Comment by Sanzig | Hacker News Reader

Comment by Sanzig | Hacker News Reader

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While this should have been better studied before the spectrum was allocated, it's not really the fault of the FCC.

These radar altimeters have very wide front-ends, as as a result are highly susceptible to out-of-band emissions. The radars are allocated from 4.2 to 4.4 GHz, so the bottom edge of the radar altimeter band is 200 MHz above the top end of the 5G band in question (3.98 GHz).

The fact that these radar altimeters are susceptible to out-of-band interference that is literally further away in the spectrum than the width of their entire allocation is IMO an engineering failure. Yes, spectrum management (rightfully) tends to err on the side of not forcing the incumbents to make modifications to existing systems. However, better filtering would resolve this issue and it is shocking that the engineers who designed them didn't foresee that other services would eventually be allocated in adjacent bands. Avionics is often NRE dominated, they couldn't have sprung for a cavity filter? Or if that was too expensive, designed more headroom in the LNA before saturation and filtered the out of band signal with a crystal or SAW filter at IF? Rejecting out of band that far away is not rocket science.

I don't think it's unreasonable in this case to temporarily suspend the rollout and give the aviation industry a deadline to fix its radars, but it should only be temporary. Any other radio service would be told to take a hike if it keeled over due to out of band interference that far away.

This seems like Boeing trying to get the government to cover their butt because they did shoddy work on the altimeters. They were allocated spectrum for the equipment and then built a system that monopolized frequencies well outside of the allocated spectrum.

I completely agree that Boeing needs to issue a recall order on all affected hardware and provide a fix. This should be painful, it is basically punishment for trying to cut corners on safety equipment.

To add: if you ever read an FCC label on a device it says “must be able to accept harmful interference” or similar. So I’d argue the Boeing devices are the ones out of spec, should be immediately revoked as out of compliance by the FCC, and be done with it so the masses can enjoy 5G in peace.

I believe no agency should consider downstream effects of applying the law, to ensure fairness. Why should Boeing get a pass but say an out of compliance robotic lawnmower gets enforced?

That's a Part 15 label (see https://www.ecfr.gov/current/title-47/chapter-I/subchapter-A...) and it doesn't apply to avionics / radio navigation systems.

(I do agree this is Boeing's issue to fix, 200MHz guard band is huge, Boeing needs filters)

That's the kind of label you put on a device that can fail safely from harmful interference, in SOHO setting. It totally does not apply to aviation systems, and FCC didn't apply any of the mitigations used elsewhere in the world when 5G deployment in 3.8-4.0GHz range was allowed (it's in limited use in UK, and with restrictions near airports in Japan)

First, these altimeters are not made by Boeing.

Second, the FCC was already involved in their development and certification.

Third, these radars have been in operation for decades.

You are talking about thousands of planes that have to be modified to accommodate telcos. If anyone should pick up the tab for this it is Verizon and AT&T.

> Third, these radars have been in operation for decades.

That makes their domination of spectrum far outside their range worse.

If anything, threaten to retroactively charge them for all that bandwidth.

This isn't commercially allocated spectrum. It's in reserved radio navigation spectrum which is used all around the world by multiple different users.

It should have similar value either way.

Yes, but it's collectively used. We don't charge anyone for it because it's a public resource.

> First, these altimeters are not made by Boeing.

Who cares? The comment you are responding to says "they did shoddy work on the altimeters." That doesn't mean that Boeing built the altimeters.

They specced them, procured, inspected, installed and sold it with the whole airplane. If the pilots' chair collapses, that's shoddy work on part of Boeing, and nobody would talk about how they are just buying the chairs from someone. Same with the radio altimeters.

> Third, these radars have been in operation for decades.

Ok? Sometimes corners cut bite you in the backside only after decades.

There is one question which matters: Are the 5G towers transmitting on frequencies assigned to the use of radio altimeters or not? If they are they must stop. If the radio altimeters degrade or might degrade because of transmission outside of their assigned range and guard then they are faulty.

> Third, these radars have been in operation for decades. You are talking about thousands of planes that have to be modified to accommodate telcos.

Perhaps Boeing should have bid of the spectrum that their products depend on being unused.

This is essentially the government leasing out previously unused federal land for mineral extraction and a nearby rancher saying "it ruins my view," only with safety concerns added in.

It’s more like the rancher has been (legally!) grazing cattle on the land for 30 years and then the BLM sold the mineral rights.

This spectrum can’t simultaneously be “unused” and a problem for radar altimeters.

They didn't cut corners at all - that's the thing.

The allocation of the spectrum is only for transmission, and the radar altimeters in question passed that spec. It says nothing about receive, and due to how radar altimeter works, it's much, much harder to filter out interfering signals from systems like 5G (5G in case of interference will just drop maximum data rate as it filters mismatched codes or incorrect checksums, radar altimeter might return incorrect altitude)

Boeing does not usually make avionics. Companies like Garmin, Collins, and Honeywell do.

Does Boeing even make the radio altimeters?

Unless the airlines are installing the altimeters themselves, it seems like Boeing is still the relevant company. If your car’s airbag has a malfunction, the car manufacturer does the recall even if the fault lies with the airbag manufacturer. Presumably Boeing would then go on to sue the altimeter manufacturer though.

Aviation doesn't work that way. Airlines have direct aftermarket relationships with all of the OEMs making the components on their airplanes. From a certification and warranty standpoint, Boeing is the design authority, and if there is an FAA mandated fix for 737 RAs, Boeing will have a role to play. Whether Boeing is responsible financially or logistically for the fix depends on the nature of the failure (is it limited to the component or is it a systems engineering issue) as well as the contracts Boeing has with their supply chain and their airline customers.

That doesn't seem to be practical. Maybe for some big parts like engine airlines have direct relationships but all parts including radar altimeters ?

An modern commercial plane has tens of thousands of suppliers supplying millions of parts. No airline [1] could even imagine to have the staff to manage OEM relationships with that many indirect suppliers. That is what you pay Boeing for.

If that was the case no airline could afford to buy more than 1 type of plane or certainly not more than one manufacturer, you would need to handle all the suppliers in the industry?! it would be incredibly inefficient to do this for them individually .

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[1] Maybe United , American or Delta could have that kind of staff, but given the airlines operating margins in the last 2-3 decades even that seems unlikely

Commercial aviation works the way it works regardless of whether you think its practical.

And you're grossly overstating how many suppliers are on a commercial aircraft. There are in total an average of 30 tier-1s on a modern airframe, and yes, airlines are expected to work with all of them. Radio Altimeters are typically made by avionics suppliers i.e. Collins, Thales, or Honeywell for Part-25 commercial airliners.

Even a private owner can effectively have (if not direct, then through agents) connections straight to OEMs, bypassing original airplane vendor.

There's a reason why the only time two airplanes are identical is when they roll from assembly line as part of the very same order with same spec, and never again.

Yes, I am simply suggesting the accusations in the first two sentences are misplaced.

They certainly certified them on the aircraft. If it means putting pressure on the subcontractor that actually built them or finding a suitable replacement then so be it.

It's Boeing's job to make sure that the altimeters they put into planes are fit for use. It doesn't matter if those altimeters are sourced from Boeing, General Electric, Uzbekistan, or the Moon - Boeing is the one who has to make sure they work. It's why they make the big bucks.

Boeing can argue with and sue their suppliers, but that's not a problem for anyone but them and their supplier.

The altimeters are fit for use.

They aren't, because they are not operating within their mandated frequency bands.

If I made routers that flood the EM spectrum with random crap, even if their wifi 'worked' they wouldn't be fit for use, either.

The altimeters are operating within their frequency bands. They are operating as designed and certified. The 5G towers are the ones doing the flooding.

The 5G towers aren't putting significant signal outside of their allocated bands. This would be a very different story if they were spilling signal outside of their allocations.

The problem is the 5G allocation. It infringes on a band that existing use cases rely on.

Given Boeing's reliability vector, the fix might be more painful to others than itself.

From an engineering perspective, I would guess, as an avionics tech, back in the age of dinosaurs, these radio altimeters are probably of a very old simplistic design as to have dead simple and reliable electronics built with an absolute minimal number of parts. The carrier generation is probably just a crystal and an analogue multiplier with lots of phase noise and wide band so a similarly simplistic receiver can pick it up.

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