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The FAA is concerned that 5G cell phone signals could adversely affect radar altimeters on aircraft. This is because 5G signals are in the 3.7 to 3.98 GHz range, while radar altimeters operate in the 4.2-4.4 GHz frequency range. The worry is that the nearby cell signals will cause interference with avionics. However the 5G roll-out in Europe is also under way but I haven't noticed any concerns raised by EASA or the CAA. Why not? Is there a difference in frequencies/equipment between the US and Europe? Or have I just missed the news stories about potential problems outside the USA?

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    $\begingroup$ "nearby cell signals" - There's 200 MHz of separation! If this sort of thing was going to be an issue, wouldn't we have long since seen issues with VORs from the FM broadcast stations in an immediately-adjacent band? $\endgroup$
    – Vikki
    Dec 21, 2021 at 23:11

3 Answers 3

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If you read the FAA’s statements (not media reports) closely, you will find their complaint is merely that it hasn’t been proven not to be a problem, not that they actually expect a problem or have any rational basis to suspect it would be.

This follows the general hyper-conservative stance of the FAA: assume everything is unsafe until proven safe (at someone else’s expense), which has not been done in this case.

As a result, operators are prohibited from relying on radioaltimeters in areas where the 3.7-3.98GHz band is being rolled out, but they are still to monitor it. If no problems are reported after some period, they will rescind the order.

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    $\begingroup$ +1 My guess was that there would be a standard 5G frequency and the FAA would look at evidence from other nation's 5G rollouts when making a decision, but regulation is never that simple in aviation. $\endgroup$ Dec 22, 2021 at 11:22
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    $\begingroup$ I'm pretty sure that the FAA assumed the 737 Max was safe until repeatedly proven unsafe. Hyper-conservative is not the word I would choose. I might consider this more of an over-reaction in the wake of the 737 Max affair. As Vikki already noticed, it's 200 MHz separation. 5G specs allow a 100PPB+15Hz frequency error, i.e. 415 Hz at 4 Ghz. That's 500.000 times less than the 200.000.000 Hz separation. $\endgroup$
    – MSalters
    Dec 22, 2021 at 20:08
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    $\begingroup$ @MSalters - I might be wrong, but I think that's just for the carrier frequency. As soon as you modulate the carrier (in order to carry information), the bandwidth increases. $\endgroup$
    – SteveSh
    Dec 22, 2021 at 22:50
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    $\begingroup$ And filters (used in receivers, such as the receive portion of an altimeter), are not perfect. That means that an altimeter using 4.2 GHz MAY see some leakage or bleed over from a 3.98 GHz cell phone signal. $\endgroup$
    – SteveSh
    Dec 22, 2021 at 22:52
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    $\begingroup$ @MSalters Boeing provided the FAA with tons of (falsified) data that proved the MAX was safe. That’s different from 5G on the 3.8GHz band, where there is currently no data at all. $\endgroup$
    – StephenS
    Dec 23, 2021 at 0:23
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Since you are asking specifically about the 3.7–3.98 GHz band, the answer is that most of the world uses the 3.3–3.8 GHz band. The 3.7–3.98 GHz band was seen as much less important until US operators (somewhat surprisingly) paid over $90 billion in an auction this year.

So, the US are the first (and at the moment only) ones to use this band, which is why it makes sense that they are also the first ones to worry about interference in this band.

However, the French DGAC has also voiced concerns.

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    $\begingroup$ "most of the world uses the 3.3–3.8 GHz band" is inaccurate. The EU utilizes 3.4-3.8. Canada allocated 3.475-4, China is 3.3-3.6, but critically Japan has allocated 3.6-4.1, and South Korea allocated generally 3.4-4. $\endgroup$
    – user71659
    Dec 21, 2021 at 16:32
  • $\begingroup$ So how is the lower 3GHz used by the US right now? US is the first region in the world to use mm wave (~30GHz) as well. Unlike other regions, US seems really short on sub-6G, e.g. lower 3GHz and 5GHz. $\endgroup$ Dec 21, 2021 at 18:10
  • $\begingroup$ @user3528438 AFAICT, the highest band currently used in the US for mobile networks is B41 (2.5GHz). $\endgroup$
    – StephenS
    Dec 21, 2021 at 19:31
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    $\begingroup$ StephenS has provided a good answer however I'm going to accept this because Jörg specifically addresses the difference between the US and the rest of the world. I guess my problem now is to find which stack I should use to ask if the handsets differ for different frequencies. $\endgroup$ Dec 22, 2021 at 11:17
  • $\begingroup$ @DaveGremlin Let me know where you ask and I’ll answer. Short version: handsets sold in different regions support different sets of bands, but today they should all include a small subset that works everywhere as a last resort. $\endgroup$
    – StephenS
    Dec 24, 2021 at 16:34
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The FAA wants to draw attention to and monitor any effect of 5G on aviation safety so it detects issues now, while they are minor, and not later, as the number of 5G devices per square mile grows and the effects get stronger.

The rest of this is just my theory on why it might be different by nation. In the USA, there has always been an abundance of extreme caution about allowing radio equipment to proliferate in the market before ruling out the potential that it could later have to be banned or heavily restricted. You want to know up front. This eliminates waste in producing such equipment. The national difference here is the "regulatory takings" doctrine, which I don't think much of Europe has. That is where, if a person or company bought a perfectly legal thing that had a perfectly legal use, and that use is later banned or restricted in a way that decimates its value, it's considered "taken" by the government, even if they didn't physically take hold of it. This means it must be for the public good, and even then, they have to pay. If you ban beef, you've "taken" the full value of all the cows currently being raised for beef, and would have to compensate farmers their full value under the Constitution. Ban driving, pay for all the cars. There are still exceptions to seat belt laws for extremely old cars that existed before the seat belt law - Congress keeps those exceptions because if they didn't have that exception, the law would be a "taking" of antique cars and they'd have to pay for them. Ban 5G later when problems are discovered, pay for all the towers (and all the phones' price difference from the 4G models most likely). Because the financial risk of proliferating infrastructure that will have to be banned falls on the agency banning it, they have an incentive to actually look ahead before more is built. If you are a European leader, ignore the problem until it is a crisis, then ban 5G, and make the telco's take the loss.

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