Basically, I am writin gsomething on safety critical systems.
What I mean by backup is something which is in place should the 'normal working one' fail.
I know you have primary and secondary radar, but I was looking for other examples.
Perhaps a way to talk to aircraft if the transmitter fails (secondary backup transmitter that doesn't get used etc.)
Sorry if it is not clear - it's approaching finals time and my head's in a mess!