From AMS Glossary
A measure of the noise power of a device or circuit.
The noise temperature is the temperature of a resistor that has noise power equal to that of the device or circuit. Specifically, the noise temperature is defined by T = N/kB, where N is the noise power within bandwidth B, and k = 1.38 × 10-23J K-1 is Boltzmann's constant. A radar system is characterized by several noise temperatures: the antenna temperature Ta, the receiver temperature Tr, and the transmission line temperature Tl. The transmission line temperature is a measure of the noise power within the receiver bandwidth generated by the resistive losses in the transmission line or waveguide between the antenna and the receiver. The transmission line temperature is frequently combined with either the antenna or the receiver temperature, depending on the reference point for the measurement. The total system temperature is T = Ta + Tl + Tr.