A New Hybrid Algorithm to Image Lightning Channels Combining the Time Difference of Arrival Technique and Electromagnetic Time Reversal Technique

Abstract

Very-high-frequency (VHF) electromagnetic signals have been well used to image lightning channels with high temporal and spatial resolution due to their capability to penetrate clouds. A lightning broadband VHF interferometer with three VHF antennas configured in a scalene-triangle shape has been installed in Lhasa since 2019, to detect the lightning VHF signals. Using the signals from the VHF interferometer, a new hybrid algorithm, called the TDOA-EMTR technique, combining the time difference of arrival (TDOA) and the electromagnetic time reversal (EMTR) technique is introduced to image the two-dimensional lightning channels. The TDOA technique is firstly applied to calculate the initial solutions for the whole lightning flash. According to the results by the TDOA method, the domain used for the EMTR technique is predetermined, and then the EMTR technique is operated to obtain the final positioning result. Unlike the original EMTR technique, the low-power frequency points for each time window are removed based on the FFT spectrum. Metrics used to filter noise events are adjusted. Detailed imaging results of a negative cloud-to-ground (CG) lightning flash and an intra-cloud (IC) lightning flash by the TDOA method and the TDOA-EMTR are presented. Compared with the original EMTR method, the positioning efficiency can be improved by more than a factor of 3 to 4, depending on the scope of the pre-determined domain. Results show that the new algorithm can obtain much weaker radiation sources and simultaneously occurring sources, compared with the TDOA method.

A Time Delay Calibration Technique for Improving Broadband Lightning Interferometer Locating

“…Wang et al explored weak signal and multi-radiation source location through the use of time-reversal methods and the multiple signal classification algorithm [29,30], while Li et al used the orthogonal propagator method [31]. The combination of time delay estimation direction finding and time-reversal direction finding techniques can alleviate the computational complexity issue that arises when using the time-reversal method alone [32].…”

3D Lightning Location Method Based on Range Difference Space Projection

“…Based on the above work, Fan et al introduced a signal processing method (empirical mode decomposition (EMD) technology) to filter the original waveform to improve the accuracy of the peak time extraction, which further improves the location accuracy [18]. Furthermore, the time reversal (TR) technology can also be used in the 3D location of lightning discharges [20][21][22]. For example, Chen et al used the TR method to find the optimal solution in the space limited by the linear initial solution of the TOA method [21].…”

“…For example, Chen et al used the TR method to find the optimal solution in the space limited by the linear initial solution of the TOA method [21]. Li et al combined the TDOA technique and electromagnetic TR technique to image lightning channels [22]. In addition, the deep-learning technology has been introduced in the location of total lightning [23].…”