ARCADE 2 Measurement of the Extra-Galactic Sky Temperature at 3-90 GHz
  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
{\it 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 $50\pm7$ mK at 3.3 GHz in addition
to a CMB temperature of $2.730\pm .004$ K. Combining the ARCADE 2 data with
data from the literature shows a background power law spectrum of $T=1.26\pm
0.09$ [K] $(\nu/\nu_0)^{-2.60\pm 0.04}$ from 22 MHz to 10 GHz ($\nu_0=1$ GHz)
in addition to a CMB temperature of $2.725\pm .001$ K.
