I am flying APM 2.5 with firmware ArduCopter v3.1.5. When I engage AltHold or Loiter mode, copter starts climbing up fast. As I have observed until now, the altitude displayed in mission planner correctly when the motors are not spinning. After take-off, the altitude takes negative values and decreases further with increased throttle. When I plot the barometer altitude, the data seems reasonable however, GPS’s RelAlt data matches with the displayed altitude data. I have tried the quad with APM 2.0 (both firmwares 3.1.5 and 3.1.2) also, but still got the same results. I am sharing my log file for APM 2.0 v 3.1.2.(Vibration data is also available)
Did not see barometer data in logs. Turn up your logging for these test so we can see all the PCIN/PCOUT data as well as the barometer data.
The log show you were in Alt Hold only a short time. See some Vibration Spikes but that does not look like the issue.
AltHod was engaged for a shor time because quad climbs fast in AltHold. Barometer altitude is available in the uploaded log file under CTUN with the name BarAlt. I am sharing the plot of BarAlt, WPAlt and GPS RelAlt together. The log shows GPS ERROR but this was the only the time I got GPS ERROR, the problem is present at all times. The problem is GPS RelAlt goes negative and WPAlt is set accordingly. I suspect vibrations were the problem but does it have anything to do with GPS RelAlt?
Note: Red for BarAlt, Green for WPAlt and Blue for RelAlt