An algorithm is presented which allows the calibration of three-dimensional eye movements in two magnetic fields with two search-coils in one eye, whose relative orientation does not have to be known. Prior to recordings a calibration cube is placed in front of the eye to measure real and apparent crosstalk, produced by imprecisely oriented horizontal or vertical magnetic fields, and to obtain a first approximation of offset voltages. For calibration it suffices if one point is fixated and, in addition, spontaneous eye movements for 30-60 sec are performed. Additional fixation points can be used to improve the calibration. Position quaternions are applied for computing eye movement recordings from man and monkey and to determine Listing's plane.
Bartl, K., Siebold, C., Glasauer, S., Helmchen, C., & Büttner, U. (1996). A simplified calibration method for three-dimensional eye movement recordings using search-coils. Vision Research, 36(7), 997–1006. https://doi.org/10.1016/0042-6989(95)00201-4