Broadband radio interferometry for lightning observations

A broad band radio interferometer for locating lightning emissions has been designed, constructed and tested. For a broad band interferometer, a single fixed pair of antennas is equivalent to having many baselines in a narrow band interferometer. So, a broad band system requires fewer antennas than a narrow band system to achieve equivalent angular resolution. In addition, frequency dependent locations of the radio emissions can be extracted for a more detailed look at the lightning breakdown processes. Such a system has been tested by a computer simulation and by measuring a man-made broad band radiation source. The system consists of two antennas which are separated vertically. Bandwidth of the system is from 40 to 350 MHz. Measurements of the man-made source indicate that with a signal-to-noise ratio (SNR) above 10 dB, the system is able to locate the source with an accuracy of about 2 degrees over the detectable frequencies. Preliminary observations of lightning discharges with this new technique appear to indicate that breakdown processes of a dart leader to: ground radiate Solely at its descending tip.