Altitude Keep changing in ground ?!

Hi all ,

I am using
Pixhack Flight controller
Ublox 7m GPS
Firmware 3.7.1

when I turn on my plane in ground and stay some time the altitude start in 0 , then start changing high and low some time it reach 100 m , while It’s in ground .

any information about this issue ?


Can you post a DataFlash log demonstrating this problem?