BliksemdetectieLast Updated on Thursday, 16 April 2009 10:11 Written by Administrator Friday, 25 April 2008 12:54
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KNMI's SAFIR/FLITS network
Het volgende stuk komt uit mijn afstudeer-rapport waarin de het SAFIR FLITS van het KNMI nader wordt beschreven (Engels)
LDS=Lightning Detection System
The SAFIR LDS developed by Dimensions SA in France is a TOTAL DF detection system that is working on the principle of interferometry in combination with a TOA sensor. Interferometry is the technique that makes use of the phase-differences from a signal between the antennas (five dipole antennas in this case, see figure) which are dependent on the direction of the signal. The azimuth of the source can therefore be determined and when combining more of these sensors in a network, triangulation can provide the localization of the lightning occurrence (see figure).
The LDS works in two different frequency ranges; the five dipole antennas for interferometry-localization are using the VHF (110 - 118 MHz) while the LF TOA sensor is measuring in the LF (300 Hz - 3 MHz) (see figure x) (beekhuis, holleman ILDC 2000). As described in the introduction the sensors in the high frequencies are able to describe more details about the propagation of lightning while the wavelength is small, the opposite holds for the low frequencies. Therefore the VHF-sensors are able to describe the path of the lightning with an accuracy of 3 meters in the horizontal plane. In the primary SAFIR system the localization of both CG and CC was done by interferometry-localization. The LF-sensor was decisive in the discrimination of CC and CG based on pre-defined criteria regarding amplitude, rise- and decay-time of the disruption. After a software-upgrade in December 2003 also TOA was made available with the LF-sensor and the system referred to as SAFIR/FLITS from then. In this report the SAFIR network refers to the old detection network before the upgrade whereas the SAFIR/FLITS or FLITS network is referring to the new detection network after the upgrade that is currently used.
In the new setup the TOA is not only responsible for discrimination between CC and CG, it also uses TOA to override the localization that was done by interferometry for CG-strikes (beekhuis, comment) while the low frequent pulse is associated and characteristic for CG-discharges. The SAFIR/FLITS system has a relatively small baseline caused by the VHF sensors installed. The SAFIR sensors are claimed to have a detection efficiency of around 90% and a localization accuracy of around 500m (Vaisala brochure, 2008).
The FLITS network (Flash Localisation by Interferometry and Time of arrival System) is the successor of the SAFIR network that was upgraded in December 2003. The network operated consists of 7 stations of which 3 station are operated in collaboration with the Belgium KMI that operates its own network. The collaboration with the Belgiums was necessary after the initial phase of the older SAFIR network; coverage of the Netherlands was not total or not sufficient for several regions which resulted in the need for additional stations. As can be seen in the figure there is a localization gap between the two stations on the left situated in the Belgium area. This is a result of the disability to detect lightning occurrences in a straight line between stations, while both stations are pointing at each other, which creates inaccuracy when localizing the event. Additional stations covering this limited area can correct for this problem.