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Very Low Frequency (VLF) waves propagate with insignificant attenuation in the Earth-ionosphere waveguide. Attenuation is, however, relatively higher on illuminated paths due to the presence of the ionospheric D region. Sub-ionospheric propagation conditions may be monitored using a narrowband receiver tuned to the stable signals from numerous VLF transmitters distributed across the surface of the Earth. The passage of the day-night terminator across the transmitter-receiver path has a characteristic signature in the amplitude and phase of the signal. Principal Component Analysis (PCA) was applied to distinguish between this regular diurnal pattern and exogenous perturbations.