Four-year Cosmology Large Angular Scale Surveyor (CLASS) Observations:
  On-sky Receiver Performance at 40, 90, 150, and 220 GHz Frequency Bands
  The Cosmology Large Angular Scale Surveyor (CLASS) observes the polarized
cosmic microwave background (CMB) over the angular scales of 1$^\circ \lesssim
\theta \leq$ 90$^\circ$ with the aim of characterizing primordial gravitational
waves and cosmic reionization. We report on the on-sky performance of the CLASS
Q-band (40 GHz), W-band (90 GHz), and dichroic G-band (150/220 GHz) receivers
that have been operational at the CLASS site in the Atacama desert since June
2016, May 2018, and September 2019, respectively. We show that the
noise-equivalent power measured by the detectors matches the expected noise
model based on on-sky optical loading and lab-measured detector parameters.
Using Moon, Venus, and Jupiter observations, we obtain
power-to-antenna-temperature calibrations and optical efficiencies for the
telescopes. From the CMB survey data, we compute instantaneous array
noise-equivalent-temperature sensitivities of 22, 19, 23, and 71 $\mathrm{\mu
K}_\mathrm{cmb}\sqrt{\mathrm{s}}$ for the 40, 90, 150, and 220 GHz frequency
bands, respectively. These noise temperatures refer to white noise amplitudes,
which contribute to sky maps at all angular scales. Future papers will assess
additional noise sources impacting larger angular scales.
