Positioning/Navigation
VLF RECEIVERS
VLF receivers are simple, yet uncommon. Consisting only of an antenna and an audio amplifier, they are sensitive to radio waves with frequencies between a few hundred Hertz and 10 kHz . For comparison, AM broadcast band radios --like the ones in most automobiles-- span the much higher frequency range 540 kHz to 1.6 MHz. Even if there is no lighting in your area, you can still hear VLF crackles from storms thousands of kilometers away. Some sferics travel all the way around the Earth! Radio waves can propagate such great distances by bouncing back and forth between our planet's surface and the ionosphere -- a layer of the atmosphere ionized by solar ultraviolet radiation. The ionosphere, which begins about 90 km above the ground and extends to thousands of kilometers in altitude, makes a good over-the-horizon reflector of low frequency radio waves. [Summary provided by NASA]