>but I can't ever recall a nearly still level hover from a quadcopter.

Seriously? Every one of the DJI products can do this out of the box using just GPS/accel/gyro/barometer. Newer ones can do it even better by adding in a camera pointing at the ground that works like an optical mouse. Some really insane stuff can be done if you throw an active positioning system into the mix: http://www.ted.com/talks/raffaello_d_andrea_the_astounding_a...

Just saw that, thanks for the link. I stand corrected, I hadn't personally seen them stay very still in a hover before, maybe it was operator inexperience.

If the quad has only an accel and gyro sensor onboard then yes, the stability will be based primarily on the skill of the pilot and how well the machine is tuned.

With a GPS and barometer thrown in, position hold should be pretty close to a still hover unless there is significant wind or unresolved problems with the power train

I'm surprised that GPS has enough accuracy to do a position hold. Why, then, my cellphone GPS is accurate only to 5-10 meters?

The copter doesn't need to know where it is, just whether it is moving and if so, how far and in what direction. Because the error in GPS measurements doesn't vary much in small time frames, that can be done more accurately than absolute position measurements. See http://en.m.wikipedia.org/wiki/Differential_GPS.

A "good" GPS system (the DJI version is ~$100 by itself I think) should be able to get you down to just a few inches of precision, but yes, the market is flooded with cheap (~$20) units that aren't nearly as useful for position hold (hovering completely still) but can still give you a good lat/long and do well with navigation, distance measuring or acting as a reference sensor for a automatic direction tracking antenna. Cell phones are similarly hit or miss-- some have nicer GPS modules than others, but ultimately the user is unlikely to notice this since smartphone style road navigation only requires 10 meters or so of accuracy and a vague single position can be greatly refined in that context by knowing the road/map layout as well as the direction and speed of travel.

That makes sense. Thanks for sharing more info, I like learning.

I've bought various helicopters through the years as gifts but they always get totalled on their 2-3 flight. The quadcopters I bought are still in good nick, are cheap to replace parts for and are very intuitive. The entry level models are £30 and robust enough for beginners.

level hover without stick input, will fly back to position if moved: [1]

[1] https://www.youtube.com/watch?v=OsATmfRC0U4

That is awesome. How is the "home" position set? Is it from where it starts or do you preprogram it? Also, how is elevation determined? GPS or an on board sensor?

Home is usually set where you power up / take off at, however there are plenty of users who can report that their misconfigured home position ended up with a drone taking off at full speed towards Shenzen or the 0,0 lat/long point off Africa.

Elevation can be determined by GPS, but a barometer works better.

Cool, that makes sense, thank you again.

I would think a barometer would work far better. GPS is not the best for vertical accuracy.

There are pros and cons to both-- PCB mount baros don't always respond well in settings with lots of moving air and various other nearby components that get warm-to-hot during operation.

Keep in mind for either method you also have an accelerometer, so even if GPS (or a crummy baro) provides a messy signal for altitude you can average it over a given timespan and compare that against how quickly you are accelerating upwards or downwards.