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I don’t know much about aviation or electrical engineering, and I’m wondering if anyone could help me understand something. It seems like the telecom and aviat

I don’t know much about aviation or electrical engineering, and I’m wondering if anyone could help me understand something. It seems like the telecom and aviation industries (along with the FCC and FAA) disagree on a question of fact: does 5G equipment interfere with radio altimeters? There’s a way that the FAA could convince me, and probably all the politicians. Shoot a video with a radio altimeter and some 5G equipment where the radio altimeter malfunctions. Put it on YouTube. Do a press release. This seems like an obvious step if the underlying issue exists. Any reason they haven’t done this?

ethbr0 5y ago
My guess is that RF interference is not binary. So the question is not "Does 5G equipment interfere with radio altimeters?" but rather "Could 5G equipment ever interfere with radio altimeters, under any set of feasible conditions?" Which is a harder question to answer, but how seriously we should (and usually do) take air safety.
- moonchrome 5y ago
  If you can't reproduce those conditions in an artificial scenario setup to exaggerate the effect - how are they feasible ?
  - bluGill 5y ago
    The question isn't can you reproduce it in an artificial setup, the question is can you do it in the real world? I can think of a lot of ways to cheat in the lab (turn the power to 11...) that create situations that in the real world wouldn't happen and so I wouldn't worry about it. However that doesn't change the fact that if you can recreate it in the real world I'm really worried.
  - markmark 5y ago
    Here's the RCTA report where they reproduced those conditions and showed they were feasible: https://www.rtca.org/wp-content/uploads/2020/10/SC-239-5G-Interference-Assessment-Report_274-20-PMC-2073_accepted_changes.pdf https://www.rtca.org/wp-content/uploads/2020/10/SC-239-5G-In... The way you test this stuff is get a rad-alt and inject known amounts of RF and see how much is needed to give false readings. Then you go and measure how much RF you'll see in the environment and see if it exceeds those levels. Here's a choice quote from the summary: "In the worst case, the safe interference limit for the fundamental emissions is exceeded by over 47 dB, and the safe interference limit for the spurious emissions is exceeded by over 27 dB."
addaon 5y ago
The question isn't really whether the radalt fails in any particular test (spoiler: it probably won't). It's whether there's an edge case where the safety case for the aircraft depends on correct operation of the radalt AND there's a failure. I haven't seen the fault tree for the 737, of course, so I don't know what reliability is required from the radalt to be safe; but with the radalts I've worked with it's not unreasonable to have an undetected failure rate requirement of 10^-8/hour or better. (Detected failure rates can be much higher.) Testing enough to show that you're not violating that is /hard/. Speaking a bit from my own experience, I suspect the guard bands on most radalts out there are wide enough that 5G interference isn't a practical issue. I've seen radalts with 30 MHz guard bands around the 4.1 GHz / 4.3 GHz limit frequencies of interest... and that's friggin' huge. (I'm also sure there are radalts with much smaller guard bands that use more of the available bandwidth for better performance.) But this is hard to analyze fully. Taking the example of the unit I'm thinking of, the center frequency is from a non-temperature-compensated oscillator -- so your worst case is an extreme temperature going to assymetric guard bands, a unit with a weak bandpass to begin with, and a 5G system pouring a lot of energy at (or even, in violation of spec, a bit beyond) the edge frequency. Probably fine. 10^8 fine?
- tjohns 5y ago
  > The question isn't really whether the radalt fails in any particular test (spoiler: it probably won't) Annecdata, but... I know two commercial pilots who have both recently run into interference with their radar altimeters during landing/takeoff. It certainly doesn't affect all aircraft (depends on the particular avionics installed), but from what I've heard it is a real issue.
kayson 5y ago
It's not so black and white. From what I've read, radio altimeters operate in the 4.2-4.4 GHz range, and the 5G spectrum in question is 3.7-3.98 GHz. RF systems generally work by choosing a center frequency (say 4.3 GHz for altimeters), and trying to filter off anything outside the band of interest (4.2-4.4 GHz). The junk that remains is noise as far as the system is concerned, and has to be budgeted for very carefully. The problem is that radio altimeters are pretty old, and their filtering technology is not great. So even though they are filtering the 3.7-3.98 GHz 5G band, it's not as effective as a modern RF system/filter would be. Previously, this wasn't the issue because nothing was broadcasting in that band at particularly high power. That's the second component of the problem: power. The radio altimeter system is capable of handling some amount of noise power caused by unwanted broadcasts, among other things. Previously, satellites were using this band, and they were so far away that between the distance-based attenuation of the signal and the limited filtering, radio altimeters could operate unaffected. 5G is a different story, though. The towers are much closer to the runway, so the leakage power is much higher. Whether this actually causes a problem for the altimeters is then dependent on a number of factors, including the distance to the 5G tower, the transmit power level, the direction of the 5G antennas, etc. Could it cause a problem? Sure. I bet if you pointed a directional 5G antenna straight at a particularly crappy radio altimeter, you could materially affect its operation. But there are many mitigation strategies that could be used, like requiring lower transmit power, further antenna distances, appropriate directional transmissions, etc. These strategies have been used successfully in other countries. The real concern should be the total lack of collaboration between the FAA and FCC. It's turning into a jurisdictional pissing contest.
- zachberger 5y ago
  > The problem is that radio altimeters are pretty old Interestingly the 737-200 is excluded from this airworthiness directive. 737-200s are the oldest, still-operating members of the 737 family.
  - bastardoperator 5y ago
    Total noob question, why isn't Boeing building/buying new radio altimeters that can filter better? Is it a money thing, a technology thing, maybe both?
gdavisson 5y ago
You're using the wrong burden of proof here. You're assuming it's safe until proven dangerous, while the FAA is (properly) treating it as dangerous until proven safe. Take the recent 737 MAX debacle as an example. Its MCAS hadn't been proven to be dangerous when it went into production, but it turned out that it was. Result: two airplanes fell out of the sky and 346 people died. It's the FAA's job to prevent this sort of thing from happening, and in this case (unlike the 737 case) they're doing that job.
- pigbearpig 5y ago
  Agreed, very surprised at the cavalier attitude of many of these comments. I'd rather my phone be slower than my plane think it's 300ft higher than it actually is. Aviation isn't the area to be making assumptions that it "should be fine."
- theYipster 5y ago
  Thank you! This is the most important and most correct comment on this forum today. Truly, it's amazing how many aerospace engineers hang out on HackerNews!\s So many assertions about what is right, what is wrong, and who is to blame. 99% of these assertions reflect a poor understanding of how commercial aviation fundamentally and functionally works. Not only is your comment 100% correct, it also strikes at the heart of the problem. Diplomatically, the issue that exists today is a clash of the very different safety cultures that exist between aviation and telco (and by extension the FAA and the FCC.) Aviation will win, btw. The question, however, is at whose expense. Btw, the root of all this can be traced back work the aviation industry had undertaken to create a standard for wireless safety critical avionics (a system and standard called WAIC.) It was about four years ago that the aviation industry tried to secure additional spectrum, was denied, and thus started researching how to use the existing 4.2-4.4ghz spectrum already dedicated to aviation. It was through this research that the industry discovered the inherent problems with radio altimeters and nearby spectrum interference. The aviation industry petitioned the FCC during the notice of comment period for the C-band spectrum sale, sharing the research and the data which shows 5G interference could be a problem given how it was planned to be deployed in the US. T-Mobile did their own preliminary study in support of the aviation industry's findings, but the rest of the telco industry attacked the research done without providing their own alternative data. The FCC, under Ajit Pai, and with support of the telco's lobbying arm CTIA, then effectively politicized any additional technical assessment or study, leading to today's brouhaha. If you want to blame anyone, blame the FCC and their former chair, who btw, is already a favorite internet punching bag. (Note that T-Mobile is the only carrier that did not purchase the spectrum, although it's not known whether this was due to T-Mobile's own supporting research.)
zw123456 5y ago
here is the report https://www.rtca.org/wp-content/uploads/2020/10/SC-239-5G-Interference-Assessment-Report_274-20-PMC-2073_accepted_changes.pdf https://www.rtca.org/wp-content/uploads/2020/10/SC-239-5G-In... which i have posted a bunch of times. I am 40+ year experience in this area, I have run the calcs, the thing they are worried about is the phones on the plane, the phone would have to be less than 2 meters from the radar antenna and at full power to come even remotely close to the interference numbers they are talking about to give a false reading, and that would be without a lot of body loss. The airlines want the telcos to pay them to swap out their old radar units, it is about money of course.
- zw123456 5y ago
  Keep on down voting me... But I am not wrong.. the airlines are asking for money.
- amirhirsch 5y ago
  I'd love to talk with you about your experience, I work at genrad.io and my email is amir at that domain.
zthrowaway 5y ago
There’s already been incidents of 5G interfering with ILS on airliners. Which is what planes use to land in bad weather. When bad weather hits a destination airport, you’ll more likely be rerouted to a different airport due to the risks of using ILS now.

ethbr0 5y ago

My guess is that RF interference is not binary. So the question is not "Does 5G equipment interfere with radio altimeters?" but rather "Could 5G equipment ever interfere with radio altimeters, under any set of feasible conditions?" Which is a harder question to answer, but how seriously we should (and usually do) take air safety.

moonchrome 5y ago
If you can't reproduce those conditions in an artificial scenario setup to exaggerate the effect - how are they feasible ?
- bluGill 5y ago
  The question isn't can you reproduce it in an artificial setup, the question is can you do it in the real world? I can think of a lot of ways to cheat in the lab (turn the power to 11...) that create situations that in the real world wouldn't happen and so I wouldn't worry about it. However that doesn't change the fact that if you can recreate it in the real world I'm really worried.
- markmark 5y ago
  Here's the RCTA report where they reproduced those conditions and showed they were feasible: https://www.rtca.org/wp-content/uploads/2020/10/SC-239-5G-Interference-Assessment-Report_274-20-PMC-2073_accepted_changes.pdf https://www.rtca.org/wp-content/uploads/2020/10/SC-239-5G-In... The way you test this stuff is get a rad-alt and inject known amounts of RF and see how much is needed to give false readings. Then you go and measure how much RF you'll see in the environment and see if it exceeds those levels. Here's a choice quote from the summary: "In the worst case, the safe interference limit for the fundamental emissions is exceeded by over 47 dB, and the safe interference limit for the spurious emissions is exceeded by over 27 dB."

addaon 5y ago

The question isn't really whether the radalt fails in any particular test (spoiler: it probably won't). It's whether there's an edge case where the safety case for the aircraft depends on correct operation of the radalt AND there's a failure. I haven't seen the fault tree for the 737, of course, so I don't know what reliability is required from the radalt to be safe; but with the radalts I've worked with it's not unreasonable to have an undetected failure rate requirement of 10^-8/hour or better. (Detected failure rates can be much higher.) Testing enough to show that you're not violating that is /hard/. Speaking a bit from my own experience, I suspect the guard bands on most radalts out there are wide enough that 5G interference isn't a practical issue. I've seen radalts with 30 MHz guard bands around the 4.1 GHz / 4.3 GHz limit frequencies of interest... and that's friggin' huge. (I'm also sure there are radalts with much smaller guard bands that use more of the available bandwidth for better performance.) But this is hard to analyze fully. Taking the example of the unit I'm thinking of, the center frequency is from a non-temperature-compensated oscillator -- so your worst case is an extreme temperature going to assymetric guard bands, a unit with a weak bandpass to begin with, and a 5G system pouring a lot of energy at (or even, in violation of spec, a bit beyond) the edge frequency. Probably fine. 10^8 fine?

tjohns 5y ago
> The question isn't really whether the radalt fails in any particular test (spoiler: it probably won't) Annecdata, but... I know two commercial pilots who have both recently run into interference with their radar altimeters during landing/takeoff. It certainly doesn't affect all aircraft (depends on the particular avionics installed), but from what I've heard it is a real issue.

kayson 5y ago

It's not so black and white. From what I've read, radio altimeters operate in the 4.2-4.4 GHz range, and the 5G spectrum in question is 3.7-3.98 GHz. RF systems generally work by choosing a center frequency (say 4.3 GHz for altimeters), and trying to filter off anything outside the band of interest (4.2-4.4 GHz). The junk that remains is noise as far as the system is concerned, and has to be budgeted for very carefully. The problem is that radio altimeters are pretty old, and their filtering technology is not great. So even though they are filtering the 3.7-3.98 GHz 5G band, it's not as effective as a modern RF system/filter would be. Previously, this wasn't the issue because nothing was broadcasting in that band at particularly high power. That's the second component of the problem: power. The radio altimeter system is capable of handling some amount of noise power caused by unwanted broadcasts, among other things. Previously, satellites were using this band, and they were so far away that between the distance-based attenuation of the signal and the limited filtering, radio altimeters could operate unaffected. 5G is a different story, though. The towers are much closer to the runway, so the leakage power is much higher. Whether this actually causes a problem for the altimeters is then dependent on a number of factors, including the distance to the 5G tower, the transmit power level, the direction of the 5G antennas, etc. Could it cause a problem? Sure. I bet if you pointed a directional 5G antenna straight at a particularly crappy radio altimeter, you could materially affect its operation. But there are many mitigation strategies that could be used, like requiring lower transmit power, further antenna distances, appropriate directional transmissions, etc. These strategies have been used successfully in other countries. The real concern should be the total lack of collaboration between the FAA and FCC. It's turning into a jurisdictional pissing contest.

zachberger 5y ago
> The problem is that radio altimeters are pretty old Interestingly the 737-200 is excluded from this airworthiness directive. 737-200s are the oldest, still-operating members of the 737 family.
- bastardoperator 5y ago
  Total noob question, why isn't Boeing building/buying new radio altimeters that can filter better? Is it a money thing, a technology thing, maybe both?

gdavisson 5y ago

You're using the wrong burden of proof here. You're assuming it's safe until proven dangerous, while the FAA is (properly) treating it as dangerous until proven safe. Take the recent 737 MAX debacle as an example. Its MCAS hadn't been proven to be dangerous when it went into production, but it turned out that it was. Result: two airplanes fell out of the sky and 346 people died. It's the FAA's job to prevent this sort of thing from happening, and in this case (unlike the 737 case) they're doing that job.

pigbearpig 5y ago
Agreed, very surprised at the cavalier attitude of many of these comments. I'd rather my phone be slower than my plane think it's 300ft higher than it actually is. Aviation isn't the area to be making assumptions that it "should be fine."
theYipster 5y ago
Thank you! This is the most important and most correct comment on this forum today. Truly, it's amazing how many aerospace engineers hang out on HackerNews!\s So many assertions about what is right, what is wrong, and who is to blame. 99% of these assertions reflect a poor understanding of how commercial aviation fundamentally and functionally works. Not only is your comment 100% correct, it also strikes at the heart of the problem. Diplomatically, the issue that exists today is a clash of the very different safety cultures that exist between aviation and telco (and by extension the FAA and the FCC.) Aviation will win, btw. The question, however, is at whose expense. Btw, the root of all this can be traced back work the aviation industry had undertaken to create a standard for wireless safety critical avionics (a system and standard called WAIC.) It was about four years ago that the aviation industry tried to secure additional spectrum, was denied, and thus started researching how to use the existing 4.2-4.4ghz spectrum already dedicated to aviation. It was through this research that the industry discovered the inherent problems with radio altimeters and nearby spectrum interference. The aviation industry petitioned the FCC during the notice of comment period for the C-band spectrum sale, sharing the research and the data which shows 5G interference could be a problem given how it was planned to be deployed in the US. T-Mobile did their own preliminary study in support of the aviation industry's findings, but the rest of the telco industry attacked the research done without providing their own alternative data. The FCC, under Ajit Pai, and with support of the telco's lobbying arm CTIA, then effectively politicized any additional technical assessment or study, leading to today's brouhaha. If you want to blame anyone, blame the FCC and their former chair, who btw, is already a favorite internet punching bag. (Note that T-Mobile is the only carrier that did not purchase the spectrum, although it's not known whether this was due to T-Mobile's own supporting research.)

zw123456 5y ago

here is the report https://www.rtca.org/wp-content/uploads/2020/10/SC-239-5G-Interference-Assessment-Report_274-20-PMC-2073_accepted_changes.pdf https://www.rtca.org/wp-content/uploads/2020/10/SC-239-5G-In... which i have posted a bunch of times. I am 40+ year experience in this area, I have run the calcs, the thing they are worried about is the phones on the plane, the phone would have to be less than 2 meters from the radar antenna and at full power to come even remotely close to the interference numbers they are talking about to give a false reading, and that would be without a lot of body loss. The airlines want the telcos to pay them to swap out their old radar units, it is about money of course.

zw123456 5y ago
Keep on down voting me... But I am not wrong.. the airlines are asking for money.
amirhirsch 5y ago
I'd love to talk with you about your experience, I work at genrad.io and my email is amir at that domain.

zthrowaway 5y ago

There’s already been incidents of 5G interfering with ILS on airliners. Which is what planes use to land in bad weather. When bad weather hits a destination airport, you’ll more likely be rerouted to a different airport due to the risks of using ILS now.