Very low frequency because VLF waves are reflected from the bottom of the ionosphere, while higher frequency shortwave signals are returned to Earth from higher layers in the ionosphere, the F1 and F2 layers, by a refraction process, and spend most of their journey in the ionosphere, so they are much more affected by ionization gradients and turbulence. Therefore, VLF transmissions are very stable and reliable, and are used for long distance communication. Propagation distances of 5000 to 20000 km have been realized. However, atmospheric noise (sferics) is high in the band, including such phenomena as "whistlers", caused by lightning. VLF waves can
Earth bulge Earth bulge Earth bulge is a term used in telecommunications, meaning the curvature of the Earth's surface, which limits the range of communication methods that require a line-of-sight path, like high frequency radio waves, microwaves, or lasers. Assuming the Earth is a perfect sphere with no terrain irregularity, the line-of-sight distance to the horizon from a transmitter located at a given altitude above the surface can readily be calculated. Let R be the radius of Earth and h be the altitude of the transmitter's antenna. Line of sight distance d of this station is given by the Pythagorean theorem; Since