The GBM includes two sets of detectors: twelve sodium iodide (NaI) scintillators, each 12.7 cm in diameter by 1.27 cm thick, and two cylindrical bismuth germanate (BGO) scintillators, each 12.7 cm in diameter and 12.7 cm in height. The NaI detectors are sensitive in the lower end of the energy range, from a few keV to about 1 MeV and provide burst triggers and locations. The BGO detectors cover the energy range ~150 keV to ~30 MeV, providing a good overlap with the NaI at the lower end and with the LAT at the high end.
GRBs will be detected by a significant change in count rate in at least two of the NaI scintillators; the trigger algorithm will be programmable. Time-tagged event data (with 5 μs resolution) will be recorded continuously to provide ~50 s of pre-trigger information for GRBs. After a trigger, the GBM processor will calculate preliminary position and spectral information for telemetry to the ground and possible autonomous repointing of Fermi. The GBM is projected to detect ~200 GRBs per year.