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Signal-to-noise ratio (SNR) is a measure of a receiver's intended signal as opposed to background noise, expressed as a ratio of the average power of each (signal-to-noise, SN). The power (of each) is directly related to the square of its root-mean-square amplitude i.e., the mean of the amplitude squared. Signal-to-noise ratio is also expressed in decibels, as 10 times the log 10 of the power ratio above. As the log of a ratio, zero means "signal equals noise", and greater than zero means the signal is stronger than the noise. The SNR of the observation of a non-varying source can be improved through longer observations or multiple observations.
In astrophysics, the signal-to-noise ratio may be used to characterize the possibility of sensing the output of an astrophysical phenomenon above the background signal and distortion. It is generally applicable but is key in all electronic sensors such as CCDs, and radio telescope antennas and receivers. As a general rule, the signal-to-noise ratio is improved by longer integration time, which in some cases can be accomplished by combining multiple observations. However, a source that is changing within the same time-frame undercuts such improvement.
What constitutes the "signal" needs considering, depending upon the intention of the observation. While discovering radio sources, the signal is the radio signal from such a source and the noise is that produced by other sources, by the sky and Earth sources and the equipment. When investigating transients, such as investigating atmospheres using transit differential photometry or spectrography, the "signal of interest" is the difference between readings during versus before/after the transit, and the noise is from sources of variation due to the sky, Earth atmosphere, and equipment, as well as those from star variability and photon noise.
The abbreviation SNR is also used for supernova remnant.