Digitization, sampling and GPS timing

After analog processing, the signal is digitized by an analog-to-digital converter. The sampling frequency is chosen so as to comfortably cover the ELF band of interest (on the order of tens of Hz), and anti-aliasing filtering is applied before conversion (see #37). The digital data then enter spectral analysis.

A key element is precise timing. Synchronization using GPS provides a common time base for geographically distant stations. This enables multi-station coherence — the comparison of signals from several observatories — and especially the determination of the time of arrival (TOA) of strong transients. From the differences in arrival times at different stations, the locations of discharge sources (e.g., large lightning strikes and Q-bursts) can be determined.

Precise absolute timing is therefore just as important as amplitude calibration (see #39): whereas calibration ensures correct field values, GPS timing ensures the correct spatial and temporal relationships between stations and the measured events.