The Accuracy of GBM GRB Locations

The Fermi Gamma-Ray Burst Monitor (GBM) locates transient gamma-ray sources using the relative rates in the twelve NaI detectors. The location algorithm is described. This method is subject to both statistical and systematic errors. Three types of locations are produced: automatic locations by the Flight Software onboard GBM, automatic locations by ground software, and human-guided locations. A Bayesian model comparison method is used to analyze the error distributions of the GBM GRB locations. The analysis uses samples of accurate reference locations provided by other instruments.