  POLAR is a new satellite-born detector aiming to measure the polarization of
an unprecedented number of Gamma-Ray Bursts in the 50-500 keV energy range. The
instrument, launched on-board the Tiangong-2 Chinese Space lab on the 15th of
September 2016, is designed to measure the polarization of the hard X-ray flux
by measuring the distribution of the azimuthal scattering angles of the
incoming photons. A detailed understanding of the polarimeter and specifically
of the systematic effects induced by the instrument's non-uniformity are
required for this purpose. In order to study the instrument's response to
polarization, POLAR underwent a beam test at the European Synchrotron Radiation
Facility in France. In this paper both the beam test and the instrument
performance will be described. This is followed by an overview of the Monte
Carlo simulation tools developed for the instrument. Finally a comparison of
the measured and simulated instrument performance will be provided and the
instrument response to polarization will be presented.

Instrumentation and Methods for Astrophysics;Instrumentation and Detectors