# L2-CalSat: A Calibration Satellite for Ultra-Sensitive CMB Polarization Space Missions
## 4 L2-CalSat Description  
In this work it is proposed the use of a calibration source embarked on an ancillary satellite (L2-CalSat) that would be in formation flying with the main CMB scientific satellite (CMB-Sat). As will be shown in Section 4.4, the unpolarised thermal emission of the calibration satellite could saturate the highest frequency band detectors on-board CMB-Sat, even when placed at the far field distances of Table 1. So, in order to avoid this issue, we propose to place L2-CalSat at the necessary distance from CMB-Sat, which results to be about one order of magnitude greater than the one shown in Table 1. For small aperture experiments (LiteBIRD), this calibration system will enable control over pointing errors and polarization systematics, allowing the calibration of the polarization angle, far field measurements of the telescope beam patterns, and the calibration of the intensity response. For larger apertures (CORE or PICO), the larger distances required to reduce thermal emissions from the calibration satellite, makes harder the achievement of the required calibration signal power level, and also the orientation accuracy. However, in such cases, it would be also possible to use the proposed calibration system by adding to L2-CalSat some kind of cooling system (for shorter distances between satellites) or a thermal calibration source able to provide more power at the highest frequency bands (for larger distances). In any case, this paper is focused in small-aperture experiments that can be calibrated with a simpler system, letting the case of larger aperture missions for a future work. In the proposed calibration system a set of microwave sources will emit linearly polarized signals at different wavelengths, covering the frequency bands of the CMB-Sat polarimeters, typically between 40 and 700 GHz. The calibration satellite will operate from far field distances, providing:  
* Narrow band signals with high signal to noise ratios, to explore in-band features of the optical elements. * Wide band signals, to calibrate the integrated bands of the instruments with a signal similar to that received from the sky. * Polarized signals, used primarily to calibrate the absolute polarization angle, but also to measure beam systematics and for intensity calibration. L2-CalSat will fly in formation with the main satellite, appearing as a distant source for the polarization sensitive detectors. It is important to note that the developments and results of the ESA technology demonstration mission PROBA-3, to be launched in 2022, will represent a clear benefit for the accurate formation flying required for L2-CalSat. Figure 6: BB signals (**a**) produced by the FSL of two non-ideal beam models (**b**) with FWHM = 30 arcmin and for a frequency of 140 GHz. Red lines (option 1): spurious signal produced by FSL placed at more than 20\({}^{\circ}\) from the beam center. Blue lines (option 2): spurious signal produced by FSL placed at more than 10\({}^{\circ}\) from the beam center. Primordial B-mode (solid black lines) for two values of \(r\) and gravitational lensing B-mode (dashed black) spectra are also plotted in (**a**) for reference. Our proposed calibration mission, far from the Earth, is very challenging because it involves formation flying without support from a GPS navigation system. Therefore, the Attitude and Orbit Control System (AOCS) has to provide not only support for the orientation of the calibration payload (source) but also navigation around the main satellite. As calibration system, the AOCS will be used to position the source and to register the polarized source direction to in sky coordinates. Two celestial coordinates (from an accurate star camera pointing direction) will be used to determine the third angle defining the rotation of the polarization plane along the line of sight of the detectors. In the absence of a GPS system, and also for security reasons, the main satellite should provide navigation information for L2-CalSat, the latter requiring also a propulsion system.