PROPAGATION EFFECT ON FIELD WAVE-FORMS IN RELATION TO TIME-OF-ARRIVALTECHNIQUE IN LIGHTNING LOCATION

The propagation effect on electromagnetic pulses associated with negat ive return strokes is evaluated from data observed by direction finder s for lightning location equipped with GPS clocks. Nine direction find ers constitute a network in the northern part of the Honshu Island of Japan, which locates lightning by a method combining the direction fin ding and the time-of-arrival (TOA) technique. The delay of the peak of an electromagnetic pulse was evaluated to be negligible in propagatio n over seawater for distances less than 300 km, and its average was ab out 1.8 mu s in the east-west propagation over land for the distance o f about 130 km. The timing error of 1.8 mu s may cause maximum locatio n error of about 0.5 km within a TOA lightning-location network, and l arger error is anticipated outside of the network. The evaluated propa gation delay in this area corresponds to the average ground conductivi ty of about 0.003 S/m, except for the southwest part with slightly low er conductivity of about 0.002 S/m. These findings were confirmed by a n analysis based on ground-truth data obtained by still cameras equipp ed with precise clocks. The measured delay in the north-south propagat ion corresponded to the average ground conductivity of about 0.003 S/m . The estimated arrival time of an electromagnetic pulse, based on the arrival time of the peak and the trigger time, gives smaller delay th an the simple arrival time of the peak, but the difference is not sign ificant. It would be better to compensate the peak time by evaluating the propagation effect in obtaining an estimate of the arrival time, t o improve the accuracy of lightning location by using the TOA techniqu e.