Large 25Kg drone MP shown height different from actual height

Yesterday we experienced a large drone gradually dropping height for a mission to test flight for an hour. When it drops height, the pilot throttles up a few meters (1-2m), and after some time, it drops height again. This was repeated and Mission Planner (MP) showed 10.x m in height at the end of the one hour flight, but physically the drone was just 2.x m above the ground all along.

Any experts can guide and narrow down what is the likely cause.
BIN file.

HW: CubeOrange
FW: Arducopter 4.1.5
configuration: Octocopter

Air pressure changes over time, changing the barometer readings.

I am still beginner in reviewing the flight log, From the flight log, any section I can guide our customer to view and prove this point?

Besides that, will mounting a rangefinder to get the height helps in this situation?

Take a look at the BARO messages in the flight log.

Yes, a rangefinder helps, if it is precise and accurate.

I guess we don’t use GPS values to correct barometer readings over time? Seems like a (heavily filtered) GPS altitude might be useful to correct barometer drift.

Maybe a barometer on the ground(lots of cell phones have them) could be used to track baro pressure changes over time to prove this is where the effect is coming from?

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