In-flight estimation of magnetic biases on board of small UAVs exploiting cooperation

This paper presents an innovative method to estimate the systematic error, i.e., bias, affecting the measurements of magnetometers on board of small UAVs, thus significantly improving magnetic heading accuracy. Excluding the presence of other external disturbances, this bias is largely produced by the magnetic field generated on board due to the presence of electric equipment, such as the payload and the engines, especially in very compact configurations (besides eventual uncompensated sensor offsets). Consequently, ground calibration techniques have limits. The proposed approach is conceived to rely on a simple and fast flight procedure during which all the devices operate in nominal conditions. It exploits the capability of the UAV whose magnetometers need to be calibrated (chief) to detect and track a cooperative vehicle (deputy) using a visual camera. Also, the two UAVs must fly under nominal GNSS coverage thus enabling relative positioning. By integrating deputy line-of-sight and chief/deputy differential GNSS information, the magnetic biases’ determination problem can be expressed as a system of non-linear equations which is solved using a customized implementation of the well-known Levenberg-Marquardt algorithm. The calibration can be carried out either off-line, using the data collected in flight, or directly on board, i.e., in real time. In the latter case, the two UAVs must be able to exchange navigation data through a reliable communication link. Demonstration of feasibility and performance assessment of this method are done using data collected by means of a flight test campaign.