Why is LINET so exact?
The mean deviation with respect to the location of lightning strokes in the LINET measurement network amounts to about 75 meters. The calculation of the geographical position of a lightning stroke entails an unavoidable degree of uncertainty due to physical reasons. Irrespective of the fact that every lightning stroke also features a certain expansion, with respect to cloud-to-ground strokes, the point of the return stroke is measured, which is situated approximately 100 meters above the ground. Thanks to the expert alignment of several parameters, nowcast has been able to hone the location accuracy within the core region (almost all of Europe) down to a mean error of 75 meters. This value is regularly verified and confirmed via comparison with confirmed strikes into towers whose exact geographical position is known.
A high degree of location accuracy is indispensible
Reliable forecasts on current and past lightning phenomena are only possible if a high degree of location accuracy is ensured.
In order to be able to link the malfunction of a transmission line with a lightning stroke for example, the measured position of the damage-inducing lightning stroke should not be far from the line itself. Only in this manner is a reliable correlation possible.
The location accuracy allows for the clear distinction of storm cells. Not only is a mean deviation of 75 meters an extremely good value – the scatter of worse location errors is also exceedingly low.
In cases of subsequent verification of damage, the importance of location accuracy is apparent. Only precise measurement allows for a reliable prediction as to whether damage was actually caused by a lightning stroke or by lightning-induced overvoltage.
Location accuracy in detail
The first lightning-detection systems used the method of angle-bearing. Due to various interferences, this method is rather inaccurate and creates location errors on the scale of several kilometers. With the availability of GPS time signals, the time-of-arrival method become customary – a technique which is considerably more exact and normally allows for a precision level of around 400 m.
nowcast GmbH has optimized this well-known technique, and now achieves standard accuracy levels of less than 100 m; in network areas adjusted for site errors, the accuracy level attained is even 75 m. This is flawlessly demonstrated by lightning strikes into towers at known locations (Betz et al., “LINET – An International VLF/LF Lightning Detection Network in Europe”, in: “Lightning: Principles, Instruments and Applications”, Eds. H.-D. Betz, U. Schumann, and P. Laroche, ch. 5, Dordrecht (NL), Springer, 2008).
Network manufacturers always specify the average statistical accuracy, but not the scatter around the mean. As such, with conventional commercial measurement networks, the deviation with respect to location errors may continue on the scale of a few kilometers – despite an average accuracy of a few hundred meters for the majority of the lightning strokes.
For many purposes of use, however, it is of great import that no major errors occur - even if this happens only in relatively few cases. It can, for example, be extremely misleading if individual, wrongly located lightning strokes are projected in areas which are actually lightning-free, and hence wrongly depict the formation of a storm. In order to avoid such location errors, nowcast has developed an efficient technique which allows for an extremely low scatter of location errors around the mean. The long-standing and continuous use of the nowcast lightning data by the German Meteorological Service (DWD – Deutscher Wetterdienst) has confirmed the effectiveness of our method.