To measure the current waveform when lightning strikes the radio tower we have installed a sensor (shunt) at the tower top.
Whenever a lightning flash hits the lightning rod or is upward initiated from one of the lightning rods at the tower top, the lightning current flows directly through a current shunt of 0,25 mΩ and of high frequency bandwidth and then the lightning current is carried to ground by the lightning protection down conductors. The measuring signal (shunt output signal) is converted to an optical signal at the tower top and transmitted to the building at the base of the tower via fiber optical cables.
In order to be able to collect data over a wide range of peak current amplitudes, two channels of distinct sensitivity are used:
The measuring system applies GPS time stamping to all recorded data to be able to time correlate the lightning current data from the tower with the data from the Austrian Lightning Location System ALDIS.
All the equipment is located inside the office building of the ORF, the Austrian TV and the operator of this radio tower.
For data acquisition we have used until 2012 PC based digitizer boards with a sampling rate of 20 MS/s for the lightning current recording. At maximum sampling rate data over duration of up to 0.8 seconds are digitized. Dead time between triggers is about 20 seconds. This time is required to download the data (32 MB per trigger) from the onboard memory to the local hard disc.
The recording system was completely renewed in 2013. It employs a 12-bit digitizer now and allows much higher sampling rates.
The continuous data acquisition over a period of 0.8 seconds allows collecting information about all different processes during a lightning flash, starting from the initialization of the upward leader till to the end of the last stroke of a flash.
The entire measuring system is configured in a way to be remotely controlled and operated by using internet connection.
A typical upward initiated lightning starts with the so called "Initial Continuous Current (ICC)" with amplitudes of a few hundred amperes and a duration of several hundreds of milliseconds. Sometimes, after a period of no current flow in the lightning channel, the ICC is followed by one or more dart leader-return stroke sequences. Those return strokes are assumed to be the same as subsequent strokes in natural cloud-to-ground (downward) lightning.
It is exactly these return strokes which are of great interest for the lightning research community. Measurements of lightning currents at tall towers, such as Gaisberg Tower, or triggered lightning are the only way to do a direct measurement of the current of a lightning discharge and gather information on different lightning parameters (amplitude, charge, ...).
Return Strokes typically show a very rapid increase of the current within 1-2 microseconds corresponding with high field changes. These field changes can induce high voltages in nearby electronic systems and cause damage.
Frequently current pulses (ICC pulses) of very different waveforms are superimposed on the initial continuing current (ICC). Some are more like the so-called M-components and others are more like return strokes. Normally M-components are distinct current pulses which occur during a continuing current phase following a return stroke.
Up to now we have registered a significant number of discharges which had virtually no current pulses (neither superimposed on the ICC nor following the ICC). They showed a more or less uniform current of a few 100 A over a period of several 100 milliseconds. Although the amplitudes in these cases are relatively low, these discharges can transfer significant amounts of charge from the cloud to ground.