[jpsb]

I am surprised there is no "dead reckoning" back up software should primary navigation fail or appear to be incorrect. That would have been simple to implement.

[jpsb]

Continuing

The aircaft maintains a series of way points
If craft find itself some delta from it's expected position
Then the craft uses dead reckoning to return to a previous way point (and tries again) or it uses dead reckoning to return to base
problem solved.