Thanks for the prompt answer.
Regarding radio-altimeter not working on cruise altitudes, I presume it is because of the time it needs for the beam to go and return back from earth to the airplane which has already "gone" forward?
Regarding altitude what about GPS as alternative? In AF447 they had a false reading that airplane is descending, so copilot puiled up and that is how the trouble started. Although GPS is not too precise reading altitude, wouldn't it at least give a "hint" that they are not descending?