The odd part is that Boeing originally proposed a 100MHz guard band. It's literally twice the headroom a major manufacturer suggested for this purpose.
Even weirder, the critical phase of all this is generally during airport approach and landing so at pretty low altitudes. In terms of reception if you can't design something to tell you if you are 100 / 50 / 20 feet off the ground with a 200Mhz guard band ON BOTH SIDES that is insane (600Mhz total bandwidth to tell me if I'm 20 feet off the ground??).
If you are ham these bandwidth needs are just nuts.
And this has been 10 years in the making. The FCC must just be rolling their eyes - why didn't FAA do something earlier.
We have coverage during landing phase with a localizer and glideslope as well, plus RNAV / WAAS enhanced GPS options etc etc + the mark 1 eyeball. Somehow in the last year the US has just become helpless to solve problems. Hard to beleive folks flew across the ocean with no GPS or moving map etc etc.
Now, single digit khz filters at RF frequencies, that's difficult. But not 200Mhz!
I guess the radio altimeter manufacturers just figured, "there's a large guard band, why play it safe and put in filters", or what?