The combined noise in the receiving system of a radio telescope. This has several sources: the receiver itself and all other telescope components; the contributions from the sky, both natural and human-generated; and the heat radiated by the telescope itself and its surroundings (so-called ground radiation). System temperature is usually measured in Kelvin degrees and is used to calculate the so-called system equivalent flux density in janskys (Jy) for use in calibration. See also background noise.