I just finished building my first quadcopter, and everything seems to be working as expected apart from the weird altitude readings I'm seeing in Mission Planner.
If I turn the power on and put the quad on the floor (with motors off), the altitude reading continually fluctuates at a rate of about +/- 0.02 meters every second. When I first turned it on about five minutes ago, the reading was 0.05 meters, but now it's 2.15 meters even though it hasn't been touched the whole time. What's more is that I'm on the second floor of my house, and I know that my true altitude is most likely somewhere around 4.00 meters off the ground.
The flight controller I'm using is a HKPilot32 (Pixhawk clone) running the latest stable version of ArduCopter. The flight controller's case is completely opaque and also came with a little block of foam that covers the barometer, so light hitting the sensor shouldn't be an issue. I also keep my motors off when I'm testing the barometer, so vibration can't be the cause either.
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Go outside. ;-) the Baro will drift a little on startup as it warms up, it's reset when it's armed. The altitude is calibrated against the reading from the GPS, and it doesn't work indoors. (if it says it's a good fix indoors, it's lying as that's down to 'multipathing' issues with GPS)
Gotcha. That makes sense.
I just found out that MP includes a terminal which I can use to view raw sensor output, and since it's cold and dark out at the moment, I decided to give that a go and take a look at what the barometer itself is actually sensing. I ran a test for about 10 minutes, and it seems as though the air pressure reading stays at around 100133 +/- 3. That's stable enough for me. :)