  Polarization leakage of foreground synchrotron emission is a critical issue
in HI intensity mapping experiments. While the sought-after HI emission is
unpolarized, polarized foregrounds such as Galactic and extragalactic
synchrotron radiation, if coupled with instrumental impurity, can mimic or
overwhelm the HI signals. In this paper we present the methodology for
polarization calibration at 700-900 MHz, applied on data obtained from the
Green Bank Telescope (GBT). We use astrophysical sources, both polarized and
unpolarized sources including quasars and pulsars, as calibrators to
characterize the polarization leakage and control systematic effects in our GBT
HI intensity mapping project. The resulting fractional errors on polarization
measurements on boresight are well controlled to within 0.6%-0.8% of their
total intensity. The polarized beam patterns are measured by performing spider
scans across both polarized quasars and pulsars. A dominant Stokes I to V
leakage feature and secondary features of Stokes I to Q and I to U leakages in
the 700-900 MHz frequency range are identified. These characterizations are
important for separating foreground polarization leakage from the HI 21 cm
signal.
