The All-Sky SignAl Short-Spacing INterferometer (ASSASSIN) I: Global sky
  measurements with the Engineering Development Array-2
  Aiming to fill a crucial gap in our observational knowledge of the early
Universe, experiments around the world continue to attempt to verify the
claimed detection of the redshifted 21-cm signal from Cosmic Dawn by the EDGES
experiment. This sky-averaged or 'global' signal from neutral hydrogen should
be detectable at low radio frequencies (50-200 MHz), but is difficult to
measure due to bright foreground emission and difficulties in reaching the
required levels of instrumental-calibration precision. In this paper we outline
our progress toward using a novel new method to measure the global redshifted
21-cm signal. Motivated by the need to use alternative methods with very
different systematic errors to EDGES for an independent result, we employ an
array of closely-spaced antennas to measure the global sky signal
interferometrically, rather than using the conventional approach with a single
antenna. We use simulations to demonstrate our newly-developed methods and show
that, for an idealised instrument, a 21-cm signal could theoretically be
extracted from the visibilities of an array of closely-spaced dipoles. We
verify that our signal-extraction methods work on real data using observations
made with a Square Kilometre-Array-like prototype; the Engineering Development
Array-2. Finally, we use the lessons learned in both our simulations and
observations to lay out a clear plan for future work, which will ultimately
lead to a new global redshifted 21-cm instrument: the All-Sky SignAl
Short-Spacing INterferometer (ASSASSIN).
