 5G" means "this airliner may not be airworthy near 5G" which means airlines must ground those planes.

No, they don't need to ground the planes.

There's different types of IFR approaches available to chose from, depending on equipment and how much visibility is reported. An inoperative radio altimeter means the aircraft couldn't land using the approaches for the lowest-visibility conditions, but doesn't mean they can't land under other less-severe IFR conditions.

In practice, this means the aircraft wouldn't even be dispatched if the weather forecast showed the destination's weather was below the aircraft's capabilities. They would also have enough fuel to divert to an alternate airport with better weather.

The idea they can have this type of interference low to the ground in a landing situation (when the reflected power is high) is a total failure on behalf of boeing. I can't even fully get my head around the physics that would get you an issue with even normal filtering, much less what once should be able to do on an airplane with high fixed costs and safety of life factors.

Amazing they can't do GPS as a cross check on indicated altitude over a terrain model.

> Amazing they can't do GPS as a cross check on indicated altitude over a terrain model.

I mean they could, but they skimped out on just getting a halfway decent filter for frequencies 200MHz away from their signals what are the odds they'd pay for that.

No, they really can't. At least not until GBAS is rolled out. GPS is notoriously unreliable for altitude data.

GPS altitude is used for LPV approaches, provided you also have a WAAS correction signal, but those require higher weather minimums than the Cat II/III approaches that you'd use a radio altimeter for.

1. This isn't just a Boeing problem. Not only does the FAA have these concerns for other jets, they've also prohibited some other jets from landing at some airports in some conditions because of it.

2. Boeing doesn't get to decide to just cross check against GPS. The FAA has rules about the equipment that can be used for various types of landings.

GPS is incapable of getting even close to the resolution required from radar altimeter, and altitude ranging in GPS was always worse than its circular error probable in 2d - and the best accepted dynamic, unaided GPS deviation is big enough to show the same result between correct landing, missed landing, and a fiery crash.

The problem is that radar altimeter is very precise device that for best precision needs to use all of its 200MHz bandwidth on transmit. And there are no methods to recognize that as part of the return in its own 200MHz band (or in the guard band) a random CDMA-encoded signal radiated outside of allocated spectrum (happens) reflected of random crap around the airport and gets interpreted as valid data. This is an analog system where the analog characteristics are core to how it works, so the methods that allow tight channel spacing in communications tech do not work for radar altimeters

I don't know much about aviation, but I wouldn't want to rely on GPS for anything important. It's easy to jam and not always reliable.

These altimeters are required for decreased ILS minimums and autoland. You can perform a Cat I ILS down to 200' AGL in 1/2-mile visibility just fine without any radar altimeters.

Am a pilot, but just PPL, from my understanding the greatest threat is forcing the autopilot to perform certain maneuvers thinking terrain is closer than it actually is.

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