System temperature ($T_{\mathrm{sys}}$)

The equivalent input noise temperature of the antenna-plus-receiver system is defined by available noise power $P_n=k_B{T}_{\mathrm{sys}}\Delta \nu$ in one matched polarization of waves mode. It includes receiver noise and the appropriate sky and instrumental contributions. It is not generally the physical temperature of a single component.

The system-equivalent flux density is the unpolarized spectral flux density that would produce an antenna temperature equal to $T_{\mathrm{sys}}$. With effective collecting area $A_e$, $\mathrm{SEFD}=2k_B{T}_{\mathrm{sys}}/A_e$. The factor two follows from one polarization of waves measurement of an unpolarized source. State effective area and polarization of waves conventions when comparing sensitivities.^[ref]