An absolute measurement of the cosmic microwave background radiation temperature at 20 centimeters

A ground-based radiometer is used to measure the absolute temperature of the cosmic microwave background radiation (CMBR) at a frequency of 1.4 GHz. The instrument comprises a corrugated horn antenna coupled to a cryogenic correlation receiver. Accurate gain calibrations are made every few minutes; the radiometer zero offset is measured at the beginning and the end of the observing season. Tip scans are made to measure the atmospheric emission. We find T_atm = 1550 ± 170 mK for a pencil beam, measured from an elevation of 836 m. The Galactic foreground emission is removed by extrapolation from a lower frequency map. Over the region observed, the Galactic component is ∼ 1 K and varies by ∼ 10%. Cold loads are-used to measure the instrumental emission. The ability to sample narrow (5 MHz) frequency bins across a 5% bandpass provides a powerful tool for studying systematic effects. For the absolute temperature of the CMBR, we find TCMBR = 2.65_-0.30^+0.33 K. This result is higher, but not significantly, than the value of T_CMBR at 1.4 GHz previously reported by Bensadoun et al.