Properties of “site error” of lightning direction-finder (DF) and its modeling

Abstract

Lightning Location Network (LLN) based on time-of-arrival/direction-finder combined technique (TOA/DF) has been used world-widely for lightning warning and protection purpose. As long as the DF technique is involved in a LLN, it suffers from the “site error”. Firstly, based on experimental data, we have examined the properties of the “site error” as a function of the source azimuth. It is found that for a given DF, the “site error” as a function of source azimuth appears as a complicated sinusoidal waveform with a dominant cycle of either 360° or 180°. A different DF has a different “site error” pattern but this pattern is timely invariant unless the DF's site environment is changed. Secondly, with the magnetic field waveforms recorded by a broadband DF, the properties of the “site error” in frequency domain have been studied. It is found that for a given lightning stroke, the source azimuth determined by the ratio of amplitude of the spectra of magnetic fields from the two orthogonal magnetic loops of a DF varies significantly against the frequency. The source azimuth (or the “site error”) as a function of the frequency usually appears as a complicated curve with some mono-polar and bi-polar impulses superposed on a relative flat line. Thirdly, theoretical explanation and modeling of the “site error” have been attempted based on an electromagnetic dipole model. Simulations with the model show that the mono-polar impulsive fluctuations of the “site error” in frequency domain are due to the reflection of lightning incident signals on “electric-dipole-wise” structures nearby the DF, while the bi-polar ones are due to that of “magnetic-dipole-wise” structures. The model can well interpret the properties of the “site error” as a function of the source azimuth too. Furthermore, possible approaches for making “site error” corrections have been discussed.