ABSOLUTE MEASUREMENT OF THE COSMIC MICROWAVE BACKGROUND AT 2 GHZ

We measured the intensity of the cosmic microwave background (CMB), radiation at a frequency of 2.0 GHz (15 cm wavelength) with a ground-based total-power radio-frequency (RF) gain radiometer calibrated at the antenna aperture with a quasi-free space cold load. The observations were performed from a remote, high-altitude site near the Amundsen-Scott South Pole Station to minimize systematic effects. The measured thermodynamic temperature of the CMB is T(CMB) = 2.55 +/- 0.14 K (68% confidence level), the limiting factors in the accuracy being the subtraction of the atmospheric and Galactic contributions. The atmospheric emission was evaluated both by direct measurements and by extrapolation from our higher frequency data. The Galactic emission was determined by scaling low-frequency maps and was checked with differential drift scans. Our result is approximately 1 sigma lower than the recent accurate value of T(CMB) obtained by COBE-FIRAS above 60 GHz. The temperature of the sky (i.e., CMB plus Galaxy and extragalactic sources) at the south celestial pole with approximately 22-degrees resolution is T(A,Sky) = 2.83 +/- 0.10 K, and its accuracy is limited by the subtraction of atmospheric emission.