Magnetic Sensor Compensation Using Factor Graph Estimation

Recently, there has been significant interest in the ability to navigate without GPS using the magnetic anomaly field of the Earth (magnav). One of the key technical bottlenecks to achieving magnav is obtaining an accurate magnetic sensor calibration, taking into account own-ship and sensor effects. The Tolles–Lawson magnetic calibration method continues to be the industry standard and was developed when airborne magnetic survey aircraft were first used over 70 years ago. In this article, we present a magnetic calibration algorithm based on a factor graph optimization using inertial measurements and inputs from both a vector and scalar magnetometer. The factor graph is well-suited for combining multiple sensor inputs, allowing accurate calibrations in the presence of large permanent moments and a time-varying external magnetic field, two problems that are difficult to solve with previous approaches. The ability to accurately calibrate a magnetic sensor in flight (i.e., with the presence of a large platform field and a varying Earth field due to movement) will allow greater flexibility in sensor mounting locations for magnetic anomaly navigation.