# L2-CalSat: A Calibration Satellite for Ultra-Sensitive CMB Polarization Space Missions
## 4 L2-CalSat Description
### Calibration Sources  
Focusing in a frequency band from about 40 to 400 GHz, and trying to optimise volume, cost and power consumption, it is proposed to cover six bands determined by the corresponding wave-guide standards: 33-50 GHz (WR22), 60-90 GHz (WR12), 90-120 GHz (WR8), 140-220 GHz (WR5), 220-325 GHz (WR3) and 325-500 GHz (WR2). Note that the 50-60 and 120-140 GHz bands are not covered by the sources. For simplicity, we have assumed that they can be omitted in this first study but, if necessary, an alternative frequency coverage could be considered in a future analysis. It is critical to include the higher (WR2) frequency band since at these frequencies it is not possible to use the orbital dipole (a Doppler effect generated by the orbital motion of the CMB mission) to calibrate the intensity, as was done by the Planck mission for lower frequencies.  
In order to generate calibration signals covering all these frequency bands, it is proposed the use of two common low-frequency sources (monochromatic frequency-variable and broadband) that can be used to generate variable signals. Then, the required bandwidth coverage will be achieved by means of power splitters and Frequency Multipliers (FM) as those supplied by, for example, Virginia Diodes [53] (USA), Viva Tech [54] (France) or Farran Tech. [55] (Ireland). Figure 8 shows a schematic of this kind of source implementation.  
It consists of a variable low-frequency Continuous Wave (CW) source, which is able to cover a given bandwidth \(BW_{LF}\), and a broadband noise source (NS) covering the same bandwidth, feeding both of them a common frequency multiplying system that translates the signals to the required frequency bands. The broadband NS can be implemented by means of a noisy diode or a 50-Ohm termination connected to a Power Amplification (PA) chain with the same bandwidth \(BW_{LF}\) that will provide the required level of signal power. A PIN (P-type, Intrinsic semiconductor, N-type) switch diode is used to select the kind of signal to be used, as a function of the particular systematic to be calibrated. A variable attenuator (VA) is used to change the power level for linearity and beam calibrations. VA applied directly to the common low-frequency signals will reduce the cost and complexity of the overall source. The signal is then divided to feed the sub-band transmitters and the frequency of the signal is multiplied using FM to get the required frequency coverage in each of them. A Band-Pass Filter (BPF) rejects undesired harmonics of the signal and a Directional Coupler (DC) is used to measure the emitted power level by connecting a Zero-Bias-Detector (ZBD) to the coupled output. The direct output is connected to a feed-horn providing the calibration signal of each sub-band. Due to the difficulties to generate microwave power at the highest frequencies, it has been proposed the use of conical horns (CH), with 20 dB gain, for the two lower frequencies bands and rectangular horns (RH), with 26 dB gain, for the rest of bands. Figure 9 shows a sketch of the frontal view of the calibration source mounted in a 2U CubeSat chassis of size 10 \(\times\) 10 \(\times\) 20 cm.  
Figure 8: Diagram of a calibration source covering frequency bands from 40 to 400 GHz.  
Another requirement of the calibration source is to provide a very pure linearly polarized signal, presenting a cross-polarization degree of the order of \(-60\) dB. This can be also achieved by means of the use of Wire-Grid Polarisers (WGP) placed in front of the source's horns and accurately aligned with their polarization axes [11; 35].  
Table 2 summarises the final main payload physical characteristics.