back to article FAA to airlines: 5G-sensitive radio altimeters have to go

The US Federal Aviation Administration (FAA) met with airline and telecom officials yesterday to present its latest solution to the instrument interference problem presented by C-band 5G: replace the affected equipment. A letter from the FAA's head of aviation safety, Chris Rocheleau, proposed the meeting to establish a …

  1. Anonymous Coward
    Anonymous Coward

    Reminds me of DOS/Windows ....

    Back in 1997, if a DOS program needed more memory, it was down to the developers to shave a byte off here and there.

    When a Windows program needed more resources, the customer was told to buy a new machine.

    I once increased a programs capacity by 1/7th by using the 8th bit in a 7-bit structure.

    1. Anonymous Coward

      Re: Reminds me of DOS/Windows ....

      A better analogy is Cat 3 cables used for data transmission. They are cheap and they work but they aren't up to modern computing standards. However there are undoubtedly cheapskates who didn't want the cost of upgrading to modern cabling.

      This is one thing if it's only affecting your office. It's quite another when it's affecting both air travel and telecommunications.

  2. Anonymous Coward Silver badge

    "C-band signals operate between 4.2 and 4.4GHz, while C-band 5G transmits between 3.7 and 3.98GHz"

    So use better filters. If your altimeter is affected by doppler shift enough to need that bandwidth, I suspect 5G interference will be the least of your worries.

    1. Headley_Grange Silver badge

      I don't think they use doppler. - they are pulsed and based on the time the pulse takes to ping off the the surface. Improved filtering might work, and it probably wouldn't be difficult to design a system with some coding in the pulse or pulse-stream so that the altimeter doesn't interpret interference as a valid return. The problem is that qualifying new stuff for aircraft is expensive, as is the swap out of kit.

      1. Alan Brown Silver badge

        You've hit the nail on the head here

        The affected kit is very VERY old and if the FCC had its way would have been ordered switched off many years ago (somehow the FAA gets to trump them "because safety", but kit this old is cantankerous anyway). Airlines whine like crazy about costs but the reality is that an instrument like this is fairly cheap compared with things like periodic engine overhauls - they just won't spend money unless absolutely forced to do so with bamboo splints under the fingernails

        As for filters - a cavity resonator is dirt cheap, weighs next to nothing and and works pretty well. Again, airlines won't fix their broken shit if they can blame other people for it and make them pay to upgrade it instead.

        If that kind of management and mindset was allowed to prevail we'd still be using 1MW spark transmitters (or Neucomen engines in a bygone age)

      2. elkster88

        Qualifying new radar altimeters.

        "The problem is that qualifying new stuff for aircraft is expensive, as is the swap out of kit."

        Spot on.

        Also- the qualification process is not terribly speedy. Which is why it's incredible that this was not being foreseen and addressed long before 5G was deployed.

        1. TheRealRoland

          Re: Qualifying new radar altimeters.

          i think it was foreseen, but not addressed because *waving hands*

          1. Anonymous Coward
            Anonymous Coward

            Re: Qualifying new radar altimeters.

            It was forseen - but the speed it takes to certify an airliner to work with a slight change in equipment is glacial (see how long a small software fix for MCAS on the MAX took) - and for good reason (see what happened when Boeing sneaked the MCAS onto the MAX without telling anyone).

            1. Xalran

              Re: Qualifying new radar altimeters.

              The small fix for MCAS on the MAX was so that it stopped killing people by plane loads.

              Here we are talking of a small fix to something that might eventually, potentially, become unreliable while performing an ILS under very specific conditions...

              And it's fun to note that the Japanese have found a solution that does not require any retrofit without any fuss despite the fact that their 5G C-band extend to 4.1GHz :

              Just ensure that the ILS and the 5G C-Band radio emitters are separated enough when seen from the glide path of the landing planes.

              1. Anonymous Coward
                Anonymous Coward

                Re: Qualifying new radar altimeters.

                As a pilot, I can tell you that losing the radar altimeter during the approach could fairly easily cause a chain of events that would result in a fatal crash. Its unlikely - but unlikely is not good enough for aviation - and therin lies the crux of this issue. "It is incredibly incredibly unlikely that this would result in a plane full of passengers dying" is good enough for the FCC, but not good enough for the FAA.

                The proposal of not permitting 5G-C band base stations on the glide path of airports was the solution suggested by the FAA initially but rejected by the FCC.

                1. Cliffwilliams44 Bronze badge

                  Re: Qualifying new radar altimeters.

                  The FCC should not have the authority to reject that proposal!

      3. Anonymous Coward Silver badge

        I didn't think that they used doppler, just that any signal is susceptible to it and so you need a bit of bandwidth to accommodate it. It'll be a few Hz under normal conditions, but if the plane is gaining altitude at a ludicrous rate (ie faster than a speeding bullet), they might need an input stage that can handle a signal 200-700MHz below the transmitted pulse.

  3. Anonymous Coward
    Anonymous Coward

    Classic industry focused answer

    Hey, look, people have to buy new gear! I suspect some backroom 'encouragement' helped this decision along..

    1. Alan Brown Silver badge

      Re: Classic industry focused answer

      In this case the airliner kit is at fault - the stuff in the neighbouring bands is perfectly compliant with emissions requirements including splatter, but the airliner receivers were pieces of shit that relied on nobody else transmitting in the adjacent bands. They weren't standards-compliant regarding adjacent-channel rejection at the time they were sold but they got away with it because the adjacent channels were sparsely populated

      1. Electronics'R'Us

        RF levels

        <Puts on RF designer hat>

        Although the 5G transmitters are within their specification, RF radiation does not suddenly stop at an arbitrary frequency.

        There will still be some 5G transmit energy in the radio altimeter band, and although that level is fairly low, most radio altimeters (regardless of the method used, either pulsed or frequency swept which are the primary techniques) have a receiver sensitivity of around -100dBm.

        Put that in perspective; that is 100 femtowatts which is not enough to warm a gnat's behind.

        Adding filters won't really help the affected devices very much. All that will do is reduce the resolution of the device and although that may be ok,, it is a material change of specification of a safety critical item which requires re-qualification to level A (failure can result in catastrophic loss of life).

        So whether a new piece of equipment or an update to existing equipment is done, there is an expensive piece of work to be done.

        The 5G antenna could be adjusted to not point at the sky, of course, which would also solve the problem.

        1. OhForF' Silver badge

          Re: RF levels

          "The 5G antenna could be adjusted to not point at the sky, of course, which would also solve the problem"

          I am pretty sure 5G stations will do their best to send the limited energy they may use where it is useful to them. To cover as much area as possible they will want their signal to travel horizontally.

          Unless a base station is there to provide service to passengers in a landing airplane it won't point at the sky.

          Not being a RF designer i might be wrong - if there is a reason why it is beneficial to focus the energy upwards feel free to correct me.

  4. Duncan Macdonald

    What a surprise

    Boeing 737 aircraft having crap radio altimeters - the frequency margin is so large (over 200MHz) that any properly designed radio altimeter would be completely unaffected by the 5G transmissions. Boeing would rather cut costs than buy good equipment.

    1. Anonymous Coward
      Anonymous Coward

      Re: What a surprise

      They have to, they need the money to pay off regulators.

      Wait, did I say that out loud?


    2. Richard Pennington 1

      Re: What a surprise

      In the case of the 737 MAX, does the existing radio altimeter go down to ground minus 500 feet?

      1. Anonymous Coward
        Anonymous Coward

        Re: What a surprise

        The death of 346 people in a completely avoidable accident is not and never will be a joke.

        1. Hubert Cumberdale Silver badge

          Re: What a surprise

          You're sort of right: the deaths are very much not funny. However, everything is fair game for humour.

          I feel completely impotent regarding current affairs such as Putin invading Ukraine and claiming they're the fascists (hilariously terrible), the present UK government being a bunch of privileged, ignorant, self-serving pr#cks who know nothing of the hardships of the general populace and who have blithely killed thousands of people in various casual ways (e.g. by underfunding the NHS while lining the pockets of their old Eton chums) without even really understanding what they've done (terribly hilarious), or the US apparently allowing a tiny, narrow-minded, right-wing branch of what they claim is "Christianity" to dictate not only what consenting adults should be doing behind closed doors but whether they should die in a back-street abortion clinic (admittedly, just terrible).

          Just because something is extremely serious, that doesn't mean it can't be the subject of humour: sometimes it's the only weapon we have left to fight incompetence, injustice, or downright evil.

        2. Anonymous Coward
          Anonymous Coward

          Re: What a surprise

          Dark humour exists for a reason (and that reason is not victimisation).

  5. This post has been deleted by its author

  6. DafyddGrif

    This is really silly. Just prevent people using 5G on flights. Simple.

    1. Xalran

      err, the issue is not caused by people using their phones ( not flight mode ) while being on planes.

      the issue is caused by the fact that the radioaltimeters used in some planes ( including B737 ) for ILS were built on the cheap side and can be interfered with (under specific circumstances ) by the base stations ( on the ground near the airport ) emitting in the 5G C-Band...

      This basically restrict the affected planes to Visual landings which means said planes can't land in fog and other foul weather where landing can only occurs through ILS.

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