Comparative studies of 3D multi-fluid simulations with Galileo magnetometer data are used to develop a quantitative model of the currents and fields within Ganymede's magnetosphere as well as its bulk plasma environment. The model includes contributions from Jupiter's magnetosphere and the flux of different ion species originating from Ganymede's ionosphere. Comparisons between the magnetometer data and the simulation demonstrate good agreement for the strength and structure of Ganymede's magnetosphere. An ionospheric outflow rate of ~1026 ions/s was found for the simulation, which is well correlated to the sputtering rate determined for the surface of Ganymede that actively supplies the ionosphere. Qualitative comparisons are made with the Hubble Space Telescope observations of Ganymede's UV aurora. The size and location of regions of Jovian magnetospheric plasma precipitation are similar to the observed UV emissions. Plasma acceleration due to reconnection and the size of Ganymede's cusps are also examined.