If Boeing needs 3.7GHz-5GHz in order to safely operate their aircraft, then Boeing needs to purchase and own the 3.7GHz-5GHz band. If they only want to pay for 4.2GHz-4.4GHz and nothing but 4.2GHz-4.4GHz, but squat on 3.7GHz-3.98GHz they can pound sand.

At the time these units were built, the filtering was sufficient to guard against the known adjacent users of the band (low-power satellite signals). Neither the FCC or the FAA set performance standards to suggest otherwise. Nobody expected at the time we'd be blasting high-power omnidirectional signals everywhere at 4 GHz - outside a few specialized uses, it's a terrible band for wide-area communication unless somebody is motivated enough to build tons of towers...which the cell companies suddenly are.

We can agree that the filters are garbage by modern standards, but they were considered adequate at the time the units were built based on the circumstances.

If Airbus has the same problem, then ground Airbus too.

> Boeing doesn't even make the avionics in question. Some of the manufacturers don't even exist anymore. It's not clear who would pay,

Surely when the parts break, they are fixed, and if they are not fixed, then the plane is either grounded or flies in conditions such that the equipment is not required. There is a somebody who repairs, services, or replaces broken parts. These parts are broken. So that somebody fixes it.

> or how you'd enforce it.

You'd enforce fixing this broken part the same way you'd enforce fixing any other broken part. When the FAA finally got around to deciding that MCAS was in fact broken, they grounded the 737 MAX until it was fixed.

Everybody's acting like it's not a big deal that this equipment can't handle a signal that is more than 220MHz away from their nominal operating frequency. Certainly if Hasbro wanted to sell Batman(TM) BatWalkie BatTalkies, but they were rendered inoperable by another signal that far from their operating frequency, the FCC would deny the device its operating license and it would be pulled from the market.

> Nobody expected at the time we'd be blasting high-power omnidirectional signals everywhere at 4 GHz

Infrastructure 5g antennas are directional. The mobile devices themselves are omnidirectional, but those are very low power devices.

At least I know why cell service at the airport is awful.

Because at the time the equipment was bought, it met every applicable federal standard. This would be unfair to the owners who had no way of knowing about the issue.

Also, in nearly every other case I can think of when spectrum changes, the FCC grandfathers in owners of legacy equipment.

> At least I know why cell service at the airport is awful.

I don't think that's at all related, given that we just started rolling out 5G. Plus the bands in question don't travel through walls very well. :/

At least in Europe, there is precedent that modern use of frequency assignments make older equipment illegal to operate. 4G and 5G have heavily eaten into the frequencies used on stage for event equipment - microphones, IEM, that sort of thing. Many, many pieces can‘t be legally operated anymore, and it‘ll probably be the same in a few years time with the current-gen devices (although I hope I can swap crystals if needed and keep using my stuff).

Radio bandwidth is a scarce resource. If the radio altimeter is transmitting well outside of its boundaries and it creates problems (for any of the parties requirements), it’s the radio altimeters that need fixing. Even if it is expensive to do so.

As I've point out in my post, why on earth wasn't all this sorted out before 5G was allocated the frequencies.

It seems to me that the the powers behind mobile services have railroaded themselves into these frequencies and that the FCC etc. haven't had the gumption to sort it out beforehand.

What do you mean by 'bad' receivers?

Spectrum management, irrespective of country, sets the parameters of every radiocommunication service by notional equipment standards - here that would be the typical intermodulation figures for radar receivers, inter alia.

The FCC and ITU through WARC/WRC - World Radiocommunication Conferences would have been aware of notional equipment standards for radar altimeters at the time the spectrum was allocated (equipment must comply with the standard or it be deemed out of spec - non compliant).

Note: adjacent services must consider interference to their adjacent services, etc. The standards associated with any service can only be deemed old or 'changed' at WRC conferences specifically to avoid the situation we now have here. The real question is 'who fucked up' - i.e.: did not abide by WRC rules (the international treaty/agreement obligations).

Remember, here international rules are paramount BECAUSE aircraft licensed in other countries that land on US territory also have to meet the same international standards for their radar altimeters as those of the US fleet - it has nothing to do with receivers being 'bad').

FYI: https://en.m.wikipedia.org/wiki/World_Radiocommunication_Con.... Note: the ITU deems that the FCC IS responsible for radio regulations within the USA.

Ah, yes. The best thing you can have is tribal mentality in your government departments. Where instead of cooperating to sort problems out you just say "fuck everyone else" and press onward.

"Airbus has issued Flight Operations Transmission 999.0002/22 (01/10/2022) which describes the currently understood effects of radar altimeter anomalies. Guidance is provided for A320 family, A330, and A340 aircraft for handing these anomalies.

"Embraer Flight Operations Letter 170-001/22 and Operational Bulletin 170-001/22 (01/02/2022): Describe the possible effects of C-Band 5G interference on its E170, E175, E190, and E195 aircraft.

"The FAA has developed a process by which better performing radar altimeters that are able to reject 5G interference can be approved to operate without regard to the AD and NOTAMs. These Alternate Methods of Compliance (AMOC) approvals will be specific to a combination of aircraft model and radar altimeter model. The method of approval will take into consideration the performance of the aircraft/radar altimeter combination, as well as the location and power of the 5G transmitter in the vicinity of the airport. Therefore, the AMOCs are being issued with a list of airports where they are effective. As of 1/25/2022, FAA has approved AMOCs for several models of radar altimeters, installed on Boeing 717, 737, 747, 757, 767, 777, 787, and MD-10/-11; Airbus A220, A300, A310, A319, A320, A321, A330, A340, A350, and A380, Embraer 120, 170, and 190, all Canadair CRJ aircraft, some DeHavilland Canada DHC-8, and some ATR aircraft. These AMOCs are limited to certain airports and specific models of radar altimeters. Pilots should ensure that they have AMOC documentation for their aircraft and airport (e.g. in a dispatch release) before operating at an airport without regard to the 5G AD/NOTAM."

https://www.alpa.org/resources/aircraft-operations-radar-alt...

Also, the FAA has no way of knowing if a particular aircraft operator is using a radio altimeter or not. Even if I have one installed, maybe I chose to switch it off. It's also possible I'm using it for situational awareness only (which is still allowed) and not using it to descend below legal approach minimums.

Keep in mind these sensors were designed and approved long before the FCC allocated this spectrum above this range to 5G. 200 Mhz may be massive for communications, but not that is not entirely true for radar.

> Radar in general is more sensitive to noise.... The noise floor is quite a bit higher before it becomes unusable for communications compared to radar

This is not true. A radar has the huge advantage of knowing what signal was sent and only looks for returns that are similar- the FMCW radars in question often mix the return with the originating signal, and outside interference just doesn't cohere and won't affect the result unless the interference is very high power. Radars do not need to look at a wider band of spectrum, reflections do not change the frequency and the Doppler shift a radar altimeter (looking straight down) needs to accommodate is minuscule. A radar that can't handle noise is poorly designed, not that they are inherently susceptible to noise. Radar does require "high" transmit power to easily detect returns, but they can be robust to noise. A radar (including RA) can integrate over a relatively large window if needed and detect with an SNR <1 if it's designed that way.

Whether the old 737 RA was properly designed and can handle the interference, or if they took shortcuts because saving a buck took precedence is a different discussion than the (incorrect) generalities about radar presented in the above comment. Whether the existing RAs can be retrofitted with better filters for cheaper than lobbying the FCC is also unclear, but we know what approach the airlines are attempting right now.

I agree with the general tenet of your post but do we know what the status of the radar's RX's front end is like in practice?

From my experience, many of the front ends of older radar receivers are primitive by comparison with today's state of the art in that they have very bad crossmoduation figures (basic diode mixers etc.).

The gap is the same size as the entire allocation, and the frequency difference is more than 4 percent. That's massive.

> Radar needs to look at a wider band of spectrum as things will get shifted more from reflection and such.

If you skew a radar signal by a thousand miles per hour, isn't that around 2 parts per million? Am I missing an effect here?

Edit: I will add the a fair amount of interference by multi path return can be mitigated by making the transmitter and receiver shielded from other directions and designed to be highly directional. This also is generally not to hard to do for an altimeter since the datatum one is measuring after all is just elevation over a small area, compared to something like radar for topography that wants to cover a large area.

RFI is a problem in these spaces [1]. It's been a problem for many years, and is an increasing one.

[1] https://www.researchgate.net/publication/283760070_The_Threa...

Franky speaking, everybody here is talking out of their asses, and just rehashing bullshit that they get told. I'll lowbrow dismissal that all day long because it doesn't deserve and inch more of effort.

This whole discussion has a really low SNR

Additionally, lot these weather radars are not trying to achieve same level of accuracy as an altimeter. If the weather formations ones monitoring is off by a little bit, but overall the data is close it's fine. Similar thing with a lot maritime radar, ships are not moving that fast so you also have a lot more time to average out the data. If an altimeter when landing is off by too much can cause a lot problems, especially when you factor in the speeds of an airplane it's you have less time to average out noise or doing other things to filter out spurious noise. Edit: Also one other thing about maritime radar there nowhere as much interference once you get outside ports or areas near land.

Also it's not just Boeing here having issues, airbus also along with Boeing has brought this to the attention the FAA. The frequency bands in other countries for 5G tend to be farther away as well from the frequencies used by altimeters so it's not been issue in other countries. It only seems to be handful of air-frames that effected.

Secondly, NOAA have been concerned about 5G, particularly near 24Ghz, again for possibly interfering with radar.

All I was saying is radar is a lot more sensitive to the noise floor and if suddenly that noise floor is higher I am not surprised some equipment will have issues depending on it's use case. Especially, when things were designed long before these new spectrum allocations.

See my post about international radio regulations, the 5G service should not have been allocated if it was going to interfere with radar altimeters as that was (and still is) a 'going' service and it takes precedence under international radio treaties.

Thus, an aircraft from a country that's not under US jurisdiction expects to be able to land in the US without its radar altimeter being jammed by some unexpected local service. (It stands to reason that the safety of an aircraft that travels across international borders should not be endangered because of inference to its navigation equipment.)

This is the precise reason why international radio regulations exist! Under the treaty the 'interference' rules apply to ALL radiocommunication services.

If such regulations (standards) are deemed 'old' then they have to be changed at an ITU WRC meeting in Geneva and the item must appear on the meeting's agenda some years before said meeting takes place to give all countries adequate time to respond (as I mentioned elsewhere, I was once involved in that process).

Aircraft flying over C band satellite dishes (what used to use the current 5G band) used to cause interference to those dishes. In that case the problem was that the transmitter was mobile. Now the C band satellite people will have to fight interference from 5G transmitters in what remains of their band.

That's part of the reason 5G is such an issue, despite the ostensibly-large guard band. You need really good filters on the cellular 5G equipment to make sure it doesn't leak into the radar band.

I think that's specifically true when you're pointing a radar at an aircraft, but is not true for radar altimeters.

When you're pointing a radar at an aircraft, the angular size of the aircraft is 1/r^2, so 1/r^2 of the emitted radiation hits the aircraft. Once emitted from the aircraft, the angular size of radar receiving dish is 1/r^2, so 1/r^2 of that radiation is detected.

With a radar altimeter, you're pointing the radar at the ground. The angular size of the ground is 50% of the sphere, whether you're one foot above the ground or 100,000 feet. (...ok, maybe it's like 49.5% of the sphere...) As long as you're pointing it down-ish, 100% of the emitted radiation hits the ground. There is only one 1/r^2 relationship, on the return leg.

This fixed angle paints a larger and larger circle as altitude increases. If you double your altitude, you quadruple the size of the surface that's reflecting radar back at you.

A bunch of stuff cancels out and you end up with 1/r^2 at the receiver, which is what we started with.

But they’re not broadcasting to the whole sphere either, because the radar instrument is not an isotropic radiator. That antenna has gain. It’s louder / more sensitive in a specific direction. Probably by a lot. You can’t make perfect antennas, but you don’t need any signal at all from most of the sphere, so you can gain many decibels.

The angle of the beam you do have is probably fixed, and being fixed would imply that the area swept by the core of the beam is indeed proportional to the square of the altitude. But I don’t think two trips through an n^2 distance is n^4 unless I’m missing some of the physics. Shouldn’t it be (2n)^2 and this still proportional to the square of the altitude not to the hypercube? (And affected by scattering and bad reflectivity from the terrain, which seems likely to be independent of altitude?) This is different for detecting random entities in free space where most of the first n^2 trip really is lost.

So it’s like you say but for different reasons??!?!

As I keep repeating here, if excessive or debilitating interference is being experienced between services under normal operating conditions then there has been a failure of Spectrum Management planning - absolutely no question about it.

OK, we know such interference has happened, so who was responsible for the planning fuck-up?

I've no argument about filters, RFI etc., what most are saying about the engineering issues makes sense.

My argument is that the spectrum planning has failed and that much of the reason can be put down to deregulation in the 1980s when many governments essentially washed their hands of Spectrum Management (succumbed to commercial pressures and overly loosened engineering standards and now we're paying the penalty).

It seems to me (given the international nature of the problem with non-US registered aircraft flying into US airspace, etc.) that we need to go back to the core of the problem and acknowledge these past 'regulations' errors before the problem can be fixed properly.

Perhaps all radar altimeters will have to be changed (respecified with much tighter filters, better intermod figures, etc.) but given the fuck-up who should pay? After all, it's not the FAA's fault nor that of the aircraft industry (see my comments in other posts about the ITU and WRC).

As I see it, this is a political issue, not a technical one.

this is the white paper that the FAA bases their concerns on https://www.rtca.org/wp-content/uploads/2020/10/SC-239-5G-In...

that is the issue they are raising

I have run the numbers and there is no way even at phone full power that it can interfere unless it was less than 2m away even then it would have to be free space, etc.

the body losses, airframe, antenna back scatter all taken into account, very very unlikely unless the radar bandpass filters are from the 1960's

Were filters particularly bad in the '60s? If you claim to have run the numbers you will have to reveal your assumptions.

The numbers are in the doc i posted.

It's a matter of understanding what the analogy is giving you. In many cases, as above, it's merely a foothold, a way of getting started understanding what something is through partial identification with something already known. From that point, the differences between the two can be clarified.

You can think of it in terms of compressing descriptions: let's say you need to describe an SUV to someone; you could start by saying "it's like a truck but [...]," and remove a lot of the redundancy you'd get by starting from scratch.

AFAICT people who think there's something wrong with reasoning-by-analogy are analyzing it with the false assumption that people are just doing "X is like Y" rather than "X is like Y EXCEPT [...]," while also having an understanding on some level of the imprecision/limits of the partial identification.

That people are out there reasoning with analogies thinking they form perfect, limitless mappings between the domains involved seems like an imaginary problem.

I hate reasoning-by-analogy in general but in this case it sounds fine to me. Boeing are grazing their cattle on someone else's land - they don't get squatters rights because no one else was using it at the time when they built their fences. Because it was contractors that built the fences and Boeing didn't perhaps know they were using someone else's land then they should get a little time to move the cattle - or the FAA/FCC can arrange a deal and give the telecom companies some new land - but it's Boeings problem.

Fair, but if we are to expect a certain level of "drift" outside of designated bands, does that mean that the gap between allocated bands should increase?

That is to say, if precision is an issue, what the most effective solution to deal with that lack of precision? From the sounds of it, the technology is unable to comply with the bands allocated, so the regulations around how those bands are distributed should be far more strict, right?

And so far, despite weeks of work, no one has shown anyone is breaking the noise generation limit.

What confuses me here is exactly the FAAs safety requirements: they've said there could be a problem. But they've also not done any research to assess it. And they're not grinding any planes. So they apparently aren't that bothered by safety!?

Here is the industry research, presented two years ago:

https://www.rtca.org/wp-content/uploads/2020/10/SC-239-5G-In...

Note the telco industry have attacked and politicized the research, but they have not offered up their own data to back up their claims. The aviation industry is on much stronger ground here.

And as another commenter put it so eloquently, the aviation industry assumes something is dangerous until proven safe, not that something is safe until proven dangerous.

There is ongoing body of work, done internationally, for several years now. They aren't grounding planes because there's no operational need to right now - worst case expect a lot of canceled flights and maybe Southwest going bust as their scheduling flaps too much (also, USA is a bit lucky in having lower need for Category II or worse approaches than some other countries).

And as someone else mentioned in comments, there are already issues cropping up with malfunctioning 5G transmitters despite 400MHz guard bands...

Douglas Hofstadter would like a word with you.

Not at 4GHz. This stuff works in terms of percentages of the frequency in use. The higher the frequency the larger the guard bands required for any sort of practical filtering.

> Radar altimeters used in civil and commercial aviation operate using either unmodulated pulse radar or frequency-modulated continuous wave (FMCW) radar technology, with FMCW being far more common among models developed in the last few decades. In either case, the above ground level (AGL) altitude of the aircraft is measured in the radar altimeter by transmitting radio frequency (RF) energy down to the ground and receiving a portion of this energy back through reflection off of the terrain or other obstacles, and determining the round-trip propagation time of the RF energy. The radiated power levels are low, typically on the order of one watt, and thus highly sensitive receivers are required for radar altimeters to function properly. As such, radar altimeters are highly susceptible to RF interference entering the receiver, which can negatively impact their performance.

The planes use the band to bounce signals off of the ground and that needs to be super sensitive. That's why they probably can't afford to properly filter within the 200 MHz guard band.