Introduction

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 kilometres from the stroke location.

Naturally occurring emissions from lightning strikes can be easily masked by active radio services. Manmade emissions are distinctly different than those from natural sources; such signals tend to be higher in power than natural emissions and can saturate the ATD receiver.

Additionally, due to propagation effects at these low frequencies, active radio services geographically separated by very large distances can still adversely affect throughput, performance and measurement capability of the ATD network system. This increases the necessity to monitor spectral emissions in a spectrum that is void of other radio transmissions as far as possible.

The optimal frequency for ATD measurements is 9.766 kHz and has been in use since 1939. Recent monitoring since 2004 has shown that it was not practical to use this frequency at all the sensor sites, because of transmissions near 10 kHz in some locations which prevented the operation of the passive sensors. This adversely impacted the performance and accuracy of the system. Due to this, the system now operates at 13.733 kHz. Note that, before this time, the lightning detection systems have coexisted with the existing services below 20 kHz. Technical characteristics of the ATD system are specified within Recommendation ITU-R RS.1881.

This contribution is intended to initiate necessary work on the identification of services and stations within the band below 20 kHz. It also seeks to illustrate the complex RF environment within which the arrival time difference lightning detection system currently operates, and to identify areas of spectrum below 20 kHz where the ATD system can operate without placing any undue constraints on existing services or allocations.