An unwanted, randomly fluctuating signal that may mask the signal from a radio source. Background noise has many origins. Noise from the random motions of electrons in the receiving apparatus (thermal noise) is often minimized by cooling. The main contributions at long wavelengths (from tenths to tens of metres) are radiation from our Galaxy (synchrotron radiation), human-generated interference, and the ionosphere. At millimetre and submillimetre wavelengths the troposphere and instrumental noise are the main contributors. For ultra-sensitive observations the cosmic microwave radiation from the Big Bang can be significant. Noise extending uniformly over a broad band of frequencies is termed white noise. See also system temperature.