Originally Posted by
snowfalcon2
A question to the radio engineers among you: If you would need to design a radio altimeter today taking account of the increasing scarcity of radio frequency spectrum - what would be the minimum needed bandwidth including guard bands? I know it is always a trade off between cost, performance and need for spectrum, but looking at the whopping ~600 MHz now used by the radio altimeter service (including guard bands) one would think there would exist more spectrum-efficient solutions?
It’s a long long time since I was in practice in another life, but with just what I remember, I would posit hardly any. The aggressive way modern comms cram as many bits as possible into each Hz of bandwidth, means that it looks effectively like a broad swathe of noise; completely different to a radio altimeter which is just trying to determine time of flight of a reflected pulse, and there are multiple ways to make this more robust. They are affected by doppler effects but not that much, given aircraft vertical speeds compared with
c. I’m sure there are technical edge cases where one could interfere with the other but from a (now) lay point-of-view, it just doesn’t feel that likely.
Filter technology, both analog and digital, is pretty advanced these days, and out-of-band rejection a foregone conclusion. I can only think that old designs of RAs are really, really bad if they are susceptible to something hundreds of MHz away; certainly the 5G transmitters won’t be leaking onto the RA band. I haven’t heard of anyone actually being able to generate false RA readings with 5G signals, so it seems to be conjecture at the moment...