Frequency Analysis of Broadband Interferometer for Lightning Observations

In a broadband interferometer technique, the discrete Fourier transform is used to analysis the whole frequency range of a lightning radiation pulse and after resolving the phase difference and incident angle, the direction of the electromagnetic source can be determined by averaging the angular position (azimuth and elevation) of all frequencies. We found that the method of averaging the angular position as proposed by previous authors has a disadvantage, that is a part of reconstructed lightning channel near the ground level often disappears due to higher frequencies are taken into account. The Fourier energy spectrum of broadband signal decreases along with increasing frequency, and frequencies having low energy spectrum contribute very small to the lightning reconstruction. For better reconstruction, a method of using a weighting function is proposed. The broadband frequencies that contribute very well to the reconstructed lightning channel should be amplified, otherwise the rest should be attenuated. By taking the cross power spectrum of broadband signals as the main consideration, we succeeded to map lightning discharge channels, especially at low elevation, in two dimensions with better reconstruction.