Simultaneous measurements of parameters of CN Tower lightning strikes have been performed since 1991. The current derivative signals measured, are corrupted by a 100 kHz oscillating interference. This noise has caused substantial limitations on the usage of the CN Tower lightning current data. As a result, we became motivated to characterize it and search for its source. Furthermore identifying the low-frequency noise is expected to help in its removal and avoid it altogether in future installations.This thesis proves that the low-frequency noise corrupting the lightning current derivative signals is the Loran-C radionavigation signal. This finding is a major contribution not only for the CN Tower lightning project but also for any other research related to measurement of lightning at tall structures. Researchers and experimentalists should be aware of the existence of the Loran-C signal and take the necessary precautions to avoid its interference effect.