The Arrival time difference (ATD) system utilizes a network of “detector” outstations to monitor spectral emissions of cloud to ground lightning strikes centred between about 5 and 20 kHz. At these frequencies the sky waves reflected off the ionosphere propagate for very large distances with relatively little attenuation, and at shorter ranges may be preceded by a ground wave. Thus, it is possible to receive the emissions from the cloud to ground strokes at thousands of kilometers from the stroke location.