  Direction-dependent instrumental polarisation introduces wide-field
polarimetric aberrations and limits the dynamic range of low-frequency
interferometric images. We therefore provide a detailed two-dimensional
analysis of the Giant Metrewave Radio Telescope (GMRT) primary beam in
full-Stokes at 325 MHz and 610 MHz. We find that the directional dependence is
essentially independent of the feed and is dominated by the curvature of the
dishes reflecting mesh. The developed beam models are used to reduce wide-field
instrumental polarisation in 610 MHz observations by subtracting the expected
response from the $uv$-data itself. Furthermore, a new technique for
polarisation angle calibration is presented that allows for calibration using
an unpolarised source and therefore can be implemented at arbitrarily low
observational frequencies. This technique has the advantage that it calibrates
the polarisation angle independently of ionospheric Faraday rotation and source
variability. It also removes the need for known polarised sources on the sky -
which are scarce at low frequencies. We use the technique to retrieve the
Rotation Measure of pulsar B1937+21 at 325 MHz, finding it to be consistent
with previous independent measurements. An extended version of this method may
be useful for verifying the calibration of other interferometers intended for
polarimetric surveys.
