The barometer's altitude and the GPS altitude are impossibly far apart

What ArduPilot says:

• GPS alt error N.Nm (see BARO_ALTERR_MAX)

With a 3D GPS fix, ArduPilot works out the height above sea level that the measured air pressure would mean in a standard atmosphere and compares it with the GPS altitude. They differ by the number of metres shown, which is more than BARO_ALTERR_MAX (2000 m by default). Weather moves this by a few hundred metres at most, so an error this large means one of the two sensors is simply wrong. It catches a barometer chip that is read with the wrong formula.

What to do, most likely first

1. The board carries an MS5607 barometer where the firmware expects an MS5611 (a common substitution on clone boards): the pressure comes out at about half.

   CheckThe pressure shown in the ground station is near 500 hPa at low altitude instead of about 1000, and the error in the message is several kilometres.

   FixSet BARO_OPTIONS bit 0 (treat MS5611 as MS5607) and reboot.

2. The GPS altitude is wildly wrong: a bad fix, or spoofing.

   CheckThe pressure looks right for where you are, but the GPS altitude in the ground station is far from the real one, or jumps.

   FixWait for a good fix in the open; do not fly where the GPS is being spoofed in a mode that relies on it.

3. The barometer is damaged or wet and reads a wrong pressure.

   CheckThe pressure in the ground station is far from the local air pressure and is not close to half of it either.

   FixDry the board. If the pressure stays wrong, replace the flight controller.

Setting BARO_ALTERR_MAX to 0 switches the test off, which is not a fix: a barometer that is wrong by kilometres cannot hold height.

Parameters to look at

Where to do it:Firmware Inspector