Noise temperature ($T_N$)

A noise temperature expresses received or internally added noise power on an equivalent temperature scale. For one matched receiving channel in the classical thermal limit, an ideal rectangular bandwidth $\Delta \nu$ gives $P=k_{\mathrm{B}}{T}_N\Delta \nu$, with Boltzmann constant $k_{\mathrm{B}}$. An equivalent noise temperature need not equal a component's physical temperature.

The system noise temperature includes all noise contributions referred to a specified receiver input. In a simple additive model with negligible intervening losses, $T_{\mathrm{sys}}=T_A+T_r$, where $T_A$ is the antenna temperature and $T_r$ the receiver noise temperature.

The receiver noise temperature represents noise added by the receiver as an equivalent noise temperature referred to its input. Separating it from the antenna temperature prevents internal receiver noise from being mistaken for radiation from the sky.