[mchusma]

Sort of unrelated, but I'm surprised aircraft need that spectrum. I'm not an expert, but my impression was that using lower frequency has lower speeds but longer range and less sensitive to interference. This seems better for the aircraft usecase.

The answer could be just that they were built using this spectrum which at the time seemed fine, and now that we want to use it for something else it's an issue. Curious if anyone knows more.

https://en.wikipedia.org/wiki/Radar_altimeter

[headass]

i mean if it’s for the radar altimeter you want it to be much quicker as it’s landing which is when it’s more prone to interference

[p_l]

For Radar, the frequency used impacts resolution and reflection rates. Thus 4.2-4.4GHz was chosen based on those characteristics and what was doable in hardware given the constraints (not just what's possible, but also what could be fitted and wouldn't for example give up in fog, while working on reasonable range of distances)