# L2-CalSat: A Calibration Satellite for Ultra-Sensitive CMB Polarization Space Missions
## 5 L2-CalSat Operation
### Access Time and Viewed Detectors  
The scanning strategy of CMB-Sat and the L2-CalSat relative position, define both, the total access time and the number of viewed detectors. The number of detectors illuminated by the calibration signal has been determined by simulation, considering different relative position between both satellites. It has been noted that the more separated the different calibration signal traces are, the greater the percentage of viewed detectors. It has also been seen that the best option to place L2-CalSat is close to the anti-Sun axis, about 10\({}^{\circ}\)-15\({}^{\circ}\) from it. Figure 11 shows the percentage of viewed detectors according to the relative position between the two satellites throughout 1.5 (a), 6 (b), 12 (c) and 24 h (d) and considering a CMB-Sat FoV of 30\({}^{\circ}\). The percentage of viewed sensors (or detectors) is around 70% for 6 h (red region of Figure 11b) but it approaches 100% in that region for 12 h even when L2-CalSat is kept at the same position. It can also be observed that during a calibration session of 24 h, all the detectors could be covered from many more different positions; however, a calibration session of only 12 h has been assumed as baseline, since this calibration time would allow, among others, the polar angle calibration of almost the complete set of detectors. The total access time is proportional to the number of accesses and how the signal crosses the focal plane. If L2-CalSat has a right ascension higher than roughly 15\({}^{\circ}\) the waiting time between accesses would be equal to one precession period. Note also that the average access time is uniform and around 30 s, regardless of the calibration satellite position [57]. Figure 12 shows the simulated total access time according to the relative position between L2-CalSat and CMB-Sat, considering again a FoV of 30\({}^{\circ}\) and throughout 6 (a), 12 (b) and 24 h (c), but also for an expanded FoV of 120\({}^{\circ}\) and throughout 12 h (d). As seen, for a FoV of 30\({}^{\circ}\) and in the positions with maximum percentage of viewed sensors, about 30 min can be used for the calibration of pointing, polar angle, etc., in a calibration  
Figure 11: Percentage of viewed detectors according to the position of L2-CalSat relative to the CMB-Sat throughout 1.5 (a), 6 (b), 12 (c) and 24 h (d). session of four precession periods (6 h), around one hour is available in a period of 12 h, and around two hours are available in a calibration session of 24 h. During the access time, in order to avoid the saturation of the detectors, the calibration signal power will be attenuated. In these calibration operations the Signal to Noise Ratios (SNR) provided by the sources on-board L2-CalSat have been estimated. Considering a Noise Equivalent Temperatures (NET) of the detectors in a range between 80 and 400 \(\mathrm{\SIUnitSymbolMicro K}/\sqrt{Hz}\) for the 40 to 400 GHz bandwidth, a fractional operational bandwidth of 30%, an integration time of 10 s, and a calibration signal power equal to half the saturation one, the calculated SNR values are between \(4\times 10^{3}\) and \(5.3\times 10^{4}\) (i.e., 36-47 dB). These values are similar to those estimated in a previous calibration satellite proposal for ground-based CMB experiments [11]. On the other hand, an expanded FoV of 120\({}^{\circ}\) has also been considered to calculate the access time available for the characterization of the near- and far-side lobes of the beams. Figure 12d shows that in a calibration period of 12 h, a total access time of more than 5 h will be available for this task. Due to the large angular values of the far-side lobes, most of the sensors can be characterised with the calibration signal simultaneously, so the required time to calibrate all of them will be much lower than the available total access time.