Keck Array is a ground based polarimeter located at the South Pole. It consists of five receivers, which are aimed to measure primordial B-mode polarization imprinted on the CMB. Each receiver consists of orthogonal pairs of antenna-coupled transition edge sensor (TES) bolometers. Together with BICEP3, the BICEP/Keck program of telescopes observe the polarization of the CMB from 95GHz to 270GHz.
Keck Array observes from the South Pole, where the atmospheric loading is consistently very low. Optics are cryogenically cooled for very low internal loading. Sub-kelvin bolometers are photon-noise limited, while low optical loading keeps photon noise low. Systematics are minimized due to using a small, on-axis refracting telescope, while the detectors can be characterized the optical far field.
Here is a nice video showing our observing strategy.