  The Wide-Area Linear Optical Polarimeter (WALOP)-South instrument is an
upcoming wide-field and high-accuracy optical polarimeter to be used as a
survey instrument for carrying out the Polar-Areas Stellar Imaging in
Polarization High Accuracy Experiment (PASIPHAE) program. Designed to operate
as a one-shot four-channel and four-camera imaging polarimeter, it will have a
field of view of $35\times 35$ arcminutes and will measure the Stokes
parameters $I$, $q$, and $u$ in a single exposure in the SDSS-r broadband
filter. The design goal for the instrument is to achieve an overall
polarimetric measurement accuracy of 0.1 % over the entire field of view. We
present here the complete polarimetric modeling of the instrument,
characterizing the amount and sources of instrumental polarization. To
accurately retrieve the real Stokes parameters of a source from the measured
values, we have developed a calibration method for the instrument. Using this
calibration method and simulated data, we demonstrate how to correct
instrumental polarization and obtain 0.1 % accuracy in the degree of
polarization, $p$. Additionally, we tested and validated the calibration method
by implementing it on a table-top WALOP-like test-bed polarimeter in the
laboratory.
