Ground truth confirmation and theoretical limits of an experimental VLF arrival time difference lightning flash locating system

During trials of a VLF Arrival Time Difference (ATD) technique for long range lightning flash location, an apparently well organized and isolated flash sequence was located. Internal ATD evidence suggests flashes were located at a thousand‐kilometre range to within 5 km. ‘Ground truth’ based on synoptic/mesoscale considerations, observer reports, and lightning damage claims confirms locations to 10–15 km. In addition, elimination of systematic effects from internal ATD evidence demonstrates relative flash location errors of 0.9‐1.8 km, and a ‘tight cluster’ of ATD fixes features scatter in location which is not inconsistent with these values. The nature of VLF lightning flash observation is examined, and shown to be consistent with errors even smaller than this.