COBE differential microwave radiometers - Calibration techniques

Abstract

The COBE spacecraft was launched 1989 November 18 UT carrying three scientific instruments into Earth orbit for studies of cosmology. One of these instruments, the Differential Microwave Radiometer (DMR), is designed to measure the large-angular-scale temperature anisotropy of the cosmic microwave background radiation at three frequencies (31.5, 53, and 90 GHz). In this paper we present three methods used to calibrate the DMR. First, the signal difference between beam-filling hot and cold targets observed on the ground provides a primary calibration that is transferred to space by noise sources internal to the instrument. Second, the Moon is used in flight as an external calibration source. Third, the signal arising from the Doppler effect due to the Earth's motion around the barycenter of the solar system is used as an external calibration source. Preliminary analysis of the external source calibration techniques confirms the accuracy of the currently more precise ground-based calibration. Assuming the noise source behavior did not change from the ground-based calibration to flight we derive a 0.1%-0.4% relative and 0.7%-2.5% absolute calibration uncertainty, depending on radiometer channel.