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Comment by gregmac | Hacker News Reader

Can someone with insight into radio technology explain this?

> The 5G network deployment in the U.S. starting on December 5 is in the 3700 to 3800-MHz bands then later in the 3700 to 3980-MHz bands. Radio altimeters use the 4200 to 4400-MHz band. [1].

Are these not sufficiently separated? Can this not be tested easily, by blasting 5G frequencies at a bunch of different planes in test flights? They've had like a decade to do this; I'd expect something more concrete then "concerns about potential interference" at this point.

[1] https://www.ainonline.com/aviation-news/air-transport/2021-1...

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There is something weird going on at FAA / NTSB etc.

In most radio applications a guard band of 220Mhz is absolutely unheard of.

You would do a guard at 10% of bandwidth lets say. On one side you'd have 5%.

So we are talking 1-2 Mhz?

My understanding was planes already had 2-3 altimeters, with guard band spacing of maybe 5Mhz (ie, planes already have devices blasting signals at frequencies much closer than 200Mhz).

There will be usually only one active radio altimeter, with center frequency around 4.3GHz.

So a guard band of 300Mhz then? What's weird is that I'd expect an altimeter to be on the order of .1 to 2MHz bandwidth max. There's just something wrong in the math here.

Radar altimeters use frequency modulated continuous wave (FMCW) and the bandwidth is 20 to 170 MHz (the larger the chirp, the more accurate it can be).

It's extremely difficult to have a 170 MHz wide filter at 4.3 GHz with a brick wall response out of band.

https://www.ntia.doc.gov/files/ntia/publications/compendium/...

It's probably more about the safety culture than the technology. This is the same industry that grounded the 737 MAX worldwide for almost two years due to some rare issues that would have been tolerated in almost every other field.

Because radio altimeters have not been required to tolerate interference above a certain threshold within ~10% of their frequency band, the assumption is that they cannot tolerate it until there is something like a decade or two of production-scale experience without any serious issues. Or until all passenger planes have radio altimeters certified for the new stricter requirements. And because the latter costs money, the industry won't do it as long as other options remain.

Those "rare issues that would have been tolerated in almost every other field" are the root cause that killed hundreds of people.

Not only that, the issue was caused by Boeing cheaping out on quantity of sensors, and then the ‘there is an issue’ light was sold as a safety upgrade. And the reason this was all needed was because Boeing wanted to escape having to recertify pilots for a new type rating.

Road traffic kills over a million people every year, many of them due to various technical and design flaws. People just tolerate those deaths far better than deaths from plane crashes.

Because road traffic deaths are rarely in the hundreds range per event due to preventable criminal negligence. When a bridge collapses and tens of people die, nobody tolerates it.

You’re making a misleading comparison.

If you want to liken the 737 MAX’s failures to road traffic, you have to ask yourself if there’d be broad acceptance of shoddily built, fault-intolerant bridges and freeways. Moreover, would that acceptance be greater or lesser if the public knew that engineers believed their designs were flawed?

Or say, a fleet of supposedly self-driving capable vehicles mayhaps?

I think this has much more to do with the fact that people drive cars and not airplanes. Most people have confidence in themselves and not others (cf. George Carlin's joke: "Have you ever noticed that anybody driving slower than you is an idiot, and anyone going faster than you is a maniac?"). So they're happy to make things safer if those changes only affect other people (as in an airplane). But when it comes to the fixes that would do the most to reduce traffic fatalities (e.g., lower speed limits on non-highways, more speed cameras, or perhaps most pertinently technology in cars that would make it more difficult to speed, etc.), those affect you too.

This is a common response, but people are usually fairly comfortable with trains and busses.

If the MAX had continued to fly without getting to the root cause it would have been a completely different story than the one that we have today, which is bad enough as it is with both Boeing and the FAA owning what they broke.

Those "rare issues" were also caused by a culture of prioritizing profit and speed of development over safety and training. More of those "rare issues" popped up months later.

Safety culture has scored some huge wins in the last 100 years. Decreased auto fatality rates, safer home materials (lead paint & asbestosis), expiration dates on food, reduced second hand smoke rates, hand washing in hospitals. Let’s not snub our nose as people that work to make a safer world.

Modern cars are ridiculously safer compared to older models. I was watching the news yesterday of someone that hit a concrete wall with a partial front-on collision at speed and just walked away. He was being interviewed at the crash site and looked fine, just a little dazed. Decades ago you'd have to have been air-lifted to hospital to have your bones joined back together with titanium plates.

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