Accurate Magnetic Field Compensation in Large Platform Fields

Abstract

Recently, the potential for using a magnetic sensor to perform navigation without GNSS signals has received significant research attention. To perform magnetic anomaly navigation, it is imperative that a proper calibration be performed so that the local magnetic effects (e.g. magnetic fields due to the aircraft) can be removed and measurements of the Earth’s anomaly field can be obtained from the magnetic sensor. More specifically, a scalar magnetometer provides very high quality measurements of the total magnetic field and is the primary input for navigation using the earth’s anomaly field, after using a calibration routine to compensate for local magnetic fields. To perform this compensation, the Tolles-Lawson model has been the standard for several decades. This method, however, was designed in a scenario where (a) the permanent magnetic field due to surrounding objects was very small and (b) the only attitude information available was from a vector magnetometer co-located with the scalar magnetometer. (Note that the vector magnetometer is typically lower quality than the scalar sensor, but is sufficient for generating some attitude information.) To navigate using magnetic fields, however, the scalar magnetometer is often placed in locations where the small permanent field assumption is broken. Furthermore, inertial sensors are available that provide significant information about the attitude of the magnetic sensor. In this paper, we will describe a different approach used to compensate a scalar magnetometer so magnetic navigation can be performed. Using both gyroscopes (for relative attitude information) and inputs from the vector magnetometer, the permanent moments of the surrounding environment can be characterized. Once characterized, this permanent field can be directly removed from the sensed magnetometer readings. We will show (a) how to approximate the permanent moments, (b) how to compensate for the permanent model using inertial and vector magnetometer information, and (c) compare this method to other proposed compensation methods.