Atmospheric Noise
The signal-to-noise-ratio determines the sensibility of telecommunicationsystems (e.g.,radio receivers). An analog signal must clearly exceed the noise amplitude in order to become visible. Atmospheric noise is one of the most important sources for the limitation of the detection of radio signals.
The steady electric discharging currents in a lightning channel cause a series of incoherent impulses in the whole frequency range, the amplitudes of which decreases approximately with the inverse frequency. In the ELF-range, technical noise from 60 Hz, natural noise from the magnetosphere, etc. dominates. In the VLF-range, there are the coherent impulses from R- and K-strokes, appearing out of the background noise. Beyond about 100 kHz, the noise amplitude becomes more and more incoherent. In addition, technical noise from electric motors, ignition systems of motor cars, etc., are superimposed. Finally, beyond the high frequency band (3–30 MHz) extraterrestrial noise (noise of galactic origin, solar noise) dominates.
The atmospheric noise depends on frequency, location and time of day and year. Worldwide measurements of that noise are documented in CCIR-reports (CCIR = "Comité Consultatif International des Radiocommunications").
Read more about this topic: Radio Atmospheric
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