Reflection of VLF radio waves from the Canadian rockies

Wideband (2-53 kHz) VLF recordings have been made on an aircraft flight from Honolulu, Hawaii, to St John's, Newfoundland, via Comox on Vancouver Island. Clear periodic variations have been observed in both the amplitude and phase of the VLF signals from both navigation and communications VLF transmitters, and these oscillations reached maximum amplitude in the region above the Coast and Rocky Mountains. We suggest these oscillations were produced by the reflection of the VLF signals from the mountains. The oscillations were seen on signals typical of single-mode propagation and on those undergoing significant modal interference. We measured reflection coefficients of up to 18% at the Omega navigational frequencies and 43% at the frequency of the NPM transmitter (23.4 kHz). The effect of the reflections at the Omega frequencies was seen more than 3000 km before the mountains, but the effect of reflections at the higher VLF frequencies, although greater in magnitude, was much more localized to the immediate region of the mountains.