The power produced by a resistor at a temperature T as a result of random thermal motions of electrons is given by
where k is Boltzmann's constant and is a given frequency interval. The equivalent noise power of a receiving system is then defined as
where is the system temperature
is the receiver temperature (which represents internal noise from the receiving amplifier), and is the "antenna temperature" which represents the unwanted noise from the antenna produced by ground radiation, atmospheric attenuation, and other sources. (Note that there is actually a second nonequivalent definition of antenna temperature.)
