# L2-CalSat: A Calibration Satellite for Ultra-Sensitive CMB Polarization Space Missions
## 2 Calibration Requirements
### Calibration Methods  
Existing methods for calibrating CMB polarimeters [36] result in limited accuracy, among other aspects, on the determination of the polarization angle and the beam patterns. This is due to several reasons, being the lack of natural reference sources one of the most important limitations. The calibration of polarized beams (which requires a very well known and purely polarized signal source placed in the far field of the telescope) and polarization angle, need of frequency-independent and time-invariant point sources. Furthermore, they should be always visible independently on the site of the telescope. Actually, Tau-A is the best astronomical candidate for polarization calibration, but presents a limited polar angle accuracy between \(1^{\circ}\) and \(0.5^{\circ}\)[10]. On the other hand, microwave detectors must be cooled down from 300 K to 100 mK [37]. As a consequence, another source of inaccuracy is coming from very small contractions of the pieces in the cryostat, which could induce over 1 mm misalignment, which represents the minimum uncertainty requested to reach the \(1^{\circ}\) accuracy in the alignment that detectors and optics must satisfy. This accuracy is difficult to achieve, even if a specific design to reduce these effects is planed, since all the devices has to be mounted at room temperature, and, during operations, external agents (like pressure and temperature) can vary, affecting the cryostat conditions. Finally, and additional critical step is to relate the orientation of the detectors relative to the telescope and the receiver mount, once the cryostat is closed. This will imply, at the end, that it is not possible to have a direct polarization angle calibration with an accuracy better than \(1^{\circ}\). Additionally, several on-going and planed CMB experiments contemplate large rotating Half-Wave Plates (HWP) to act as polarization modulator systems [38]. This is a natural choice, since their large size allow them to determine the relative polarization angle with high accuracy, avoiding that edge effects could introduce, for instance, optical aberrations. However, in practice, actual HWPs present also non-idealities that can degrade the accuracy. Other calibration strategies are based on the use of polarization grids placed between the optics and polarimeters but, as a result, the don't allow to characterize beam systematic errors provided by those optics. Many experiments use a method called self-calibration [39] in order to overcome the lack of good astronomical calibrators. This method uses one prediction of the SCM to calibrate CMB polarimeters, which conclude that the TB and EB signals identically vanish due to the correlation absence between odd- and even-parity primordial signals. However, physical phenomena as CB could generate non-null TB and EB signals. In such a case, self-calibration prevents the study of these phenomena and introduces errors on the cosmological parameters. For the particular case of CB, it has been recently shown [26] that it is possible to disentangle miscalibrated polarization angles from CB due to the fact that both effects rotate the CMB polarization angles but only the miscalibration angle rotates those of the Galactic foreground emission. While this result represents an improvement on our ability to constrain parity-violating physics with CMB polarization data, a precise calibration of the absolute polarization angle for the different frequency bands of a given experiment remains as the optimal solution for the scientific exploitation of the data. In the case of previous CMB missions (Planck (ESA) [40] and WMAP (NASA) [41]) which presented a reduced number of detectors, celestial sources could be used for in-flightcalibrations of the beam and the polar angle. As previously noted, for the polar angle, Tau-A is known to date with an accuracy of about 0.5\({}^{\circ}\).