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 idealized 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).