The effective temperature measured in the absence of a signal. It is defined as
where is the receiver temperature, which represents internal noise from the receiving amplifier, and is the noise "antenna temperature" which represents the unwanted noise from the antenna produced by ground radiation, atmospheric attenuation, and other sources. A more complete description is
Table 7-1 in Crane and Napier (1989) gives (now outdated) values for each parameter at the VLA for various frequencies. The system noise for an antenna with a system temperature is, using
is the receiver temperature, which represents internal noise from the receiving amplifier, and 
is the noise "antenna temperature" which represents the unwanted noise from the antenna produced by ground radiation, atmospheric attenuation, and other sources. A more complete description is
is, using
