A narrow-band receiver is operated at Halley, measuring the phase and amplitude of
selected VLF transmissions. The purpose is to detect lightning-induced electron
precipitation (LEP) into the ionosphere by means of the transient perturbations
caused to sub-ionospheric VLF propagation (the Trimpi effect)
(Smith, 1996). The original
OPAL (Omega Phase and Amplitude Logger) design of receiver described by
Smith and Cotton (1990) was
superseded by the OMSK (Omega and MSK) which could also receiver MSK transmissions,
with a resulting improvement in time resolution and signal-noise ratio
(Friedel et al., 1993;
Smith et al., 1993).
A further enhancement (OmniPAL), designed by the University of Otago, New Zealand,
is due for deployment in 1997.