Greetings
ANP, is actually elaborated by the FMS on the AIRBUS and most probably by AIMS on B777 (I have to check this out) so if both FMC are inop you cannot have an ANP.
But you can enter lat and long.
You could use your GPS position versus your Backup position to see how far apart they are from that deduce if your are within the RNP "tube" here I am thinking RNP 4.
But unfortunatly these things cannot be tested unless the FMC are inop
...
I will try in the sim.
