The initial and yet fundamental process in a typical cloud-to-ground lightning strike includes the
propagation of a very faint and charged channel which is called stepped leader. The exact
mechanism for the step leaders is not understood. The reason for this is that the temporal and/or
spatial resolution of the devices exploited for observing this phenomenon has not been sufficient.
The radio interferometric array of LOFAR however is capable to measure radio signals with 1 ns
temporal resolution. Thus LOFAR can measure the radio pulses emitted by stepped leaders at
multiple times during the formation of the steps and locate the positions of the pulses with sub-
meter accuracy. This provides with new possibilities to test and probe the theories explaining the
propagation of a stepped leader. We are currently processing the data measured by LOFAR, and
in addition, are preparing a simulation tool to calculate the radio signals expected on the basis of
current models. By comparing the simulation and experimental data, we aim to determine the
characteristics of stepped leader formation.