GroundBIRD is a ground-based cosmic microwave background (CMB) experiment for observing the polarization pattern imprinted on large angular scales (ℓ > 6) from the Teide Observatory in Tenerife, Spain. Our primary scientific objective is a precise measurement of the optical depth τ (σ(τ) ∼ 0.01) to the reionization epoch of the Universe to cross-check systematic effects in the measurements made by previous experiments. GroundBIRD observes a wide sky area in the Northern Hemisphere (∼ 40% of the full sky) while continuously rotating the telescope at a high speed of up to 20 rotations per minute (rpm) to overcome the fluctuations of atmospheric radiation. We have adopted the NbTiN/Al hybrid microwave kinetic inductance detectors (MKIDs) as focal plane detectors. We observe two frequency bands centered at 145 GHz and 220 GHz. The 145 GHz band picks up the peak frequency of the CMB spectrum. The 220 GHz band helps accurate removal of the contamination of thermal emission from the Galactic interstellar dust. The MKID arrays (138 MKIDs for 145GHz and 23 MKIDs for 220GHz) were designed and optimized so as to minimize the contamination of the two-level-system noise and maximize the sensitivity. The MKID arrays were successfully installed in May 2023 after the performance verification tests were performed at a laboratory. GroundBIRD has been upgraded to use the full MKID arrays, and scientific observations are now underway. The telescope is automated, so that all observations are performed remotely. Initial validations, including polarization response tests and observations of Jupiter and the moon, have been completed successfully. We are now running scientific observations.

GroundBIRD is a millimeter-wave telescope to observe the polarization patterns of the cosmic microwave background (CMB). The target science topics are primordial gravitational waves from cosmic inflation and reionization optical depth. Therefore, this telescope is designed to achieve the highest sensitivity at large angular scales, ℓ = 6 - 300. For wide sky observations (∼40% full-sky), scanning at a high rotation speed (120°/s) is important to remove atmospheric fluctuations. Microwave kinetic inductance detector (MKID) is utilized with the fast GroundBIRD rotation since its good time response. We have started the commissioning run at the Teide Observatory in the Canary Islands. We report the performance of the telescope, receiver, and data acquisition system, including cooling achievements, observations of astronomical objects, and observations taken during several days ahead of our main survey observations.