Continuous calibration of ground-based magnetic-observatory fluxgate magnetometers

Continuous data collection at magnetic observatories, such as those operated by the USGS and other Intermagnet programs, is accomplished using fluxgate magnetometers. Since these sensors are subject to baseline drift, periodic absolute measurements are made at each observatory. This is accomplished using a proton magnetometer, which can deliver accurate intensity data, and a theodolite coupled to a single-axis fluxgate magnetometer (DIM), which can deliver accurate magnetic directional data, although the theodolite measurements must currently be made by hand. Through data processing, the absolute data can be used to adjust for the response drift of the fluxgate sensor and thereby place the variational data onto an absolute scale. Drawing lessons from satellite magnetometer calibration methods, we show how to use absolute data to not only correct for fluxgate sensor response drift, but also how to correct for both small rotations in the orientation of the fluxgate and for nonorthogonality between the sensor elements of the fluxgate. This calibration of fluxgate data for response, rotation, and nonorthogonality, can be accomplished continuously in time by inverting a 3x3 matrix and using a set of basis functions consisting of B-splines. The near-real-time calibrated data are of potential interest to the directional-drilling and satellite-magnetometer communities.