What do you think about this airline vs 5G brouhaha?

I really don't know anything about it. It seems really late to be having this fight now, right?



Here’s a recent PCmag editorial on the subject, and it seems like many people want to put Internet speed above airline safety:


This issue definitely impacts network operations for 5G providers, so makes sense to discuss here.

Here’s a comment from a friend of mine who has been both a network engineer and a pilot for United Airlines, posted on the article linked above:

“As a pilot, I can tell you that landing in instrument conditions is by far the most critical flight regime possible, during which the radar altimeter reports are a matter of life and death. There is no alternative technology, such as GPS, with the required accuracy and reliability, to provide approach guidance down to the runway in zero-zero weather, which is what the radar altimeter does.

The collective tech industry needs to admit that it made a huge blunder when it urged the FCC’s clueless Ajit Pai to “blow off” the clearly demonstrated FAA spectrum conflict. Sorry, passengers, but if you look out your window, you’ll see that aviation owns this spectrum and is entitled to interference-free operation. Replacing all radar altimeters isn’t going to happen in time for 5G anyway — it took more than ten years just to deploy anti-collision technology. So do what you should have done from the beginning: follow the FCC rules of non-interference to existing users, who have clear priority in this case.”

I tend to agree with him, and it looks like the 5G providers and FAA agreed last week to put some buffer safety zones around runway approaches at 50 major airports:



Is this the band that has really really short range for 5G? If so, it doesn't seem like a very big deal to give them the airspace on approaches. I mean, if you live under a flight path by the airport, not getting fast 5G is hardly your biggest problem.


The issue seems to be old aviation equipment that has poor receiver selectivity on its radio (not radar) altimeter. This is, apparently, a secondary, but still very important, instrument for instrument approaches upon landing.

This older equipment can be subject to meaningful interference by signals as much as 500MHz outside the actual assigned radio altimeter band limit. Note that the radio altimeter band is only about 500MHz wide itself, so even a naive single-conversion receiver could/should have better selectivity that this. The reason for this poor selectivity seems to simply be that, at the time, there was nothing else using the RF spectrum nearby, so they could get away with it, and it made the receiver somewhat simpler.

The system apparently also responds poorly to both narrowband and wideband jammers i.e. it does not employ what we'd consider robust, modern error-correction or coding systems or even digital error checking techniques.

Both of these are basically issues with how old the system is and how old a large amount of deployed equipment using it is. The former is probably hard to fix in a backwards compatible way, but the latter is mostly a matter of upgrading your instruments more than once every 25 years which, for planes that are actually routinely making use of this system (largely commercial and charter operators), doesn't really seem like that big of an ask.

I think the issue is that the FCC did some rulemaking assuming that existing service users were being reasonable with their equipment design, then a giant game of chicken got started, and nobody blinked in time for anything to get done until a collision was imminent.

The C-band spectrum at issue here has become very valuable, both economically and from a public usage perspective, for mid- and short-range wireless communications. The FCC allocated some of it based on "reasonable" expectations of existing users and provided an ample (arguably rather large) guard band between services.

In the end, I'd say that aviation folks are in the wrong, here, but they also have a lot of history to contend with and a large install base of gear that, whether it "should" or not, apparently does need to be upgraded to prevent detrimental interference to an important flight safety and operations facility. A pause in deployment seems reasonable in that light, though it would have been nice if folks could have gotten this resolved sooner.

This is the C-band spectrum near 4GHz. The super short-range (or, rather, highly direction and subject to attenuation by almost anything) that you're probably thinking of is likely the UWB mmWave band up at ~30GHz.

C-band has moderate structure penetration and limited foliage penetration. With line of sight and at the power levels the carriers would consider running, several miles of usable range would be unsurprising, though I suspect many typical deployments would have design cells smaller than that while using existing "4g" (and newly opened ex-TV broadcast space) low- and mid-band frequencies for wider area coverage at reduced speeds. Interference considerations, especially high above the horizon (planes...) would be present for potentially dozens of miles away.


Bo, it’s the radar altimeter, not the barometric altimeter. This is a radar distance measurement device for determine the precise height above the ground, critical for low-visibility approaches.

Where frequency interference is concerned, under FCC rules the existing users have priority, and are entitled to interference-free operation.

-mel via cell


An article I read said that other countries are accommodating them. What are they doing different?


The thing is aviation DOESN’T own this spectrum, they just assumed it would always be unused. And they failed to mention it would be a problem during the last 5 years of discussion regarding the use of this spectrum.



Incorrect. Owning spectrum also includes the right to interference-free operation. And you imply that the FAA and airline industry has done nothing, when in reality it’s the FCC who has done nothing. the FAA sponsored extensive engineering tests that demonstrate the interference is a concern, and they notified all the parties well in advance. The fCC et al chose to do no research of their own, and are basing all their assumptions on operation in other countries, which even you must admit can’t really be congruent with the US.

-mel via cell

Apples and oranges Michael. The US domestic aviation environment is quite different than even Europe or and especially smaller countries overseas. And how long has 5G been out anyway? I hardly think that’s been available for enough of a safety track record in any country.

-mel via cell

What I’ve seen so far from the airline industry is a joke.

The original fixed satellite comms (space-to-earth) allocation was 3700-4200MHz, which was split into two parts in 2020: a mobile wireless spectrum allocation on 3700MHz to 4000MHz (for 5G) with 4000-4200MHz remaining allocated to satellite comms. The 4200-4400MHz range is allocated to aeronautical navigation and is used for radio altimeters.

So by rights, aviation doesn't now and never did own this spectrum. That said, spectrum bleed on radio transmitters is something that happens, and I've no doubt that there are plenty of broken altimeter receiver antennas out there which will pick up signals outside their formal allocation of 4200-4400MHz. Regularly tested band pass filters should deal with most of this.

Even if technically the aeronautical sector doesn't own this spectrum, the consequences of transmitter or receiver bleed from nearby allocations could be serious for the same reason that if someone walks out on a pedestrian crossing without checking and gets mown down by a drunk driver, they're not going to be jubilantly talking at their funeral about how at least they were acting within their rights.


From a technical standpoint it seems to me to be a non-issue. There's a 220 MHz guard band. 5G signals top out at 3980 MHz and radar altimeters operate between 4200 and 4400 MHz.

If a signal 220 MHz away is going to interfere, then radar altimeters on other aircraft operating in the same band would clearly be a far greater threat, and those radar altimeter signals will be rather numerous near airports. In other words, if non-correlated signals 220 MHz away are going to interfere, then signals within the same band are going to be a far greater source of interference.

Radar receivers are typically some form of direct conversion with rather good selectivity, synchronized to the frequency of the transmitted pulse. In addition, radar altimeter antennas are pointed at the ground, perpendicular to the horizon. Cell site antennas by design are aimed more or less toward the horizon, not pointed straight up at the sky.

There's also an existing FCC mobile allocation from 4400 to 4500 MHz directly adjacent to the aeronautical radar band on the high side with no guard band, yet no complaints about that.

IMNSHO, the concern that 5G cellular signals will cause airplanes to fall out of the sky has about this >< much more credence than the concern that 5G signals cause coronavirus.

It shouldn't be that hard to instrument an aircraft with test equipment, buzz a few operating cell towers, and come up with hard data.

Except that the FAA isn’t claiming interference in their LICENSED band, they are claiming interference OUTSIDE their licensed band. You can’t squat on a frequency and then expect the licensed users to accommodate you.


Owning spectrum includes the right to interference-free operations from IN BAND interference (or not, depending on how you "own" it).

The FAA and airlines are (presumably) correct that there is de-facto an interference issue. The FCC is also (presumably) correct that it's "not their problem" as the interference is due to grossly out-of-band signals, and the FCC has provided what they believe to be (and, according to most RF engineering practices I know of, is) a more than sufficient guard band between the two users.

Interference from out-of-band sources is on the operator of the receiving equipment to correct EVEN IF they are a licensed, primary user of their spectrum since the interference is from outside their allocation. This is always true so long as the folks sourcing the interference are complying with the limits of their spectrum (there are some other wiggles for Part 15 unlicensed users) including power limits and applicable transmit spectrum masks.

The FCC's job is to make sure that they set the rules such that folks with licensed spectrum do not experience practical problems when presented with out-of-band signals. When doing this, they attempt to use established guidelines of good engineering practice as well as reports from "the field", but they can't possibly account for users with what is arguably simply (very) faulty equipment.

If my 1kW HAM FM radio transmitter on 145MHz causes receive problems on your aviation band AM receiver (108-137MHz), that is YOUR problem as long as I'm complying with all the rules and regs of part 97. That is, your receiver sucks, and you need to fix it - possibly by replacing it. Likewise, if I'm getting receiver desense issues on my 145MHz FM handheld near the airport because of ATC's AM transmitter a few dozen MHz down, it's on ME to fix it (or live with it).

The issue that cropped up appears to be that, since the C-band spectrum under discussion went unused for so long, a LOT of sucky receivers got deployed, and nobody really noticed or cared. Now, it's a big deal to try to replace them all, and it's made even worse by how difficult changing anything in aviation is and how comparatively old and hence simple (perhaps too simple) the radio altimeter RF physical layer apparently is.

New to the public eye but not orgs like AOPA who’ve been fighting since 2020 but there not multi billion dollar lobby groups. US is more affected because we have more general aviation, and an older fleet overall.

And it’s not cheap to replace these radio altimeters (but that’s kind of like everything aviation)

Hmm, I'm seeing that "radar altimeter" and "radio altimeter" can indeed refer to the same class of instrument, so perhaps there's confusion (perhaps including by myself).

Nonetheless, while indeed existing users are granted some reprieve from interference by new users of other services, this is mostly in the planning stage of things and not the actual operations. The time to get this addressed would have been back when this portion of the band was re-allocated to wireless systems (from space-to-ground satellite systems) several years ago.

Further, it seems that good engineering practice was not used in the design of these vulnerable systems and that they are subject to interference from broad-spectrum "jammers" (i.e. signals that, in terms of modulation and timing, don't necessarily correspond to what they're expecting to receive) transmitting well outside their allocated band (by separation comparable to the entire band in which they operate) let alone outside the expected, tuned frequency of signal reception. All of these are typically very high on the list of consideration when designing an RF receiver and seem to have been either ignored entirely or at least discounted in the design of these instruments from what I'm hearing.

That is, I have yet to see any source (even from the aviation industry) claiming that there is in-band interference issues from the new wireless systems or that these radio altimeter systems somehow need such extreme receiver sensitivity that a several hundred-MHz guard band between services (with an existing service in between, albeit one with the transmitter usually in the other direction) is not sufficient to ensure proper receiver isolation from unwanted signals.

This simply doesn't make sense. Radar receivers are usually direct conversion driven from the same frequency source as the transmitter, meaning that they are going to have rather good selectivity with regard to frequency.

Furthermore, a radio altimeter used for approach and landing is going to have a very short time window. I'm by no means familiar with the internal workings of these devices, their specifications, or their effective range, but if the altitude to be measured is 5000 feet or less the device will send a pulse and then open a receive window of no more than about 11 microseconds to look for its return. If you're only concerned about being 1000 feet or less above terrain, the window is about 2 microseconds. The pulses are presumably sent relatively frequently, probably several times a second, and the results averaged. In addition, the radar antenna beamwidth is going to be relatively small and pointed more or less straight down.

Intentional broadband jamming isn't going to be very effective against an airplane as the jammer would need to be directly beneath a fast moving target and get the timing exactly right with microsecond accuracy.

Accidental interference from a source at least 220MHz out of band with a beam pointed at the horizon is even more far-fetched unless, as you say, the radar unit's receiver is complete garbage in which case how did it get a TSO in the first place? Avionics equipment that is critical to a precision approach isn't, or at least shouldn't be, crap.

Just to clarify, I wasn't referring to intentional (and naive) "jammers" that simply attempt to disable a system, here, but rather using a more academic notion of the concept to refer to a 5G NR system acting in an unintentional context with the same outcome similar to how one might consider modern OFDM-based WiFi a "jammer" to a conventional narrowband communication system operating on the same or a nearby carrier frequency like the classic Bluetooth PHY.

5G NR is (or should be, from what I know of it and its relation to other OFDM systems) a pretty broad-band, flat-spectrum PHY operating at only moderate power and for essentially infinite duration in the scope of a radar receiver. It would by no means be an ideal means to disable such a system, but it does represent RF energy that the receiver needs to contend with.