# L2-CalSat: A Calibration Satellite for Ultra-Sensitive CMB Polarization Space Missions
## 2 Calibration Requirements  
The information obtained from the intensity and the polarization of the CMB have provided a unique source for probing different cosmological scenarios and capital aspects of fundamental physics. The reasons is that the physical mechanisms that take place in the early Universe, and that affected the CMB radiation during its path to reach us, have left very weak but significant anisotropies on the otherwise uniform microwave background. Many properties of our Universe have been revealed (e.g., flatness of the space-time, age, and energy and matter content [20]). Electric-like (E) and magnetic-like (B) modes of polarization are distinguished by their behaviour under a parity transformation \(n\rightarrow-n\). The harmonic coefficients of the E-mode show \((-1)^{l}\) parity whereas the ones of the B-mode behave as \((-1)^{l+1}\). As a consequence, a parity transformation affects only the sign of B-modes but not E-modes. The Standard Cosmological Model (SCM) establishes that temperature and E-mode polarization anisotropies were originated by density perturbations while the B-modes are only sourced by tensor perturbations [21] (despite a contribution coming from E-modes due to gravitational lensing).  
However, while recent polarization experiments are able to detect E-mode signals that supports the SCM [22], primordial B-mode signals have yet to be detected [23]. They are fainter and can be easily contaminated (in particular, by the lensing of the E-mode polarization and by astrophysical emissions from our Galaxy and extra-galactic sources) but they can provide essential information about inflation, the signal level of the primordial gravitational wave background (PGWB), the species and mass of neutrinos [24] or the magnetic fields, both primordial (PMF) and also those present in galaxies and galaxy clusters [25]. They may also be used to study other crucial aspects as cosmic birefringence (CB) [26; 27; 28], dark matter or extensions of General Relativity [18; 19]. As mentioned above, polarization B-modes are sourced by different mechanisms, like the PGWB originated at inflation, primordial magnetic fields, the lensing of the CMB, or astrophysical foregrounds (notably dust and synchrotron emissions). Indeed, in 2014 the CMB community was attracted by the claim of a B-mode detection above the lensing level reported by the BICEP2 experiment [29]. This detection was afterwards explained by an accounted dust contamination [30], showing the important role that dealing with foregrounds contamination has in the study of the B-mode science. However, these genuine sky signals can be confused with spurious instrumental effects. In general, the spurious B-modes originating from polarization systematics cause a bias that hinders the extraction of cosmological information from the observed sky signal. For instance, the use of a continuously-rotating HWP coupled to fluctuations in the time constants of the detectors can lead to apparent shifts in the polarization angles of the detectors. Additionally, in-flight variations of detector responsivity with respect to the laboratory calibration as well as responsivity stability, also require a precise calibration during observation.  
Moving forward, the high sensitivity levels required by current and future ground [31; 32; 33; 34] and space-based CMB polarization experiments make that, systematic effects, previously only partially considered due to higher statistical uncertainties, now become the most significant limitation. Such experiments need an independent experimental calibration approach allowing one to trace systematics affecting, among others, to intensity, polarization and radiation pattern [11; 35].