  The second generation Absolute Radiometer for Cosmology, Astrophysics, and
Diffuse Emission (ARCADE 2) instrument is a balloon-borne experiment to measure
the radiometric temperature of the cosmic microwave background and Galactic and
extra-Galactic emission at six frequencies from 3 to 90 GHz. ARCADE 2 utilizes
a double-nulled design where emission from the sky is compared to that from an
external cryogenic full-aperture blackbody calibrator by cryogenic switching
radiometers containing internal blackbody reference loads. In order to further
minimize sources of systematic error, ARCADE 2 features a cold fully open
aperture with all radiometrically active components maintained at near 2.7 K
without windows or other warm objects, achieved through a novel thermal design.
We discuss the design and performance of the ARCADE 2 instrument in its 2005
and 2006 flights.
