  We propose a new scheme for an improved determination of the Newtonian
gravitational constant G and evaluate it by numerical simulations. Cold atoms
in free fall are probed by atom interferometry measurements to characterize the
gravitational field generated by external source masses. Two source mass
configurations having different geometry and using different materials are
compared to identify an optimized experimental setup for the G measurement. The
effects of the magnetic fields used to manipulate the atoms and to control the
interferometer phase are also characterized.
