tacybo:
my father and I had a discussion the other day. We were talking about navigation and GPS systems when I stated that nowadays it is possible to navigate with GPS. My father replied that this would be possible but is currently not allowed by the LBA (the German FAA). All aircraft operating RNAV are getting their data via the inertial reference system and the GPS system is only a backup but still not available for RNAV...
Ask him for an authoritative reference to support his position.