Of all the things I was researching for control projects for helicopters and planes, the hardest thing to do is to make it stay hover or a piece of ground (or fly a preset path over ground) because the aircraft moves relative to the air, not the ground below it. At worst, there will be wind, at best the helicopter will be blowing itself to the side due to it's tail rotor among other things. The helicopter has no way to keep track of the ground unless you use GPS. Two fancier and more self-contained alternatives are fusing gyros and accelerometers together with double integration and rotational coordinate math but this still suffers from drift and integration error even if you pour the dollars into it. And, of course, vision has it's own problems.
You're pretty much stuck with GPS.