  The design of spaceborne high-energy (E>100 MeV) gamma-ray detectors depends
on two principal factors: (1) the basic physics of detecting and measuring the
properties of the gamma rays; and (2) the constraints of operating such a
detector in space for an extended period. Improvements in technology have
enabled major advances in detector performance, as illustrated by two
successful instruments, EGRET on the Compton Gamma Ray Observatory and LAT on
the Fermi Gamma-ray Space Telescope.
