ARCADE (for Absolute Radiometer for Cosmology, Astrophysics, and Diffuse Emission) was an instrument package (virtually, a telescope) flown in a balloon to about 120,000 feet. It was flown four times, in 2001, 2003, 2005, and 2006, then decommissioned. The latter two flights had an improved instrument package named ARCADE 2.
It sensed microwave from the sky in the range of centimeter wavelengths. It used radiometers cooled to 3K, attempting to measure temperature differences of 0.001K. Effectively, it was viewing the cosmic microwave background, but was looking for variations corresponding to the heat of early stars and galaxies, and also EMR from early universe particle interactions.