Fluxgate magnetometers are the most frequently used vector magnetometers in current observatory practice. However, at more and more sites, alternative solutions, e.g. dldD magnetometers are also applied to record the geomagnetic variation. In this paper we demonstrate the advantage of the simultaneous use of a fluxgate and a dldD by introducing a cross-calibration between these instruments. The calibration of a fluxgate is a process to determine 9 parameters including 3 scale factors, 3 offset values, 3 orthogonality parameters. As follows from the dldD principles, the dldD is void from offset and scale factor errors. Therefore, to calibrate the dldD one has to determine one single orthogonality error; the other orthogonality conditions are satisfied automatically since the 3rd dldD axis is defined mathematically as perpendicular to the two physical axes of the instrument. Furthermore, the intercalibration can yield information on relative orientation errors, i.e. the orientation of one instrument can be determined relative to a well-oriented reference instrument.
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