## 2 Instrument Design
### Block Diagram
#### 2.1.3 Calibration Assembly  
Internal calibration noise sources are used to determine the absolute values of noise temperatures, \(T_{\text{RM,in}}\) and \(T_{\text{RM,in}}\), applying at the radiometer input ports for horizontal and vertical polarization, respectively. As depicted in Figure 2, the input noise switch allows switching between \(T_{\text{RM,in}}\) and \(T_{\text{RM,in}}\) at the input ports and the internal calibration sources mounted on the Calibration Assembly (CA) depicted in Figure 3. The design is such that the losses between the radiometer input ports and the corresponding inputs of the switch are identical (\(\approx\)0.05 dB). The same applies to the losses between the outputs of the three calibration sources and the inputs of the switch. The CA consists of a heavy copper block (1.7 kg) on which the calibration sources and the two amplifiers (AMP1/2) used in the MA are mounted. A Thermo Electric Cooler (TEC) and a temperature sensor (T-sensor) are used for the thermal stabilization of the CA. This is crucial to maintain constant gains and noise added due to losses in the MA front-end. For the typical set point \(T_{0}\) = 313 K (40 \({}^{\circ}\)C), the temperature is measured to be within \(\pm\)0.1 K. Furthermore, the CA is designed as a separate module to allow for independent operation for cross calibration among other L-band radiometers. \begin{table}
\begin{tabular}{l|c|c|c|c|c} \hline
**MA input noise [K]** & \(P_{\text{front}}\) [dBm]** & \(P_{\text{PDA}}\) [dBm]** & \(P_{\text{PDA}}\) [\(\upmu\)W]** & \(U_{\text{PDA}}\) [V]** \\ \hline \(T_{\text{sky, in}}\) & \(=10\) & -103.8 & -34.8 & 0.328 & 0.324 \\ \(T_{\text{ACS}}\) & \(=48\) & -102.8 & -33.8 & 0.420 & 0.416 \\ \(T_{\text{scene, min}}\) & \(=100\) & -101.6 & -32.6 & 0.545 & 0.540 \\ \(T_{\text{scene, max}}\) & \(=300\) & -98.9 & -29.9 & 1.029 & 1.019 \\ \(T_{\text{RS}}\) & \(=313\) & -98.8 & -29.7 & 1.060 & 1.050 \\ \(T_{\text{HS}}\) & \(=630\) & -96.4 & -27.4 & 1.827 & 1.810 \\ \hline \end{tabular}
\end{table} 
Table 1: Power levels \(P_{\text{front}}\) at the output of the MA front-end and \(P_{\text{PDA}}\) fed to the PDA for typical input noise temperatures at the MA. PDA output voltages \(U_{\text{PDA}}\), considering the measured PDA sensitivity (7), are shown in the last column. The resistive source (RS) consists of a standard 50 \(\Omega\) SMA resistor tightly pressed into a borehole in the copper block to keep it constantly at the temperature \(T_{0}\). Accordingly, this results in the noise temperature \(T_{\mathrm{RS}}=T_{0}\) of the RS equal to the CA temperature. The hot source (HS) is made up of a commercial noise diode (ND) with the output attenuated by 6 dB. Given the noise temperature \(T_{\mathrm{ND}}\approx 1575\) K of the factory-calibrated ND and the physical temperature \(T_{0}=313\) K of the -6 dB attenuator (with power transmission factor \(t_{\mathrm{\text{-6 dB}}}\approx 0.251\)), the noise temperature \(T_{\mathrm{HS}}\) of the HS is estimated as the transmitted part of \(T_{\mathrm{ND}}\) plus the thermal noise of the attenuator. In case of perfect match between the components (no reflections) \(T_{\mathrm{HS}}\) is:  
\[T_{\mathrm{HS}}=T_{\mathrm{ND}}\cdot t_{\mathrm{\text{-6 dB}}}+T_{0}\cdot\left( 1-t_{\mathrm{\text{-6 dB}}}\right)\approx 630\;\mathrm{K} \tag{4}\]  
The active cold source (ACS) is implemented with a low-noise amplifier (AMP5) with an isolator (ISO2) attached to the input and terminated with 50 \(\Omega\). The idea of this design is to use the low noise level of the amplifier (\(T_{\mathrm{AMP5}}\approx 34\) K at \(T_{0}=313\) K) as a cold source. The isolator provides a good 50 \(\Omega\) match between the ACS and the MA. In accordance with (4), the noise temperature \(T_{\mathrm{ACS}}\) of the ACS is:  
\[T_{\mathrm{ACS}}=T_{\mathrm{AMP5}}\cdot t_{\mathrm{ISO2}}+T_{0}\cdot\left(1-t _{\mathrm{ISO2}}\right)\approx 48\;\mathrm{K} \tag{5}\]  
The estimated \(T_{\mathrm{ACS}}\) for \(T_{0}=313\) K is based on the component specifications and assumes perfect match between the components.