HMC5883 External Magnetometer Compass for APM

If you’re having trouble with magnetic noise affecting your flight controller's on-board compass, you may want to switch to an external compass that you can mount further away from noise sources such as motors, ESCs and your power distribution system.

This Magnetometer is designed for use with the APM FC. When using on APM2.5+ boards you need to disable internal compass. APM2.6/2.8 board you only need to activate if from Mission Planner.

Ardupilot Setup Wiki