Calibration of antennas and the instrument chain

In order to obtain absolute field values from the measured voltages (pT for the magnetic component, nV/m for the electric one), the antennas and the entire instrument chain must be calibrated Nickolaenko Hayakawa 2014. Without calibration, only relative changes can be tracked, not the true physical amplitudes.

Calibration accounts for the frequency-dependent sensitivity. For an induction coil the induced voltage is proportional to \mathrm{d}B/\mathrm{d}t (see #35), so the transfer characteristic increases with frequency; this must be measured precisely as part of the calibration and compensated in the data. Likewise, the transfer of the amplifiers and filters of the analog front-end must be known (see #37).

A calibrated instrument chain is a prerequisite for comparing data between different stations and for absolute inverse problems, for example for reconstructing global lightning activity from the measured amplitudes. Precise knowledge of the transfer function of each channel is therefore just as important as the sensitivity of the antennas themselves.