Mosts questions on the Global Positioning System (GPS) are related to the actual status of the system. For a given position on the surface of the Earth, the actual number of visible satellites, their trajectories, the time of their rising and setting, the actual receiver position, the actual GMT, etc are the parameters of interest. The MONITOR displays all these issues on a carefully designed graphical user interface. The establishment of the MONITOR is based on a complete understanding of how GPS works. This conceptual undertstanding is converted into a comprehensive computation.