Assessment of the Projection-induced Polarimetry Technique for
  Constraining the Foreground Spectrum in Global 21 cm Cosmology
  Detecting the cosmological sky-averaged (global) 21 cm signal as a function
of observed frequency will provide a powerful tool to study the ionization and
thermal history of the intergalactic medium (IGM) in the early Universe ($\sim$
400 million years after the Big Bang). The greatest challenge in conventional
total-power global 21 cm experiments is the removal of the foreground
synchrotron emission ($\sim 10^3$-$10^4$ K) to uncover the weak cosmological
signal (tens to hundreds of mK), especially since the intrinsic smoothness of
the foreground spectrum is corrupted by instrumental effects. Although the
EDGES team has recently reported an absorption profile at 78 MHz in the
sky-averaged spectrum, it is necessary to confirm this detection with an
independent approach. The projection effect from observing anisotropic
foreground source emission with a wide-view antenna pointing at the North
Celestial Pole (NCP) can induce a net polarization, referred as the
Projection-Induced Polarization Effect (PIPE). Due to Earth's rotation,
observation centered at the circumpolar region will impose a dynamic sky
modulation on the net polarization's waveforms which is unique to the
foreground component. In this study, we review the implementation practicality
and underlying instrumental effects of this new polarimetry-based technique
with detailed numerical simulation and a testbed instrument, the Cosmic
Twilight Polarimeter (CTP). In addition, we explore an SVD-based analysis
approach for separating the foreground and instrumental effects from the
background global 21 cm signal using the sky-modulated PIPE.
