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In this work we present a new approach to estimate the power spectrum $P({\bfk})$ of redshifted HI 21-cm brightness temperature fluctuations. The MAPS$C_{\ell}(\nu_a,\nu_b)$ completely quantifies the second order statistics ofthe sky signal under the assumption that the signal is statisticallyhomogeneous and isotropic on the sky. Here we generalize an already existingvisibility based estimator for $C_{\ell}$, namely TGE, to develop an estimatorfor $C_{\ell}(\nu_a,\nu_b)$. The 21-cm power spectrum is the Fourier transformof $C_{\ell}(\Delta \nu)$ with respect to $\Delta \nu=\mid \nu_a-\nu_b \mid$,and we use this to estimate $P({\bf k})$. Using simulations of $150 \, {\rmMHz}$ GMRT observations, we find that this estimator is able to recover $P(k)$with an accuracy of $5-20 \%$ over a reasonably large $k$ range even when thedata in $80 \%$ randomly chosen frequency channels is flagged.

A Tapered Gridded Estimator (TGE) for the Multi-Frequency Angular Power Spectrum (MAPS) and the Cosmological HI 21-cm Power Spectrum