The CGR4 lightning sensor

The design of the CGR4 lightning sensor is based on several observed characteristics of the electric field changes caused by nearby lightning, and the radiofrequency noise bursts from lightning. The electric field signals from lightning are detected by a simple aerial, and processed by a combination of analogue and digital circuitry under microprocessor control to determine if there is a nearby lightning flash, and, if so, to assign the type of flash as negative ground flash (NGF), positive ground flash (PGF), or cloud flash (CF). These types have distinctive features in their electric field change waveforms, enabling an assignment of type of lightning to be made by obtaining several measures of the waveform characteristics and applying an appropriate set of rules. A method of estimating the sensor's effective range and area has been developed using data for a set of nearby lightning events. This involves the recording of the variation of electric field with time, the response of a lightning sensor, and the distance to the lightning flash using thunder ranging. As the sensor is able to apply different threshold overall field changes for each type of lightning detected, a single effective range of 11.3 km (effective area 400 km) for all types of lightning was achieved by using threshold field changes of 520 volts per metre (V/m) for CF, 760 V/m for NGF, and 1200 V/m for PGF. The sensor responds to about 90 per cent of all flashes at 8 km, about 40 per cent at 11 km, and approaching zero above 16 km.