We present in this paper the results of a comprehensive GEANT simulation of the Gamma-Ray Imaging Detector ( GRID ) being developed for the AGILE space astrophysics mission .
The GRID is designed to be sensitive in the \sim 30 MeV–50 GeV energy range , with excellent imaging and timing capabilities , and with a very large field-of-view ( \sim 3 sr ) .
In this paper ( Paper I ) we present the GRID baseline geometry , a model for the charged particle and albedo-photon backgrounds for an equatorial orbit of 550-600 km altitude , and the main results of the first level ( Level-1 Trigger ) on-board data processing .
Our simulations show that the GRID Level-1 data processing is expected to be capable of decreasing by a factor of \sim 20 the charged particle background ( from \sim 2 kHz to below 100 Hz ) , and by a factor of \sim 30 the albedo-photon background .
The gamma-ray photon detection efficiency by the imaging GRID is simulated to be particularly efficient , varying between 39 % and 17 % depending on photon energies and incident directions .