![]() VLF waves have very low path attenuation, 2–3 dB per 1,000 km, with little of the "fading" experienced at higher frequencies. The waves travel in a zig-zag path around the Earth, reflected alternately by the Earth and the ionosphere, in transverse magnetic (TM) mode. ![]() The conductive ionosphere and the conductive Earth form a horizontal "duct" a few VLF wavelengths high, which acts as a waveguide confining the waves so they don't escape into space. The Earth is surrounded by a conductive layer of electrons and ions in the upper atmosphere at the bottom of the ionosphere called the D layer at 60–90 km (37–56 miles) altitude, which reflects VLF radio waves. Ground waves are absorbed by the resistance of the Earth and are less important beyond several hundred to a thousand miles, and the main mode of long-distance propagation is an Earth-ionosphere waveguide mechanism. 4.3 Geophysical and atmospheric measurementīecause of their long wavelengths, VLF radio waves can diffract around large obstacles and so are not blocked by mountain ranges, and can propagate as ground waves following the curvature of the Earth and so are not limited by the horizon.4.2 Navigation beacons and time signals.
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