We present results of a study of the localization capability of Fermi-LAT, using a large set of blazars with precise radio locations. Since the width of the PSF decreases with energy, the performance is typically dominated by a few high energy photons, so it is important to properly characterize the high-energy PSF. Using such data, we have found a need to modify the pre-launch high-energy (greater than a few GeV) PSF derived from extensive Monte Carlo simulations of particle interactions in the LAT; the resulting data-based PSF is shown
@article{arxiv.0912.3855,
title = {Measurement of the Fermi-LAT Localization Performance},
author = {T. H. Burnett and M. Kerr and M. Roth},
journal= {arXiv preprint arXiv:0912.3855},
year = {2009}
}