Overview of GLAST Simulation and Reconstruction

GLAST is a pair conversion telescope due to be launched into low earth orbit in late 2006. We give here an overview of the simulation and reconstruction packages. GLAST has adopted several external packages to assist in their creation. We have used the Gaudi framework to create a set of Algorithms which tie together the GEANT4 simulation engine with a common xml geometry description used by both simulation and reconstruction. A simple interface Algorithm has been developed to control GEANT4, while a C++ visitor implements the geometry generically in GEANT4. Persistency is provided via Root I/O. A MySql database provides the metadata to the Calibration service to provide access to xml and Root calibration files. The tracker reconstruction provides a unique challenge for precision tracking in a pair conversion telescope. We have exploited Gaudi Tools extensively to design our reconstruction to maximize the flexibility for mixing and matching new techniques as we develop them.