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The redshifted 21cm signal of neutral hydrogen from the epoch of reionization(EoR) is extremely weak and its first detection is therefore expected to bestatistical with first-generation low-frequency radio interferometers. In thisletter we propose a method to extract the angular power spectrum of EoR fromthe visibility correlation coefficients p_{ij}(u,v), instead of thevisibilities V_{ij}(u,v) measured directly by radio interferometers inconventional algorithm. The visibility correlation coefficients are defined asp_{ij}(u,v)=V_{ij}(u,v)/\sqrt{|V_{ii}||V_{jj}|} by introducing theauto-correlation terms V_{ii} and V_{jj} such that the angular power spectrumC_{\ell} can be obtained through C_{\ell}=4pi^2T_0^2<|p_{ij}(u,v)|^2>,independently of the primary beams of antennas. This also removes partially theinfluence of receiver gains in the measurement of C_{\ell} because theamplitudes of the gains cancel each other out in the statistical averageoperation of <|p_{ij}(u,v)|^2>.We use the average system temperature T_0 as acalibrator of C_{\ell}, which is dominated by the Milky Way and extragalacticsources in our interested frequency range below 200 MHz. Finally we demonstratethe feasibility of the novel method using the simulated sky maps as targets andthe 21 CentiMeter Array (21CMA) as interferometer.

A Method to Extract the Angular Power Spectrum of the Epoch of Reionization from Low-Frequency Radio Interferometers