The tracking tapered gridded estimator for the power spectrum from drift scan observations

Intensity mapping with the redshifted 21-cm line is an emerging tool in cosmology. Drift scan observations, where the antennas are fixed to the ground and the telescope's pointing centre (PC) changes continuously on the sky due to earth's rotation, provide broad sk y co v erage and sustained instrumental stability needed for 21-cm intensity mapping. Here, we present the Tracking Tapered Grided Estimator (TTGE) to quantify the power spectrum of the sky signal estimated directly from the visibilities measured in drift scan radio interferometric observations. The TTGE uses the data from the different PC to estimate the power spectrum of the signal from a small angular region located around a fixed tracking centre (TC). The size of this angular region is decided by a suitably chosen tapering window function that serves to reduce the foreground contamination from bright sources located at large angles from the TC. It is possible to co v er the angular footprint of the drift scan observations using multiple TC, and combine the estimated power spectra to increase the signal-to-noise ratio. Here, we have validated the TTGE using simulations of 154 MHz Murchison Wide-field Array drift scan observations. We show that the TTGE can reco v er the input model angular power spectrum C-l within 20 per cent accuracy over the l range 40 < l < 700.