The Cosmic Microwave Background Radiation (CMBR) shows some small temperature differences of the order of one part in 100000, that were measured for the first time by the COBE satellite [8]. These so-called anisotropies trace the fluctuations of the density of matter that are thought to be the origin, by gravitational collapse, of the large-scale structure of the Universe (galaxies, clusters,...) that we observe today. Its pattern can also yield an indirect measurement of the density, age and curvature of the Universe (see e.g. [5]). There have been many experiments that have already measured these anisotropies with various techniques, angular resolution, noise and scanning strategy. Mosqt recent ones (e.g. TOCO, Boomerang [3,7], and Maxima [4,6]) have improved on COBE results by the wavelength coverage, the sensitivity and the angular resolution.
The Archeops experiment aims at mapping the anisotropies of the cosmic microwave background from small to large scales at the same time. For this purpose, a beam of about 8 arcminutes is swept through the sky by spinning a 1.5 m telescope pointing at 41 degree elevation around its vertical axis. A large fraction of the sky is covered when the rotation of the Earth makes the swept circle drift across the celestial sphere. This is only possible if the observations are done during the Arctic night and on a balloon where neither the Sun nor the atmosphere disturb the measurements. Ozone cloud emission and residual winds can be avoided with a high altitude strastospheric balloon. From the Swedish balloon and rocket base in Esrange near Kiruna, in cooperation with Russian scientists, the CNES balloon team can launch balloons in the polar night, with a typical trajectory ending just before the Ural mountains in Russia. Integration times can be up to 24 hours in the December-January campaigns.