  One of the scientific objectives of NASA's Fermi Gamma-ray Space Telescope is
the study of Gamma-Ray Bursts (GRBs). The Fermi Gamma-Ray Burst Monitor (GBM)
was designed to detect and localize bursts for the Fermi mission. By means of
an array of 12 NaI(Tl) (8 keV to 1 MeV) and two BGO (0.2 to 40 MeV)
scintillation detectors, GBM extends the energy range (20 MeV to > 300 GeV) of
Fermi's main instrument, the Large Area Telescope, into the traditional range
of current GRB databases. The physical detector response of the GBM instrument
to GRBs is determined with the help of Monte Carlo simulations, which are
supported and verified by on-ground individual detector calibration
measurements. We present the principal instrument properties, which have been
determined as a function of energy and angle, including the channel-energy
relation, the energy resolution, the effective area and the spatial
homogeneity.
