  We present a determination by the Archeops experiment of the angular power
spectrum of the cosmic microwave background anisotropy in 16 bins over the
multipole range l=15-350. Archeops was conceived as a precursor of the Planck
HFI instrument by using the same optical design and the same technology for the
detectors and their cooling. Archeops is a balloon-borne instrument consisting
of a 1.5 m aperture diameter telescope and an array of 21 photometers
maintained at ~100 mK that are operating in 4 frequency bands centered at 143,
217, 353 and 545 GHz. The data were taken during the Arctic night of February
7, 2002 after the instrument was launched by CNES from Esrange base (Sweden).
The entire data cover ~ 30% of the sky.This first analysis was obtained with a
small subset of the dataset using the most sensitive photometer in each CMB
band (143 and 217 GHz) and 12.6% of the sky at galactic latitudes above 30
degrees where the foreground contamination is measured to be negligible. The
large sky coverage and medium resolution (better than 15 arcminutes) provide
for the first time a high signal-to-noise ratio determination of the power
spectrum over angular scales that include both the first acoustic peak and
scales probed by COBE/DMR. With a binning of Delta(l)=7 to 25 the error bars
are dominated by sample variance for l below 200. A companion paper details the
cosmological implications.
