Receiver antenna detects not only radio waves but alo various noises.
The examples of the noise are:
- Galactic noise
- Solar noise
- Noise radiated by clouds and rain
- Atmospheric noise
Noise temperature received by the antenna \(T_a\) is:
\[T_a = T_s + (1-\eta)T_0\]
where \(T_s\) is the noise temperature, \(T_0\) is the ambient temperature, and \(\eta\) is the radiation efficiency of the antenna.