Thermal design of a passively cooled 138 K differential microwave radiometer

The mission of the Cosmic Background Explorer (COBE) satellite has the objective to map the spectrum and angular distribution of diffuse radiation over a wavelength range from 1 micron to 1.3 cm. The satellite will have three types of instruments, including the Diffuse Infrared Background Experiment (DIRBE), the Far Infrared Absolute Spectrophotometer (FIRAS), and the Differential Microwave Radiometer (DMR). In order to achieve a minimum operating temperature, the cold DMR design utilizes four consecutively colder radiator stages between the electronic equipment and the antenna. The results of a thermal analysis show that the design objective of a temperature range from 130 K to 150 K for the antenna assembly can be met. According to a baseline analysis, the antenna temperature can be controlled at 138.8 K with a five degree margin for error included.