Determining the direction of a geometrical/optical reference axis in the coordinate system of a triaxial magnetometer sensor

The reference coordinate axes of a magnetic vector field sensor are related to the instrument digital output vector Ū by the calibration matrix and the offset vector Ō. In addition, this reference coordinate system must be related to (at least) two externally accessible optical or geometrical axes in order to be able to determine the precise orientation of the magnetic coordinate axes in an external reference system. Two methods for determining a reference axis in the sensor coordinates are discussed: (1) using a triaxial coil facility to measure the sensor orientation for two different positions, rotated about a fixed reference axis; (2) in the Earth's field the magnetometer sensor is rotated about a fixed axis into a number of (at least three) positions.