  The Global Positioning System (GPS) is a U.S. space-based radionavigation
system that provides reliable positioning, navigation, and timing services to
civilian users on a continuous worldwide basis -- freely available to all. GPS
provides specially coded satellite signals that can be processed in a GPS
receiver, enabling the receiver to compute position, velocity and time.
Basically GPS works by using four GPS satellite signals to compute positions in
three dimensions (and the time offset) in the receiver clock. GPS provides
accurate location and time information for an unlimited number of people in all
weather, day and night, anywhere in the world. Anyone who needs to keep track
of where he or she is, to find his or her way to a specified location, or know
what direction and how fast he or she is going can utilize the benefits of the
global positioning system. Everyday activities such as banking, mobile phone
operations, and even the control of power grids, are facilitated by the
accurate timing provided by GPS.
