Hi! I have a copter with optical flow and GPS. I am trying to fly it from an open space with good GPS signal to a place where GPS signal may be lost (like a place blocked by buildings or trees). If I use loiter or poshold mode, when the aircraft loses GPS signal, will the flight control automatically switch the nav source from GPS to optical flow? Will this switch be smooth? Or will there be relatively large jitter?
Thx！ 
Yes that is possible. @rmackay9 has worked on that and did some documentation as well. Have you searched in the ArduPilot documentation for "gps denied Navigation " ?
It also explains how to switch.
And AFAIK you should use 4.6.0-beta2 for best results
Hi @airbunny,
Yes, as @amilcarlucas says we’ve got some information here on the wiki:
and we have this lua script to handle transitions between GPS and optical flow.
There shouldn’t be any twitch when switching from GPS->optical flow but optical flow->GPS can cause the vehicle to move depending upon the flight mode. Loiter and PosHold shouldn’t move but Guided mode and Auto may because they attempt to maintain their location based on lat,lon coordinates.