TLDR; the aviation industry had a decent technical study done on radio altimeter interference from 5G and the FCC determined it didn’t show radio altimeter interference from 5G was likely. Then an aggregated summary of that technical data was shared with an aviation industry group (whose members include radio altimeter manufacturers), and that group used the aggregated data to claim any 5G use would likely lead to catastrophic crashes and multiple fatalities.

As a private pilot and software engineer interested in spectrum policy, I’ve been following this closely for years as it’s wound it’s way through the FAA. I have yet to see any convincing, reproducible evidence that any radio altimeters that are operating within specification (filtering out all signal below 4.2 GHz) have malfunctioned due to interference from 5G cellular signals. The one thing the FAA and airline industry claim as evidence is a study by an industry group (Radio Technical Committee for Aeronautics), whose members include radio altimeter manufacturers who would benefit from large retrofits/upgrades. The RTCA didn’t actually do any of their own testing, they received aggregated data from AVSI, a Texas A&M aerospace research group, which had done a study in 2019 on radio altimeters interference. In 2020 the FCC determined that AVSI study “does not demonstrate that harmful interference would likely result under reasonable scenarios (or even reasonably 'foreseeable' scenarios to use the parlance of AVSI)”. Then, two years later when the FAA went to the NTIA (National Telecommunications and Information Administration), which is the agency tasked with managing federal spectrum and agency spectrum disputes, and demanded they stop the FCC from letting carriers turn on networks in the C-band, the NTIA refused because its own technical experts had already evaluated the technical data that RTCA used as evidence that any 5G use of 3.7 GHz - 3.98 GHz band would cause interference severe enough to cause a catastrophic crash resulting hull loss.

Also, when this whole fiasco unfolded neither Verizon or AT&T had spectrum even close to the radio altimeters band. Of the C-band spectrum (3.7 GHz to 3.98 GHz) that was auctioned off a few years ago, only Block A which is between 3.7 GHz and 3.8 GHz was being used. So in addition to the existing, and potentially larger than technically required, 220 MHz guard band between 4.2 GHz (where radio altimeters start) and 3.98 GHz, when the airlines were threatening to cancel service to the US and the FAA was waging a PR war against the FCC the spectrum in question was 3.7 GHz - 3.8 GHz. Even if Verizon and AT&T rolled out 100% of the 5G spectrum they had the ability to use in 2021, it would have meant 5G cell service was within 400 MHz of the radio altimeter spectrum. The entire allocated band for radio altimeters is only 200 MHz (4.2 GHz - 4.4 GHz). Additionally, other countries like Japan have cellular providers operating in 4.0 GHz - 4.1 GHz. Japanese aviation officials (unlike the FAA) actually configured different types of 5G equipment and radio altimeters and tested them, including to see the minimum guard band needed. They found 60 MHz was the minimum guard band needed and therefore the “standard” 100 MHz guard band would be fine. They also found you shouldn’t install 5G towers directly below the approach path of an airport, but that even high powered 5G base stations won’t interfere with radio altimeters if they are 200m away.

I think by far the most comprehensive explainer for this whole saga is by Harold Feld of public knowledge, which if you’re not familiar is a nonprofit advocating for an open internet which includes white papers and FCC testimony on spectrum policy they view as beneficial the public, such as allocating more of the federal government, specifically the military’s, huge bands of essentially unused spectrum. It’s on his personal blog. https://wetmachine.com/tales-of-the-sausage-factory/what-the...