  GroundBIRD is a ground-based cosmic microwave background (CMB) experiment for
observing the polarization pattern imprinted on large angular scales ($\ell >
6$ ) from the Teide Observatory in Tenerife, Spain. Our primary scientific
objective is a precise measurement of the optical depth $\tau$ ($\sigma(\tau)
\sim 0.01$) 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 ($\sim 40\%$ 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.
