Drone-based polarization calibration source for mm-wave telescopes

To probe alternative cosmologies, future CMB experiments will require an exquisite absolute polarization angle calibration. The scarce number natural polarized calibrators makes artificial alternatives appealing. Moreover, ground loading restrictions force these sources to be located at high elevations. We have shown that commercial drones can operate at altitudes above 5200 meters, lifting 4 kg payloads for 10 minutes, being capable of reaching the far-field of small aperture telescopes. A drone-based calibration source, equipped with a high-precision metrology system based on an RTK GPS, a photogrammetry camera and a geo-referenced reconstruction of the site, can provide an absolute polarization angle reference accurate to 0.1 degrees. The metrology data must be synchronized in time with the detector time-streams to compensate the source movements. We implemented an electronically chopped, linearly polarized 150 GHz coherent source, for serving the experiments CLASS, ACT, Simons Array and Simons Observatory.