  (Abridged) Noise considerations for experiments that aim to statistically
estimate the 21 cm signal from high redshift neutral hydrogen during the Epoch
of Reionisation (EoR) using interferometric data are typically computed
assuming a tracked observation, where the telescope pointing centre and
instrument phase centre are the same over the observation. Current low
frequency interferometers use aperture arrays of fixed dipoles, which are
steered electronically on the sky, and have different properties to
mechanically-steered single apertures, such as reduced sensitivity away from
zenith, and discrete pointing positions on the sky. Use of two additional
observing modes is encouraged: (1) zenith drift, where the pointing centre
remains fixed at the zenith, and the phase centre tracks the sky, and (2)
drift+shift, where the telescope uses discrete pointing centres, and the sky
drifts during each pointing. The coherence of measurements in these modes
dictates the optimal reduction in thermal noise by combination of coherent
modes, and the reduction in cosmic variance by combination of incoherent modes
(views of different patches of the sky). The balance between these noise
components provides one measure for the utility of these three modes for
measuring a statistical EoR signature. We provide a general framework for
estimating the uncertainty in the power spectrum for a given observing mode,
telescope beam shape, and antenna distribution. We apply this framework to the
Murchison Widefield Array (MWA) using an analysis of the 2D and 1D power
spectra for 900 hours of observing. We demonstrate that zenith drift scans can
yield marginally lower uncertainty in the signal power compared with tracked
scans for the MWA EoR experiment.
