back to article Japan solves 5G airliner conundrum: Keep mobe masts 200m from airport approach paths. That's it

American aviation regulators have banned the use of autoland at some of their country's airports as the local debate about 5G phone mast emissions and airliners continues – while Japan claims to have solved the problem a year ago. This morning Emirates, the largest airline of the United Arab Emirates, declared it was …

  1. Yet Another Anonymous coward Silver badge

    Protectionism ?

    >mobile industry to cough up $100m or so to pay for altimeter upgrades.

    And that money will be distributed to everyone?

    The FCC will pay for Chinese and Gulf airlines to update their planes - or will only US airlines be able to land at SFO in fog ?

    1. Rich 2 Silver badge

      Re: Protectionism ?

      Quite. As this seems to be a problem very much local to the US, the rest of the world’s airlines and aircraft makers are not going to be willing to pay for it. Nor should they.

      1. DS999 Silver badge

        Re: Protectionism ?

        If they are willing and able to land in Japan, they won't have any cause for complaint about landing in the US where there is a bigger gap between 5G and altimeter frequencies and the 5G antennas were required to be further away than 200 meters from the approach path even before all the recent shouting.

        1. Mishak Silver badge

          Re: Protectionism ?

          Isn't the real issue that they will only be allowed (by regulation) to make Cat I approaches, so they will have to divert to their alternate if the weather is poor?

          In reality, this may mean that the departure is delayed or cancelled as diversions incur costs and result in aircraft being out of position.

      2. Malcolm Weir Silver badge

        Re: Protectionism ?

        With the latest update from the FAA (which has now cleared more specific RADAR altimeters for use), this seems just like a pissing contest for "who pays to (re-)certify RADAR altimeters that were not properly tested before because the chunk of C-band that they encroached on was previously only used by TV broadcast downlinks..."

        There is no serious _technical_ doubt that, regardless of the power level of the 5G signal, that a properly designed RADAR altimeter should function perfectly well. And the FAA implicitly acknowledge that, by clearing some altimeter's for use in the 5G environment... but not all. So what we're left with is whether the FCC should pay the avionics manufacturers to fix their devices (which used to work just fine, remember... because there were no widely-deployed lower-C-Band transmitters) or whether the avionics folks should fix their own darn products.

        Now, as it happens, the senior Republican US Senator is from Iowa (Chuck Grassley). And entirely coincidentally, Rockwell-Collins is based in Iowa.

        Guess who makes RADAR Altimeters for the B777...?

        1. Anonymous Coward
          Anonymous Coward

          Re: Protectionism ?

          There's a flipside to this, though: why should manufacturers be forced to change a product that was working perfectly well because telcos have decided they rather have a profit than actually checking first if their use of frequencies doesn't interfere with existing safety critical platforms?

          To me this feels like the telcos went full steam ahead pretty much ignoring the problem (which was known - hence the Japanese proper study and prescribed measures) until they were stopped just before they went live, and now whinge about losing gazillions a day by not being able to charge the punters at the airport because they couldn't be asked to do proper spectrum analysis.

          They knew, or should have known. That leaves very little in the way of excuse IMHO.

          1. Malcolm Weir Silver badge

            Re: Protectionism ?

            Nope. There's no "flipside".

            You're ignoring the key fact: _some_ RADAR Altimeters (RAs) work just fine in the 5G environment.

            Bear in mind that an RA doesn't really care what frequency it uses, as it is just measuring the return of it's own signal. So a fully-compliant RA using a frequency in the 4.2-4.4GHz signal will function no differently than a non-compliant RA using a frequency outside that band (say, 4.02GHz). The latter may cause and suffer interference with the 5G cell frequencies. There's no interoperability issue with an RA: it's pinging away, and will ping just as happily at 4.0GHz as at 4.6GHz, except that those are outside the frequency band allocated for the purpose.

            Your argument is similar to someone making lots of noise in their home, and then objecting to new neighbors complaining on the grounds that there didn't used to be neighbors to hear the racket...

            1. Anonymous Coward
              Anonymous Coward

              Re: Protectionism ?

              The concern isn't that radars are operating outside their assigned band, the concern is that RF energy in an adjacent band could swamp the radar (highly technical and scientifically accurate terms here).

              For an easier to observe situation, tune your car radio to an FM station and drive close to a commercial FM transmitter on a nearby frequency. Most car radios will lose the first station when they get within a a couple hundred meters of the tower.

              Japan is saying the concern is easy to mitigate.

              I haven't looked at the allowable 5G C band emissions or the input specs of radar altimiters, so I can't say if the concerns are valid or not.

              1. Robert 22

                Re: Protectionism ?

                "For an easier to observe situation, tune your car radio to an FM station and drive close to a commercial FM transmitter on a nearby frequency. Most car radios will lose the first station when they get within a a couple hundred meters of the tower."

                That is known as desensitization - it is a well known behavior of radios, particularly low cost units since their designers will have accepted various compromises to reduce cost. (BTW, an FM radio station will typically operate at much higher power levels than a 5G base station).

                The thing that absolutely baffles me is that most countries take spectrum management seriously. Experimental measurements and simulations are used to determine whether a change will result in interference with existing systems and users. This is pretty much essential if you are going to manage the spectrum to (a) maximize economic benefits (provide services that users are willing to use) and (b) avoid interference (so that legal users can actually use the spectrum without bad things happening).

                Did the telecom industry use political influence to bypass or override consideration by the people involved in spectrum management?

              2. Dagg Silver badge

                Re: Protectionism ?

                I haven't looked at the allowable 5G C band emissions or the input specs of radar altimiters, so I can't say if the concerns are valid or not.

                One issue that hasn't been brought up is where the 5G service has a xmitter or antenna fault and is splatting spurious emissions that may appear in the radar altimeter receiver passband.

                Something like a dry joint or corrosion in a connector can make a nice little diode mixer.

                1. Anonymous Coward
                  Anonymous Coward

                  Re: Protectionism ?

                  The UK BBC 200kHz (ca 500kw) transmitter at Droitwich was known to have such an effect on nearby fields' wire fences - and also household iron appliances. Corroded joints rectified the signal to produce the programmes' audio as well a mixed spurious RFI.

        2. Charlie Clark Silver badge

          Re: Protectionism ?

          FWIW I live about 5 km from a reasonably busy airport runway and many WiFi 5GHz channels are routinely disabled on my router at the behest of the airport following the DFS protocol. If necessary, this is easy enough to replicate for 5G services. The only problem here being that my Apple devices don't cope well with the enforced switching, so I have to used fixed channels or LAN cables.

    2. vtcodger Silver badge

      Re: Protectionism ?

      This whole thing seems a bit surreal. Seems to me like I'm being asked to believe that the FCC doesn't know how to allocate frequencies and/or that cell phone companies don't know how to test their transmitters for spurious emissions even when they are told what band(s) to stay out of and/or that RA designers can't/don't know how to design bandpass filters and don't have some way to distinguish their return signals from interference as, for example, from the RAs in nearby aircraft.

      I don't think that I believe any of those things.

      What am I missing?

      1. Dinanziame Silver badge

        Re: Protectionism ?

        I think everybody agrees on those facts, which makes it even more surprising that the rollout is having such issues in the US, and nowhere else.

        1. Dan 55 Silver badge

          Re: Protectionism ?

          We have to remember that this was done under the Trump administration, so I guess the judicious use of brown envelopes from one of the interested parties solved interference problems. Sadly Ajit Pai and friends left and took the knowledge of how this works with them, so that option is no longer available.

          1. Charlie Clark Silver badge

            Re: Protectionism ?

            Ah, yes. How we miss Ajit Pai and his friends…

            Never fear, they're bound to pop up again sooner or later. This is one of the principles of revolving door democracy: regulator, lobbyist, businessman, politician…

      2. SundogUK Silver badge

        Re: Protectionism ?

        The issue seems to be shoddy radar altimeters unable to guarantee they aren't picking up transmissions from outside their allocate spectrum. They didn't need to in the past, so they cut costs. That is now coming back to bite their arses.

        1. Electronics'R'Us


          It all depends on the standards in force when they were certified.

          They are classified as level A devices (safety critical) and standards move on.

          The relevant ones now are RTCA-DO155 and ARINC 707 but the requirements on receiver filtering may well not have been as stringent in the past.

          The current requirements for out of band receiver attenuation (beyond the 4.2 to 4.4GHz range) are quite strict (24dB / octave IIRC).

          I have experienced standards change over the years, with some getting easier to meet and others getting more difficult.

        2. anothercynic Silver badge

          Re: Protectionism ?

          Shoddy? Debatable. Remember that in the US, there are still planes of various ages in use (thank God the old Mad Dogs have been retired), and thus I suspect it's more those than the modern ones installed in the most modern of planes. In Japan, they tend to get rid of older ones fairly quickly, and in Europe, it's also debatable to a degree.

          But, it would not surprise me if people like Rockwell Collins never re-tested their stuff when new regulations came in (you would expect them to, right, after all this *is* their wheelhouse), because they probably would be grandfathered in (but that does not mean you just don't retest at all).

          Emirates's prez Tim Clarke throwing his toys out of the pram like he did was not particularly helpful. That no-one at Emirates followed this up with their equipment manufacturer or OEM and chased that to make sure the airline was sufficiently prepared boggles the mind. Or maybe they did and he's just happy to throw a wobbly because...

          1. batfink

            Re: Protectionism ?

            You'd think that the older aircraft would be more likely to have the problem, but Boeing have issued a safety warning about the 777 being affected.

            1. Robert 22

              Re: Protectionism ?

              You might want to talk to the cost engineers. But seriously, it does sound like someone cut a few corners.

            2. Rob Daglish

              Re: Protectionism ?

              Yeah, but to be fair to Boeing, it's because they want to make sure they are the only people responsible for their planes having unexpected interfaces with the ground...

            3. anothercynic Silver badge

              Re: Protectionism ?

              The B777 *is* old (it's now 22 and finally is allowed to drink in the US).

              Its design is from the 1980s with its first flight in the 1990s. The 777X however is another beast. :-)

              If they had issued alerts for the B787, I would have been *very* surprised.

          2. Electronics'R'Us

            Re: Protectionism ?

            But, it would not surprise me if people like Rockwell Collins never re-tested their stuff when new regulations came in (you would expect them to, right, after all this *is* their wheelhouse)

            That is actually not how it works in most cases. The product supplied into a particular aircraft type will retain the same characteristics for all production items of that specific type as it will have been certified into that specific aircraft.

            Newer models would be expected to meet the (then) current standards.

            As these are level A subsystems, changing the RadAlt model on an aircraft is a very expensive business.

            1. anothercynic Silver badge

              Re: Protectionism ?

              i.e. The standard is grandfathered in. No-one checks if newer regulations could be applying to it and fixing newer models (or notifying the authorities that newer models should be installed).

          3. Malcolm Weir Silver badge

            Re: Protectionism ?

            Sorry to break it to you, but the MD-80's haven't all been retired! (To be fair, it seems like there are more operating in Iran than any other country, but there are still a few US-flagged models in revenue service).

            My guess is that Clarke was just applying the maximum amount of pressure... what we don't know is how badly Boeing/Collins/FAA have been dragging their feet, so it's entirely possible that he didn't have any options (because an operator can't just replace a safety-critical device without paper from the certifying agency: until Boeing/FAA approves an alternate, the operator is stuck...).

        3. Dan 55 Silver badge

          Re: Protectionism ?

          The issue seems to be shoddy radar altimeters unable to guarantee they aren't picking up transmissions from outside their allocate spectrum. They didn't need to in the past, so they cut costs. That is now coming back to bite their arses.

          It seems other countries were aware of this and made sure that the frequencies in question were not available for anyone to use, meaning current equipment does not need to be replaced. A date could be set in the future to phase them out, this date could be timed to match how long planes are expected to be in service.

          The US decided to allocate the frequencies to 5G now and will either have take the frequencies back off the telecos or force airlines around the world to undertake an expensive equipment replacement programme before time.

          1. Malcolm Weir Silver badge

            Re: Protectionism ?

            Kinda, but... Japan uses similar frequencies, but at a lower power (because of the higher population density, Japan doesn't need the same transmission power) and has a mast-siting/orientation policy that satisfies the Japanese safety issues.

            Since the FAA is happy with _some_ RADAR altimeters operating in the new environment, it's clear that the issue is a marginal one: some specific RA's _could_ pick up signals from a 5G mast 200MHz away.

            As others have noted, this looks like a design issue with specific RA's that the FAA/Boeing wants the FCC/telco's to fix (because "Safety!!!"), but airworthiness directives to "patch" vulnerabilities in a certified aircraft are a dime a dozen. If I had to guess, the core issue is that the operators (i.e. the airlines) were uninterested in paying Boeing/Collins/Whoever to fix the issue (not wholly unreasonably, especially given the terrible financial cost of the pandemic), and Boeing isn't eager (especially given the 737Max/787 Deliveries issues and costs), so it's come down to a game of chicken.

  2. JassMan

    Hindsight is a wonderful thing.

    Disrupting radio altimeters potentially prohibits the use of automatic landing capabilities, as used during bad weather when pilots can't see the airport.

    The simple answer which should have been done when 5G was first mooted :- Test to see if there is any interference only on days when there is good visibility. Any pilot trying to blame interference on his radio altimeter when he can see the ground rushing up shouldn't be flying.

    Yes I know that certain sizes of raindrops scatter radio in differing amounts and differing directions (otherwise weather radio wouldn't work), but presumably it does not affect the 4.2-4.4GHz band used by airliners' radio altimeters, otherwise they wouldn't work when most needed. Presumably this is how the frequency was allocated as you don't want an altimeter which gives a different result during weather. I would further postulate that the frequencies wouldn't have been chosen for 5G if there was a weather problem, 'cos the telcos would be seriously cheesed off if everyone phones up to say their phones don't work in the rain.

  3. HarryBl

    " Their US colleagues ought to be taking notes from the rest of the world."

    Good luck with that...

    1. pavel.petrman

      Maybe they did, but forgot to recalculate the ERP levels from SI units to the local standard unit for equivalent emissions: inches of mercury over isotropic square pound. Hence the twice as high emission limit (mentioned elsewhere in this forum) and associated band spillover problems. Perhaps a Gimli glider of radioelectronics, if you will.

  4. iron Silver badge

    If radalts can't filter out a signal over 200MHz out of band there is something very wrong with them.

    Who allowed such shoddy equipment to be used to fly hundreds of people in a metal tube ffs?

    1. Yet Another Anonymous coward Silver badge

      Assuming the 5G aren't leaking signal 200MHz out of band.

      Always wondered that for FCC certification, you certify that your device doesn't emit harmfull RF and you also certify that you aren't affected by external RF

      1. Woodnag

        "certify that your device doesn't emit harmfull RF" has a similar ring to "we take your privacy very seriously".

        I have a roll of CE Mark stickers to give away.

      2. SundogUK Silver badge

        Everything i have heard about this says it's the radar altimeters at fault - manufacturers cut costs because previously the spectrum they were bleeding in to weren't being used.

      3. Anonymous Coward
        Anonymous Coward

        even my 40 year old CB can operate on adjacent channels to people. 1-4 for a copy?

    2. jtaylor

      If radalts can't filter out a signal over 200MHz out of band there is something very wrong with them. Who allowed such shoddy equipment to be used to fly hundreds of people in a metal tube ffs?

      It's not just what the radio altimeter can accept, it's also what might be sent to it.

      Airplane equipment is safety critical. It's regularly re-tested. Airport electronics like ILS transmitters are too. In fact, not only are the transmitters re-tested regularly, but they are also tested from specially equipped airplanes to detect if something else is screwing with the signal.

      In the US, telcos are lighting up powerful radios intended to transmit on frequencies close to air navigation frequencies, at locations close to airports. What if one malfunctions or the signal gets distorted? Will these be treated as safety-critical equipment?

      Manufacturers, the FAA, and airlines around the world have expressed serious and specific concerns about the safety of this rollout. If that's not sufficient to take this seriously, what is?

      1. Malcolm Weir Silver badge

        This is the FAA's spin/pitch. But why are they happy with some 62% of the US fleet, but worried about the remaining 38%?

        Oddly enough, by the way, "airlines around the world" do NOT seem to have expressed serious concerns about the actual _safety_, but about their ability to operate in the USA without the all-important sign-off from the FAA...

        Japan (as the article points out) has already done all this, and (as the article points out) no-one noticed...

      2. Dagg Silver badge

        Also the problem may be environmental. A nice piece of rust where two parts of the antenna mast are joined can act as a diode and mix several different xmitter signals together and radiate spurious signals across a wide frequency range.

        Depending on the physical distance it could even be a bit of rusty fence!

  5. nerdbert

    One other minor issue in the US relative to the world

    The US FCC has authorized those same frequencies - but has allowed its carriers in those bands (Verizon and AT&T) to use double the power of the rest of the world. Yeah, even if you guardband with the same mask the rest of the more sane world does, there's more spillover into adjacent frequencies if you double the power.

    Never attribute to malice what could be attributed to rank stupidity, ESPECIALLY if the government is involved. Actually, if the government is involved, stupidity is usually tied with corruption/regulatory capture as an explanation, but that's for a longer rant. The really sorry thing is that the FCC actually looks competent when compared to the FAA.

    1. A.P. Veening Silver badge

      Re: One other minor issue in the US relative to the world

      The really sorry thing is that the FCC actually looks competent when compared to the FAA.

      And that is damning with very faint praise.

  6. Down not across

    If its Boeing... (a gift that keeps giving)

    "Boeing has announced flight restrictions on all airlines operating the Boeing 777 aircraft, and we have cancelled or changed the aircraft for some flights to/from the US based on the announcement by Boeing," an ANA spokesman told the Mail. We have asked Boeing for comment.

    More beancounter inteference? Or just state of their engineering these days.

    (yeah I know cheap shot but I couldn't resist)

    1. A.P. Veening Silver badge

      Re: If its Boeing... (a gift that keeps giving)

      More beancounter inteference?

      More like button sorter stupidity, Airbus is grateful.

      Or just state of their engineering these days.

      Probably a contributing factor.

      (yeah I know cheap shot but I couldn't resist)

      Nothing wrong with a cheap shot, especially not if it is a perfect hit.

  7. The Man Who Fell To Earth Silver badge

    "Their US colleagues ought to be taking notes from the rest of the world."

    But the rest of the world is full of damn foreigners.

  8. Yes Me Silver badge

    Who stands to gain?

    This whole thing seems to be a moral panic based on no evidence. Where's the science? More interestingly, what is the motivation for this moral panic? Who stands to gain from this particular bit of fake news? Just the radio altimeter manufacturers, or someone else?

    1. hoola Silver badge

      Re: Who stands to gain?

      I am not sure it is as simple as that.

      The 200m is based on a lower power & the antenna being configured correctly.

      Now in the grand scheme of things, it something does go wrong the consequences are potentially catastrophic.

      Given that 5G is a new service and it appears that the driver and the organisations claiming that it is safe are the same people, there is a bit of a conflict of interests.

      So, is it an over reaction by the FAA? possibly but they are under a significant amount of scrutiny still after the 737 Max fiasco.

      At the end of the day, aircraft safety surely has to take precedence of an additional form of connecting phone to data......

  9. cortland

    Too wide bandwidth in the aircraft system

    Looking at the 5G bandwidth and the aircraft navigation, it's very likely manufacturers haven't made later receivers better over the years. Look at the very wide filtering at:


    1. Nifty

      Re: Too wide bandwidth in the aircraft system

      In the same way that many vintage radio sets have vastly better RF selectivity than modern ones.

    2. Fursty Ferret

      Re: Too wide bandwidth in the aircraft system

      They are selective - the frequency band is quite wide because two radio altimeters can't transmit on the same frequency, and there are a lot of radio altimeters around.

      The in-depth report suggests that it's not necessary that radio altimeters are particularly vulnerable and more down to the fact that some of the 5G transmitters are broadcasting at reasonable powers directly in the protected band.

      1. Malcolm Weir Silver badge

        Re: Too wide bandwidth in the aircraft system

        This is false. None of the 5G transmitters are anywhere near the protected band: the telco frequencies end at 3.97GHz, the RADAR Altimeter frequencies begin at 4.2GHz.

  10. Pascal Monett Silver badge

    "accusing the FAA of not carrying out proper testing"

    Well duh.

    The FAA is nothing more than Boeing's rubber stamp these days.

    Mine's the one with the self-certification in the pocket.

  11. Clausewitz 4.0

    Messing with Altimeters

    Let me get this straight, from the hacking point of view.

    Instead of training 30 guys for over a year using Microsoft Flight Simulator, one can build a gadget to install it within 200m of the path to reliably crash a plane in a foggy, no-visibility night?

    Never thought planes were so "safe"

  12. Nigel Sedgwick

    Using a Passive Front-End Filter

    Though only on the periphery of my work experience, I suspect (in agreement with others) that lack of adequate front-end filtering could be the main issue with 5G interfering with radio altimeters. However, I have an additional thought on the differences between passive and active filters.

    I wrote on this a couple of days ago, on Tim Worstall's website: I don't know if anyone (more informed than me) has a view, but will repeat the gist of my view here.

    Consider the wanted radio altimeter ground-reflected signal (so significantly attenuated from that transmitted from the aircraft) within an around 200MHz bandwidth (4200 MHz to 4400 MHz) is received together with a much more powerful direct path 5G signal (within the different bandwidth of 3700MHz to 3980MHz). Without adequate very-front-end filtering, there is risk of the unwanted 3700..3980MHz signal saturating (the amplitude of) the first front-end active circuit (that is supposed to filter out that unwanted signal). This means that the 3700..3980MHz unwanted signal would be clipped (even if the active filter would otherwise have provided adequate attenuation. This would cause the lower level wanted signal (ground reflection) to be overwhelmingly distorted by the (non-linear) clipping. Alternatively there might be no clipping but the wanted signal could be attenuated to a low level that becomes dominated by circuit noise. Alternatively a combination of these two highly undesirable effects could occur.

    Note that the clipping explanation does not require the 5G antenna to be transmitting out of its allocated band. It only requires the received (RX) 5G signal level to be so much above expected out-of-band signal levels that clipping occurs in the first active circuits of the radio altimeter. Such active circuits are likely to be for a mix of amplification, automatic gain control and active (and so stronger) filtering of out-of-band signals.

    If this is the cause, it is quite likely that the problem itself can be largely suppressed or totally eliminated by installing a passive bandpass filter into the RX antenna lead, that significantly attenuates much of the interfering 5G signal before the first RX active circuit in the radio altimeter electronics.

    The key issue in this is that the very-front-end should include a passive filter rather than there first being an active filter. This is so the filter operates against the interfering 5G signal before there is clipping or loss of dynamic range. [Note: without such a passive filter, there would be some filtering from the RX antenna, but not enough for adequate suppression of other signals at adjacent frequencies.]

    Upgrade with insertion of an in-line passive filter into each antenna cable strikes me as worth serious consideration as a practical and cost-effective solution for existing equipment. Obviously a designed-together pair of filters (passive first, then active) would be an even better solution, for new radio altimeter equipments. If there is already some front-end passive filtering, more passive filtering would provide a solution tolerant of higher 5G transmission levels.

    Insertion of such a passive filter would improve air altimeter tolerance of 5G signals that are at a very high level, even though they have correct 5G design to avoid out-of-band transmissions. There would be problems with even higher level 5G signals, but perhaps not of those that would be desired in practice.

    Best regards

    1. Chairman of the Bored

      Re: Using a Passive Front-End Filter

      Upvote for an excellent post! You're quite right that frontend saturation is the main RF concern here.

      A relatively cheap surface- or bulk- acoustic wave filter can do the filtering job but would add a fair amount of insertion loss - several dB - and that directly reduces the signal/noise ratio of a receiver. These would cost from single digit dollars to a few tens of dollars each. The systems engineering question becomes whether one is willing to accept the loss of SNR.

      A cavity, ceramic resonator, or printed circuit "stripline" filter would work nicely. I did some back-of-envelope calcs that suggest a relatively low complexity design would provide >52dB of rejection over the 5G lower band (*). Cavities tend to cost around $100+ in bulk, ceramics probably $20-$50, integrated stripline varies depending on how much one would have to modify their existing PCB design to accommodate.

      Another approach is to use low noise amplifiers on the frontend that have high dynamic range. Marginal cost for such an upgrade is minor, but this involves spinning new PCBs - effectively a new product.

      So here is the real problem: radalt are safety critical. If I add a filter, mod a PCB, look at it funny, or even think dirty thoughts in it's vicinity... The FAA regs will require a recertification. Thats extremely expensive. Is have to draft maintenance instructions for all the radalt in the field, and these would need to be reviewed. The radalts FCC certs, CE certs, MIL-STD-461 EMI testing, MIL-STD-810 shake n bake testing, etc would have to be re-done. Somebody has to pay for the labor to installl, inspect the installation, certify it's done to spec... None of that is cheap.

      What's going on here is an effort to get someone else to pay for the systems engineering to figure out if there even is an issue, and be on the hook for any mitigation if there is. Lastly, people want a paper trail so that there is someone else to chuck under the bus if an airliner ever has an issue.

      (*) Butterworth, fifth order, would work nicely.

      1. Electronics'R'Us

        Re: Using a Passive Front-End Filter

        At these frequencies, the RF front end is usually limited to an LNA and a mixer if it a classic analog design. The undesirable signals can be very effectively filtered in the IF stage if it is done properly.

        The front end sensitivity for most of these is between -90dBm and -100dBm (lets put that in perspective: 0.1 picowatt to 1 picowatt) and receiver saturation is pretty standard at -30dBm (1 microwatt).

        The older front ends had methods for eliminating (well, significantly attenuating them by using a double superhet architecture) out of band signals (the two of interest are image frequency and adjacent channel) but with the aircraft being reported as susceptible being rather more modern I suspect that direct conversion is being done.

        Direct conversion might possibly permit aliasing in band to occur and it is much more difficult to design the RF front end for those than the classic analog front ends.

      2. Malcolm Weir Silver badge

        Re: Using a Passive Front-End Filter

        Just a trivial point: MIL-STD-461/MIL-STD-810 don't apply, but the RTCA's DO-160 does, Same game, different local rules!

  13. big_D Silver badge


    I have heard there is high demand for Douglas DC2s, Ford Trimotors and they are being unmothballed.

    Aviation museums are looking at the sudden uptake in the requirements for older models as a way to compensate for lower visitor figures during the pandemic by leasing out their display stock to airlines.

    1. jtaylor

      Re: Unmothballing

      Throw in a Travel Air and a Dragon Rapide, my wallet is ready!

      Radio interference? Pah! Just tune to an AM station near your destination, then follow the railroad tracks into town. Ah, the good old days, when men were men and sheep were scared...of crashing airplanes.

      1. big_D Silver badge

        Re: Unmothballing

        Now you are talking. I'd love to do a flight in a Rapide.

  14. Uncle Ron

    5G Isn't 5G Isn't 5G

    All airplanes with radar altimeters (the avionics in question here) use the same frequency spectrum for their operation. In the US, the spectrum from 5G is "the next one up" from radar altimeters. 5G around the world DOES NOT use the same spectrum as the US. I don't know about Japan, but I know that EU 5G spectrum is much further (higher) away from radar altimeter frequency than 5G in the US. The problem of 5G interfering with radar altimeters in the EU (and probably Japan) and other places is effectively non-existent. Seems like the authorities in the US should have foreseen this MANY, MANY years ago. Huh?

    1. A.P. Veening Silver badge

      Re: 5G Isn't 5G Isn't 5G

      I recommend you read the article again, in Japan the separation between 5G and radalt is (slightly) less than in the USA.

  15. MadAsHell

    Not comparing apples with apples

    If Nerdbert is correct - that the FCC have allowed twice the RF power from the 5G masts, and we acknowledge that USA 5G frequencies do go much higher than rest-of-world, then they have a problem.

    FCC versus FAA.

    So you choose: do you wish to land at a USA airport in a 777 (when the folk who actually build them say that they wouldn't let their own families fly in them) when the radar altimeter might well give erroneous readings? Die Hard with a Vengeance? 787-MAX?

  16. big_al

    Another difference with US 5G

    Unlike the rest of the world where 5g antennas are tilted slightly toward the ground (which limits how far the signal can propagate), US 5G antennas are set vertically at 90° to the ground.

    So not only are US 5G signals being broadcast at twice the power of other countries, they also impact a much wider area around the antenna itself.

  17. Anonymous Coward
    Anonymous Coward


    Many commenters and "experts" are forgetting the Doppler shift effects involved when an airplane is traveling at various speeds relative to fixed base transmitters. While a minimal guard band is sufficient for maintaining freedom from interference for fixed location or slow moving objects a great deal more is needed for protection between relatively fast moving objects at different distances. It's not simple and not constant. The altimeters involved likely reject interference from the mmWave 5g just fine when staticly located. Also please stop acting like it's all 5g. It's not it's just the mmWave frequencies used in the U.S. by ATT and Verizon. T-Mobile 5g is unaffected as it uses different frequencies and has been actively working for users for over a year.

    1. Malcolm Weir Silver badge

      Re: Forgetful

      ... but apparently those Doppler effects only occur with 38% of the US fleet; the remaining 62% are OK.

      (But you're right about the ATT/Verizon/TMobile point!!!)

    2. G.Y.

      1ppm Re: Forgetful

      The Doppler shift at airline speeds is about 1ppm, I.e 4Khz

  18. Joe Gurman

    I’m not close to being an expert

    ….but I have read elsewhere that not only are US 5G transmitters operating at a different frequency range from that in other places, as noted here, but also operating at something like 2x the radiated power. Perhaps something to do with the more spread-out. Suture of some American cities?

    That aside, it appears to that parts of the US (and other) airline industry has had years to replace their radar altimeters with ones with no sideband issues vis a vis 5G, but have chosen to allow the issue to become literally a last-minute confrontation, in the hope of soaking the taxpayer for what ought to be a shareholder/passenger financial responsibility.

    And not just the US carriers: business news outlets carried a good deal of coverage yesterday of the Emirates CEO bloviating about an “issue” his airline had lots of time to mitigate.

    1. A.P. Veening Silver badge

      Re: I’m not close to being an expert

      I'd say it is a bit of a coincidence that this problem only occurs at/near American airports and only with planes of an American manufacturer.

  19. DownUndaRob

    Design Flaw much

    2.2Ghz away that's like telling me my 27Mhz CB Radio can listen to Airband, if you cant reject signals that far away from your system, get out of the kitchen...

    or double the entire frequency range of the 2.4Ghz wifi band (including Japan's channel 14)

  20. Jaybus

    Wait a sec....

    If the US aircraft industry is seriously concerned, then has anyone considered that all a terrorist has to do is setup a C-band transmitter anywhere near an airport to cause all sorts of chaos? Wouldn't it be safer if the RADAR altimeters were a bit more resistant to interference than that? This sounds more like a bureaucratic issue, rather than an actual safety issue.

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