Magnetometer
From Dronepedia
Magnetometers, as opposed to the accelerometers and gyrocompasses that accompany them in Micro-Electro-Mechanical-System (MEMS) Inertial Measurement Units, measure a vehicle's orientation relative to the Earth's magnetic field. For flying in one location, this is a fixed reference which can be used to correct drift in the other instruments.
In effect, they function as accurate, fast compasses which are resistant to vibration.
Care must be taken not to place them too close to the magnetic fields of nearby motors.
