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The ARCADE 2 instrument has measured the absolute temperature of the sky at frequencies 3, 8, 10, 30, and 90 GHz, using an open-aperture cryogenic instrument observing at balloon altitudes with no emissive windows between the beam-forming optics and the sky. An external blackbody calibrator provides an in situ reference. Systematic errors were greatly reduced by using differential radiometers and cooling all critical components to physical temperatures approximating the CMB temperature. A linear model is used to compare the output of each radiometer to a set of thermometers on the instrument. Small corrections are made for the residual emission from the flight train, balloon, atmosphere, and foreground Galactic emission. The ARCADE 2 data alone show an extragalactic rise of 507 mK at 3. 3 GHz in addition to a CMB temperature of 2. 730. 004 K. Combining the ARCADE 2 data with data from the literature shows a background power law spectrum of T=1. 26 0. 09 K (ν/ν₀) ^-2. 60 0. 04 from 22 MHz to 10 GHz (ν₀=1 GHz) in addition to a CMB temperature of 2. 725. 001 K.
Fixsen et al. (Mon,) studied this question.
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