Rocket measurement of the cosmic-background-radiation mm-wave spectrum

We report here the most precise constraint to date on the spectrum of the cosmic background radiation (CBR), obtained from measurements made with a liquid-helium-cooled spectrometer carried above the atmosphere on a rocket. The spectrum is very well fitted by a Planck function of temperature T=2.736 K. The scatter of the equivalent temperature in the band 3--16 cm^-1 is ±{}10 mK, about 1/3% of the mean whereas the estimated overall accuracy of the mean temperature is ±{}17 mK. These results are inconsistent with a previously reported excess IR intensity in the CBR but are in good agreement with COBE results.