# L2-CalSat: A Calibration Satellite for Ultra-Sensitive CMB Polarization Space Missions  
[PERSON]  
1 Instituto de Fisica de Cantabria (IFCA), CSIC-UC, Avda. de los Castros s/n, 39005 Santander, Spain;  
[PERSON]DDRESS] (E.M.-G.); [EMAIL_ADDRESS] (E.M.-G.); [PERSON]DRESS] (R.B. );  
[PERSON]  
2 Instituto Universitario Ignacio Du Rino (IDR/UPM), Universidad Politecnica de Madrid, Plaza Cadrenal Cisneros, 3, 28040 Madrid, Spain; [EMAIL_ADDRESS] (J.B.-B. ); [PERSON]S] (S.G.);  
[EMAIL_ADDRESS] (J.C.); [PERSON]A.S.)  
[PERSON]  
2 Instituto Universitario Ignacio Du Rino (IDR/UPM), Universidad Politecnica de Madrid, Plaza Cadrenal Cisneros, 3, 28040 Madrid, Spain; [EMAIL_ADDRESS] (J.B.-B. ); [PERSON]S] (S.G.);  
[EMAIL_ADDRESS] (J.C.); [PERSON]A.S.)  
[PERSON]  
2 Instituto Universitario Ignacio Du Rino (IDR/UPM), Universidad Politecnica de Madrid, Plaza Cadrenal Cisneros, 3, 28040 Madrid, Spain; [EMAIL_ADDRESS] (J.B.-B. ); [PERSON]S] (S.G.);  
[PERSON]  
2 Instituto Universitario Ignacio Du Rino (IDR/UPM), Universidad Politecnica de Madrid, Plaza Cadrenal Cisneros, 3, 28040 Madrid, Spain; [EMAIL_ADDRESS] (J.B.-B. ); [EMAIL_ADDRESS] (S.G.);  
[EMAIL_ADDRESS] (J.C.); [PERSON]A.S.)  
[PERSON]  
1 Instituto de Fisica de Cantabria (IFCA), CSIC-UC, Avda. de los Castros s/n, 39005 Santander, Spain;  
[EMAIL_ADDRESS] (E.M.-G.); [PERSON]L_ADDRESS] (E.M.-G.); [PERSON]DRESS] (E.M.-G.); [PERSON]DRESS] (E.M.-G.); [PERSON]DRESS] (R.B.) [PERSON]  
1 Instituto de Fisica de Cantabria (IFCA), CSIC-UC, Avda. de los Castros s/n, 39005 Santander, Spain;  
[PERSON]DDRESS] (E.M.-G.); [PERSON]L_ADDRESS] (E.M.-G.); [PERSON]DRESS] (E.  
an important role in measuring repeatedly the same sky area in different times and with different orientations of the focal plane, helping to mitigate systematic errors induced by the instruments. Celestial sources are often used as an external reference for calibration after launch, but the characterization of these sources is not sufficiently good to match the calibration requirements of CMB polarization experiments (see e.g., [10; 11] for Tau A polarization angle uncertainties). In this paper we study the concept of a novel calibration system for the next generation of CMB polarization space missions, based on the use of a calibration satellite (L2-CalSat) flying in formation with a CMB telescope (CMB-Sat), in an orbit located at the second Lagrange point (Figure 1). During the calibration periods, L2-CalSat keeps the calibration sources within view of CMB-Sat, emitting purely polarized microwave radiation from the telescope's far-field. As a result, the observations will benefit, among other things, from the absolute calibration of the polarization angle and the detectors beam response, meeting the accuracy requirements for the next generation of CMB experiments. As a baseline option, the calibration satellite is based on the CubeSat standard carrying a perfectly known source able to emit a reference signal to reduce CMB polarization measurements uncertainty. CubeSat has become a satellite concept with increasing interest for space research and exploration. Key elements of its success have been standardization, easy design and low cost. Recent missions such as CanX-4 & 5 [12] have shown that CubeSats already have the necessary technology to carry out formation flight. Formation flying multiplies the possibilities of performing science with small satellites. As an example, the future ESA's technology demonstration mission PROBA-3 [13] can be mentioned. In this mission, one of the satellites of the formation flight acts as a parasol, hiding the solar disk so that the other can study the solar corona. PROBA-3 is composed of two smallsats (less than 500 kg), but it is not a CubeSat mission. Nevertheless, a growing number of missions using formation flight are based on CubeSat technology [14]. The scientific scope of the CubeSat concept is limited by its constraints of space, weight and power, and the consequent impact in the scientific instrumentation that can be accommodated.