The decision to sell frequencies as close as 220 MHz away from lower end of radar altimeter band came only in 2019, and aviation world already been working on possible interference at the time - which led to much higher guard bands everywhere else and/or extra requirements on use of 3.8-4.0GHz allocations, especially in terms of usage near airports.

The issue is that the FCC sale of frequency spectrum up to 3.98GHz doesn't have such limitations, so essentially in 2019 aviation got an unique problem of extra heavy interference in USA

"5G" doesn't refer to one specific set of bands. Radar has been operating this way for decades, and only last year did the FAA decide to allocate the nearby spectrum to high power transmitters.

Yeah so my question is (how) did they not see this problem last year? What made it so unpredictable until Feb 2022?

Here's an article from 2020 [0]. I'm sure plenty of this prior research was classified.

0 - https://www.defensenews.com/2020/12/21/the-military-is-scram...

They did know. Both the FAA and the FCC knew.

Yeah so we circle back to my original question: What did they expect would happen instead? Why didn't they do anything? The problem wasn't going to just disappear randomly right?

Two different government agencies with different incentives.

The FCC really wants to proceed with the 5G rollout. The FAA really wants to make sure aircraft don't crash. They do not agree with each other on this, so they each proceeded in the only way they administratively could...

The FCC continued with the 5G rollout, the the FAA recently banned use of approaches requiring a radio altimeter until each individual model can get recertified or upgraded - and the public will just have to tolerate additional travel delays for the next couple years as a result.

FAA issued NOTAMs. They don’t have the authority to stop the FCC from doing anything.

FCC was warned by industry groups but dismissed the warnings.

We’re watching the process play out.