Cruise missiles have navigated with terrain-matching for ages now. INS is a thing too 
Planes went everywhere before we had gps
SR71 had a camera looking at stars
doesnt work above the clouds
drop down and read the road signs (did that in helicopters!)
It’s a bit limited what you can fit onto a drone of this size:
PS> Our son use his caps as a “landing platform” when he is demonstrating the capabilities of the Black Hornet around the world.
Works if you are going slow
IFR is better, I Follow Railways