Differential Microwave Radiometers (DMR)

Description

The differential microwave radiometer (DMR) investigation uses three differential radiometers to map the sky at 31.4, 53, and 90 GHz. The radiometers are distributed around the outer surface of the cryostat. Each radiometer employs a pair of horn antennas viewing at 30 deg from the spin axis of the spacecraft, measuring the differential temperature between points in the sky separated by 60 deg. At each frequency there are two channels for dual polarization measurements for improved sensitivity and for reliability. Each radiometer is a microwave receiver whose input is switched rapidly between the two horn antennas, obtaining the difference in brightness of two fields of view 7 deg in diameter located 60 deg apart and 30 deg from the axis of the spacecraft. High sensitivity is achieved by temperature stabilization (at 300 deg K for 31.4 GHz and at 140 deg K for 53 and 90 GHz), by spacecraft spin, and by the ability to integrate over the entire year. Sensitivity to large-scale anisotropies is about 3E-5 deg K. The instrument weighs 120 kg, uses 114 W, and has a data rate of 500 bps.