  The Absolute Radiometer for Cosmology, Astrophysics, and Diffuse Emission
(ARCADE) is a balloon-borne instrument to measure the temperature of the cosmic
microwave background at centimeter wavelengths. ARCADE uses narrow-band
cryogenic radiometers to compare the sky to an external full-aperture
calibrator. To minimize potential sources of systematic error, ARCADE uses a
novel open-aperture design which maintains the antennas and calibrator at
temperatures near 3 K at the mouth of an open bucket Dewar, without windows or
other warm objects between the antennas and the sky. We discuss the design and
performance of the ARCADE instrument from its 2001 and 2003 flights.
