An Absolute Measurement of the Cosmic Microwave Background Radiation Temperature at 10.7 GHz

A balloon-borne experiment has measured the absolute temperature of the cosmic microwave background radiation (CMBR) at 10.7 GHz to be TCMBR = 2.730 ± 0.014 K. The error is the quadratic sum of several systematic errors, with a statistical error of less than 0.1 mK. The instrument is made up of a cooled corrugated horn antenna coupled to a total power radiometer. A cryogenic mechanical waveguide switch alternately connects the radiometer to the horn and to an internal reference load. The small measured temperature difference (≲20 mK) between the sky signal and the reference load in conjunction with the use of a cold front end keeps systematic instrumental corrections small. Atmospheric and window emission are minimized by flying the instrument at 24 km altitude. A large outer ground screen and smaller inner screen shield the instrument from stray radiation from the ground and the balloon. In-flight tests constrain the magnitude of ground radiation contamination, and low-level interference is monitored through observations in several narrow-frequency bands.