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We present the angular power spectrum derived from the first-year WilkinsonMicrowave Anisotropy Probe (WMAP) sky maps. We study a variety of powerspectrum estimation methods and data combinations and demonstrate that theresults are robust. The data are modestly contaminated by diffuse Galacticforeground emission, but we show that a simple Galactic template model issufficient to remove the signal. Point sources produce a modest contaminationin the low frequency data. After masking ~700 known bright sources from themaps, we estimate residual sources contribute ~3500 uK^2 at 41 GHz, and ~130uK^2 at 94 GHz, to the power spectrum l*(l+1)*C_l/(2*pi) at l=1000. Systematicerrors are negligible compared to the (modest) level of foreground emission.Our best estimate of the power spectrum is derived from 28 cross-power spectraof statistically independent channels. The final spectrum is essentiallyindependent of the noise properties of an individual radiometer. The resultingspectrum provides a definitive measurement of the CMB power spectrum, withuncertainties limited by cosmic variance, up to l~350. The spectrum clearlyexhibits a first acoustic peak at l=220 and a second acoustic peak at l~540 andit provides strong support for adiabatic initial conditions. Kogut et al.(2003) analyze the C_l^TE power spectrum, and present evidence for a relativelyhigh optical depth, and an early period of cosmic reionization. Among otherthings, this implies that the temperature power spectrum has been suppressed by\~30% on degree angular scales, due to secondary scattering.

First‐Year Wilkinson Microwave Anisotropy Probe ( WMAP ) Observations: The Angular Power Spectrum