The system temperature is a summary of the noise added by the system. This
noise comes from 1) the receiver and the optics, 2) the emission of the
sky, and 3) the emission picked up by the secondary side lobes of the
telescope. It is usual to approximate it (in the
scale) with
| (18) |
The
value is expressed so that all these terms are corrected for
the attenuation by the atmosphere, the coupling of the antenna to the sky,
and the side-band rejection. In other words, the system temperature is
given in units that assume a perfect antenna (coupling equal to 1) located
outside the atmosphere for a single-sideband signal.