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Detecting the cosmological sky-averaged (global) 21 cm signal as a functionof observed frequency will provide a powerful tool to study the ionization andthermal history of the intergalactic medium (IGM) in the early Universe ($\sim$400 million years after the Big Bang). The greatest challenge in conventionaltotal-power global 21 cm experiments is the removal of the foregroundsynchrotron emission ($\sim 10^3$-$10^4$ K) to uncover the weak cosmologicalsignal (tens to hundreds of mK), especially since the intrinsic smoothness ofthe foreground spectrum is corrupted by instrumental effects. Although theEDGES team has recently reported an absorption profile at 78 MHz in thesky-averaged spectrum, it is necessary to confirm this detection with anindependent approach. The projection effect from observing anisotropicforeground source emission with a wide-view antenna pointing at the NorthCelestial Pole (NCP) can induce a net polarization, referred as theProjection-Induced Polarization Effect (PIPE). Due to Earth's rotation,observation centered at the circumpolar region will impose a dynamic skymodulation on the net polarization's waveforms which is unique to theforeground component. In this study, we review the implementation practicalityand underlying instrumental effects of this new polarimetry-based techniquewith detailed numerical simulation and a testbed instrument, the CosmicTwilight Polarimeter (CTP). In addition, we explore an SVD-based analysisapproach for separating the foreground and instrumental effects from thebackground global 21 cm signal using the sky-modulated PIPE.

Assessment of the Projection-induced Polarimetry Technique for Constraining the Foreground Spectrum in Global 21 cm Cosmology