# L2-CalSat: A Calibration Satellite for Ultra-Sensitive CMB Polarization Space Missions
## 4 L2-CalSat Description
### Overall Features and Implementation Options  
Taking into account all the previous information, the overall characteristics of the preliminary design for L2-CalSat are summarised here. The calibration satellite total mass will be 7.193 kg (7.07 Kg of dry mass and 0.123 kg of propellant). The required electrical power will be provided by 2 orientable 6U-CubeSat solar panels with triple-junction solar cells by SpectroLabs and accumulated in a battery module (see Figure 7). The solar panels will continuously generate 30 W, enough to feed all the subsystems of L2-CalSat and charge the battery. During calibration, the solar panels and battery combined will provide a peak power of 62 W to supply the payload and the rest of subsystems. The final main characteristics of L2-CalSat are summarised in Table 4.  
The calibration system proposed here could reduce the uncertainty in polar angle to 1 arc-minute (0.017\({}^{\circ}\)) due to the accurate alignment [11] of the source components with the structure of L2-CalSat and the precision provided by its ranging and pointing systems. Additionally, as can be seen from Table 3, it would provide a polarized calibration signal allowing a noise floor level reduction of about \(-\)22 dB, in the 100 GHz band, respect to the values achieved by using Jupiter as a thermal source, taking as a reference the values achieved by the Planck mission [40]. In fact, the next section will show that the calibration satellite could provide noise floor levels lower than \(-\)70 dB in the overall band of example \begin{table}
\begin{tabular}{c c c} \hline \hline
**Characteristics** & **Values** & **Units** \\ \hline Mass & 7.193 & [Kg] \\ Power Consumption (Maximum at Calibration) & 62 & [W] \\ Dimensions & \(30\times 20\times 10\) & [cm] \\ Temperature Range & 0–50 & [\({}^{\circ}\)C] \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Final L2-CalSat characteristics. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Freq. (GHz)** & **L2-CalSat (Cal.)** & **L2-CalSat (Th.)** & **Jupiter** & **CMB-Pow** & **Sat-Pow** \\ \hline
40 & 40.8 & \(3\times 10^{-4}\) & \(2.2\times 10^{-3}\) & 0.16 & 0.5 \\
100 & 5.1 & \(4.7\times 10^{-3}\) & \(34\times 10^{-3}\) & 0.21 & 0.8 \\
200 & 5.1 & \(37.5\times 10^{-3}\) & 0.27 & 0.12 & 0.9 \\
400 & 5.1 & 0.3 & 2.17 & \(15\times 10^{-3}\) & 1.2 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Power arriving from Jupiter and L2-CalSat to a focal plane with 0.3 m aperture diameter. The maximum power of the calibration signal (second column), the thermal power from L2-CalSat (third column) and the thermal power from Jupiter (fourth column) are shown. Additionally, the power from CMB and the detectors saturation power are shown in the last two columns. All power values are given in pW. A Jupiter temperature of 160 K, fractional bandwidths of 0.25, Jupiter-Earth distance of 4.2 A.U. and distance between spacecrafts of 6600 m, have been considered in the calculations. (from 40 to 400 GHz), assuring so the complete and accurate characterization of the beam and in particular of the far side-lobes. With this level of systematic errors characterization, future CMB missions will be able to reach easily values of 0.001 in the tensor-to-scalar ratio \(r\), which is a fundamental requirement in all of them. Let us finally remark that if the piggy-back implementation option were not viable, the concept of L2-CalSat could be extended to a micro-satellite capable of reaching L2 autonomously from the same launcher. The modification would affect mainly to the propulsion and communication subsystems. The propulsion subsystem would be composed of two different types of thrusters, monopropellant (Hydrazine) and MEMS (\(H_{2}O_{2}\)). The communication subsystem should be capable of establishing contact with Earth by itself, unlike in the CubeSat implementation option where it has been assumed that the CMB-Sat should implement the communication interface between Earth and the calibration satellite.