Other hissband is generated farther out in Earth's magnetosphere including the bow-shock region of the magnetosphere facing the incoming Solar Wind. These are the sounds of the worldwide Omega Radionavigation System. Omega receivers used to sample the relative phase and timing of each Omega signal. Omega was subject to the same VLF propagation disturbances which affect Natural Radio, particularly during magnetic storms. GPS does not suffer any of these propagation errors as does Omega.
Alpha has only three transmitters sending three short pulses of approx.
100 milli-second duration followed by a pauseof a couple seconds' duration. Alpha is much weaker in strength than Omega, particularly for listeners previously within a couple thousand kilometres of an Omega transmitter. With AC, the polarity changes a many times a second. In North America and in most Central and South American countries, it is at 60 Hz. It has been found that within southwest London's Richmond Park and Battersea Park, quite a few remarkably low-hum listening spots exist.