20m altitude difference POS ALT vs GPS ALT

[ChrisOlson]

@Andreas_Krauchi

And surely the difference is huge. In certain parts more then 25m!
Is this because of baro drift? In arduplane I remebere using the alt_mix parameter but this does not exist in arducopter.

Likely due to GPS inaccuracy, not the baro. GPS is notoriously inaccurate in height estimates. I have had this problem with helicopters on high speed flights making lot of turns.

Loss Of Altitude in Auto with Copter 3.5

This is not a problem unique to helicopter. Multi-rotor pilots have noted this as well but it’s usually blamed on the aircraft not being able to maintain altitude due to running out of power and dismissed. With helicopters we don’t have that problem so I can more accurately illustrate what is going on. I noted this during testing and tuning the WPNAV_ACCEL on this heli and dismissed it at the time as a one-time anomoly. But it turns out it’s not. This is a screenshot summary of the flight plan …

There is a patch in 3.6-dev for this to set the EKF height origin using baro, but it is not in 3.5 yet. I built my custom helicopter code with the patch on 3.5 and have not had the problem since.