Differential Microwave Radiometers (DMRs) at frequencies of 31.5, 53, and 90 GHz (9.5, 5.7, and 3.3 mm) have been designed and built to map the large angular scale variations in the brightness temperature of the cosmic microwave background radiation. The instrument is being flown aboard NASA's Cosmic Background Explorer (COBE) satellite, launched 1989 November 18. Each receiver input is switched between two antennas pointing 60° apart on the sky. The satellite spins at 0.8 revolutions per minute to interchange the antenna directions every half-rotation. The satellite is in near-polar orbit with the orbital plane precessing at 1̊ per day, causing the beams to scan the entire sky in 6 months. In 1 year of observation, the instruments are capable of mapping the sky to an rms sensitivity of 0.1 mK per 7° field of view. This translates to a per pixel sensitivity of ΔT/TCMB = 4 × 10⁻⁵ (1 σ) and a dipole sensitivity of 5 × 10⁻⁶ (1 σ) in 1 year. The mission and the instrument have been carefully designed to minimize the need for systematic corrections to the data.
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Smoot et al. (1990) studied this question.