The operational ‘cathode ray direction finding’ (CRDF) network for the location of lightning flashes at ranges up to thousands of kilometres can be replaced by a system using the ‘arrival time difference’ (ATD) technique. The ATD technique has theoretical advantages over CRDF, which could lead to substantially improved flash location, but the extent to which it suffers from certain propagation effects in practice must be quantified.
In an experiment using the ATD technique, equipments were deployed in the U.K. and Gibraltar. These captured the vertical electric field of VLF electromagnetic radiation from individual lightning flashes, synchronized to CRDF-selected flashes and recorded against an accurate timebase. By analysing the data using techniques analogous to those of hyperbolic navigation, the flash location was inferred, together with a measure of the data consistency.
ATD location of independent flashes observed in time proximity frequently gave the tight grouping expected from an isolated active thunderstorm cell. Groups of diameter 2–10 km were observed at megametre ranges.
ATD flash locations were compared with CRDF and other meteorological data. Positioning agreed within the limited accuracy of current techniques, but the CRDF system's potential for poor accuracy was highlighted.
An analysis method was developed relating the internal consistency of the ATD flash data to geographical location accuracy, and ATD accuracy of less than 5 μs was measured experimentally. On this basis, an ATD system can locate flashes over a synoptic-scale area with errors comparable with a storm cell diameter.
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