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One of the main challenges for future 21 cm observations is to removeforegrounds which are several orders of magnitude more intense than the HIsignal. We propose a new technique for removing foregrounds of the redshifted21 cm observations. We consider multi-frequency interferometer observations. Weassume that the 21 cm signals in different frequency channels are uncorrelatedand the foreground signals change slowly as a function of frequency. When weadd the visibilities of all channels, the foreground signals increase roughlyby a factor of ~N because they are highly correlated. However, the 21 cmsignals increase by a factor of ~\sqrt{N} because the signals in differentchannels contribute randomly. This enables us to obtain an accurate shape ofthe foreground angular power spectrum. Then, we obtain the 21-cm power spectrumby subtracting the foreground power spectrum obtained this way. We describe howto obtain the average power spectrum of the 21 cm signal.

A Technique for Foreground Subtraction in Redshifted 21 cm Observations