From an engineering perspective, I would guess, as an avionics tech, back in the age of dinosaurs, these radio altimeters are probably of a very old simplistic

From an engineering perspective, I would guess, as an avionics tech, back in the age of dinosaurs, these radio altimeters are probably of a very old simplistic design as to have dead simple and reliable electronics built with an absolute minimal number of parts. The carrier generation is probably just a crystal and an analogue multiplier with lots of phase noise and wide band so a similarly simplistic receiver can pick it up.