Radio-frequency interference detection and flagging is one of the most difficult and urgent problems in 21 cm Epoch of Reionisation research. In this work, we present $\chi^2$ from redundant calibration as a novel method for RFI detection and flagging, demonstrating it to be complementary to current state-of-the-art flagging algorithms. Beginning with a brief overview of redundant calibration and the meaning of the $\chi^2$ metric, we demonstrate a two-step RFI flagging algorithm which uses the values of this metric to detect faint RFI. We find that roughly 27.4% of observations have RFI from digital television channel 7 detected by at least one algorithm of the three tested: 18.0% of observations are flagged by the novel $\chi^2$ algorithm, 16.5% are flagged by SSINS, and 6.8% are flagged by AOFlagger (there is significant overlap in these percentages). Of the 27.4% of observations with detected DTV channel 7 RFI, 37.1% (10.2% of the total observations) are detected by $\chi^2$ alone, and not by either SSINS or AOFlagger, demonstrating a significant population of as-yet undetected RFI. We find that $\chi^2$ is able to detect RFI events which remain undetectable to SSINS and AOFlagger, especially in the domain of long-duration, weak RFI from digital television. We also discuss the shortcomings of this approach and discuss examples of RFI which seems undetectable using $\chi^2$ while being successfully flagged by SSINS and/or AOFlagger.

Precise instrumental calibration is of crucial importance to 21-cm cosmology experiments. The Murchison Widefield Array’s (MWA) Phase II compact configuration offers us opportunities for both redundant calibration and sky-based calibration algorithms; using the two in tandem is a potential approach to mitigate calibration errors caused by inaccurate sky models. The MWA Epoch of Reionization (EoR) experiment targets three patches of the sky (dubbed EoR0, EoR1, and EoR2) with deep observations. Previous work in Li et al. (2018) and (2019) studied the effect of tandem calibration on the EoR0 field and found that it yielded no significant improvement in the power spectrum (PS) over sky-based calibration alone. In this work, we apply similar techniques to the EoR1 field and find a distinct result: the improvements in the PS from tandem calibration are significant. To understand this result, we analyse both the calibration solutions themselves and the effects on the PS over three nights of EoR1 observations. We conclude that the presence of the bright radio galaxy Fornax A in EoR1 degrades the performance of sky-based calibration, which in turn enables redundant calibration to have a larger impact. These results suggest that redundant calibration can indeed mitigate some level of model incompleteness error.

The Precision Array for Probing the Epoch of Reionization (PAPER) was a first-generation 21 cm cosmology experiment with the specific goal of detecting the power spectrum of the 21 cm emission from the Epoch of Reionization. Analysis of PAPER data is still ongoing, but lessons learned from PAPER to date have played a critical role in designing the next-generation Hydrogen Epoch of Reionization Array (HERA) experiment. This article reviews five key design choices made by PAPER: use of a non-imaging configuration, redundancy, short baselines, small antenna elements, and a large instantaneous bandwidth. We describe the impact of these choices and the role they played in designing HERA.