A balloon-borne experiment has measured the absolute temperature of the cosmic microwave background radiation ( CMBR ) at 10.7 GHz to be T _ { \mbox { \scriptsize CMBR } } = 2.730 \pm 0.014 K. The error is the quadratic sum of several systematic errors , with statistical error of less than 0.1 mK .
The instrument comprises 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 ( \stackrel { < } { { } _ { \sim } } 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 .