VehicleMagnetometer (UORB message) 
c
uint64 timestamp            # time since system start (microseconds)
uint64 timestamp_sample     # the timestamp of the raw data (microseconds)
uint32 device_id            # unique device ID for the selected magnetometer
float32[3] magnetometer_ga  # Magnetic field in the FRD body frame XYZ-axis in Gauss
uint8 calibration_count     # Calibration changed counter. Monotonically increases whenever calibration changes.