Recovery of Large Angular Scale CMB Polarization for Instruments
  Employing Variable-delay Polarization Modulators
  Variable-delay Polarization Modulators (VPMs) are currently being implemented
in experiments designed to measure the polarization of the cosmic microwave
background on large angular scales because of their capability for providing
rapid, front-end polarization modulation and control over systematic errors.
Despite the advantages provided by the VPM, it is important to identify and
mitigate any time-varying effects that leak into the synchronously modulated
component of the signal. In this paper, the effect of emission from a $300$ K
VPM on the system performance is considered and addressed. Though instrument
design can greatly reduce the influence of modulated VPM emission, some
residual modulated signal is expected. VPM emission is treated in the presence
of rotational misalignments and temperature variation. Simulations of
time-ordered data are used to evaluate the effect of these residual errors on
the power spectrum. The analysis and modeling in this paper guides
experimentalists on the critical aspects of observations using VPMs as
front-end modulators. By implementing the characterizations and controls as
described, front-end VPM modulation can be very powerful for mitigating $1/f$
noise in large angular scale polarimetric surveys. None of the systematic
errors studied fundamentally limit the detection and characterization of
B-modes on large scales for a tensor-to-scalar ratio of $r=0.01$. Indeed,
$r<0.01$ is achievable with commensurately improved characterizations and
controls.
