The radiation measured in radio astronomy has, in almost all cases, a Gaussian probability distribution in amplitude. Except in the case of narrow-band spectral line emissions, it has the same statistical characteristics as thermal noise radiation from the Earth, its atmosphere, or noise generated in a receiver itself. Moreover, the cosmic radio emissions are very weak. In radio astronomy observations, the S/N in the RF and IF parts of the receiver is typically in the range -20 dB to –60 dB, i.e. the power contributed by the source under study is a factor of 10–2 to 10–6 lower than the unwanted noise power from the atmosphere, the ground, and the receiver circuits. In most communication systems the corresponding S/N is of the order of unity or greater. Because radio astronomy signals are so weak in comparison to those of other services, radio astronomy observations are highly vulnerable to radio interference and, except in the case of pulsars, cosmic signals generally have no characteristic modulation that would help to distinguish them from noise or from many forms of interfering signals.