Radiometer

A radiometer measures electromagnetic radiation through its received power. A radio radiometer can express that power as a noise temperature, allowing an unknown source to be compared with calibrated reference temperatures.

A noise temperature expresses received or internally added noise power on an equivalent temperature scale. For one matched receiving channel in the classical thermal limit, an ideal rectangular bandwidth $\Delta \nu$ gives $P=k_{\mathrm{B}}{T}_N\Delta \nu$, with Boltzmann constant $k_{\mathrm{B}}$. An equivalent noise temperature need not equal a component's physical temperature.

The system noise temperature includes all noise contributions referred to a specified receiver input. In a simple additive model with negligible intervening losses, $T_{\mathrm{sys}}=T_A+T_r$, where $T_A$ is the antenna temperature and $T_r$ the receiver noise temperature.

The receiver noise temperature represents noise added by the receiver as an equivalent noise temperature referred to its input. Separating it from the antenna temperature prevents internal receiver noise from being mistaken for radiation from the sky.

For a stable radiometer with affine output $S=g(T+T_r)$, let $g>0$ be the fixed response coefficient and $T_r$ the receiver noise temperature. Two reference loads with known equivalent input noise temperatures $T_c<T_h$ give outputs $S_c$ and $S_h$. Subtracting their outputs eliminates $T_r$; dividing source and reference differences eliminates $g$:This two-load radiometer calibration assumes unchanged response and receiver noise temperature between readings, with the loads and source referred to the same input. It also eliminates an additional constant output offset. Physical load temperatures must first be converted to the applicable equivalent input noise temperatures when the classical thermal approximation fails. The Penzias-Wilson microwave background experiment gives an original worked example.