Three-Dimensional Magnetometer Calibration With Small Space Coverage for Pedestrians

Magnetometers are sensors that can sense the earth magnetic field from which heading can be determined. Gyroscopes can provide the angular rate from which the heading can be calculated, they can be available in low cost and light weight. Nevertheless, the problem of using gyroscopes as a sole source of heading is that gyroscopes readings are drifting with time in addition to the accumulated errors due to mathematical integration operation. Magnetometer is available in low cost, it does not suffer from mathematical integration errors, and it can provide an absolute heading from magnetic north. Magnetometer readings are usually affected by magnetic fields, other than the earth´s field, and by other error sources. Therefore, magnetometer calibration is required to use magnetometer as a reliable source of heading. In this paper, two techniques are proposed for fast automatic 3D-space magnetometer calibration requiring small space coverage. There is no user involvement during calibration and there are no required specific movements. The proposed techniques perform 3D-space magnetometer calibration by calibrating the three magnetometer readings in the device frame, which makes the magnetometer useful for determining heading in untethered devices, especially in pedestrian navigation, in which, portable navigation devices (such as smartphones) are always moving freely between the user´s hands, belt, pocket, or placed on the ear during phone calls. The two proposed magnetometer calibration techniques are also capable of calibrating magnetometers in tethered devices, given that the platform to which the device is tethered is capable of performing 3D motion.