All discussed noise temperatures , from an antenna or the equivalent noise source, come from the Dicke system of microwave radiometers. In a Dicke radiometer, an input switch alternatively connects to receiver input the antenna output, and, the output from the equivalent noise source (exactly known). If both noise temperatures are equal, the instrument is "balanced", or, its output is close to zero. By this, the equality of both input noise temperatures is confirmed.
In practice, antenna noise output varies; the radiometer is therefore calibrated by replacing the antenna by a variable (or several) noise source(s) with different noise-temperature outputs chosen over a desired range.
Typically, microwave radiometers can linearly respond to input noise temperature over 0 to 500 Kelvins, or, a selected narrower range.