# L2-CalSat: A Calibration Satellite for Ultra-Sensitive CMB Polarization Space Missions
## 4 L2-CalSat Description
### Thermal Control and Power Budget  
The thermal design of L2-CalSat will maintain the payload within a range of temperatures in order to meet survival and operational requirements during all mission phases, which are to be established to ensure the stability of the payload. In order to validate the design and characterize the performance of the payload under thermal loads during its operation, functional tests and thermal cycle tests (rapid changes between hot and cold temperatures to stress the hardware) will be carried out. A preliminary thermal design of the calibration satellite has been implemented in the ESATAN(r) software. Model simulation shows that a maximum temperature of 39 \({}^{\circ}\)C is reached in part of the surface of the solar panels that is facing the sun. In order to be conservative, this temperature value has been set for the complete surface of the panels, and the corresponding thermal radiation reaching the focal plane at the frequencies of interest has been calculated. Table 3 shows the maximum power of the calibration signal from L2-CalSat (second column) compared with the thermal power arriving from Jupiter (fourth column) to the focal plane, for 4 frequency bands required to characterise the synchrotron (40 GHz), CMB (100 and 200 GHz) and thermal dust (400 GHz) emissions. In order to reduce the power consumption of the payload, except for the 40 GHz frequency band, passive frequency multipliers, which provide a limited power level, have been considered. Additionally, in the third column, the unpolarized thermal power arriving from the calibration satellite in the worst-case scenario (the surface of the solar panels is facing perfectly the telescope aperture), is shown. It is important to note that, considering an uncontrolled tilting angle of L2-CalSat of 3 arc-minutes, the resulting thermal power variation is not considered to be an issue because it represents only about a 0.004 % of the values in Table 3.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Characteristics** & **Values** & **Units** \\ \hline Mass & 1.3 & [Kg] \\ Power Consumption (Average at Calibration) & 50 & [W] \\ Dimensions & \(20\times 10\times 10\) & [cm] \\ Temperature Range & 0–50 & [\({}^{\circ}\)C] \\ \hline \hline \end{tabular}
\end{table}
Table 2: Final payload characteristics.  
Figure 9: Frontal view of the flying calibration source showing the feed-horns sizes (in blue) and the corresponding wave-guide flanges (red lines), which determine the size envelope at the highest frequencies.  
On the other hand, Table 3 also shows that the thermal signal from L2-CalSat is always lower than the one from Jupiter, considering a distance of 6600 m between both satellites. In order to check the incidence of this signal over the detection stage, the last column of Table 3 shows the power from CMB and the saturation power level of each detector [56]. It can be observed that the thermal power from the satellite is below the saturation power of the detectors, even for the highest frequency band (400 GHz).  
Finally, it is also important to take into account the effect of the reflected signals from the solar panels, because this signal is perfectly (100 %) polarized in the previously cited worst-case scenario. However, the power level of that kind of signal has been calculated in those conditions, resulting about four orders of magnitude lower than the thermal emission from the panels, at all frequencies. Thus, these reflected signals have been discarded as a source of interference, as in the worst case (400 GHz) it would be three orders of magnitude below the CMB signal.