Elevation was change suddenly

I just upgrade my quadcopter to AC-3.1 yesterday. All tunes were done. It’s quite stable flight, but sometime in stable mode the elevation of the copter was decrease 1-2 meter suddenly and return to stable. During the flight, I did switch to Autotune mode by CH7.

Do you guy have any idea why does it happen? Thanks.

Were you flying in and out of shadows? We know that there are issues with bright light on the APM and mal readings in the barometer in those situations. Check the logs for jumps in the barometer.

Thanks for your answer.
Right! I fly at night and under the street light.
Is this the new feature in 3.1?
I found 3.01 do not have this problem!

Hmmm… well, it would depend how closely you were flying to the light. Here was the example that I first saw (and ultimately let me to paint the interior of my case: youtube.com/watch?v=n4wizkYGVxs

It is difficult to say. I would think that if you were flying in an area almost devoid of light, a bright source like a streetlamp would have the ability to impact the controller (i.e., light to dark would be much more contrasting) than more light to a little less light, as in outdoors. Certainly worth more looking into. Odd, though, that this didn’t occur in 3.01.