A global positioning system uses the characteristics of radio transmissions for location determination. Unlike previous navigation systems using ground based transmitters, satellite based transmitters are used to cover earth with higher accuracy that that available from the land based systems. The satellites transmit timing information, satellite location information and satellite health information. The Space Segment is technical term for the satellites that belong to the system.
The user requires a special radio receiver - a GPS receiver - to receive the transmissions from the satellite. The GPS receiver contains a specialized computer that calculates the location based on the satellite signals. The user does not have to transmit anything to the satellite and the satellite does not know the user is there. There is no limit to the number of users that can be using the system at any one time. The users with their receivers are known as the User Segment.
The satellites are controlled and monitored from ground stations (the Control Segment). The control stations monitor the satellites for health and accuracy. Maintenance commands, orbital parameters and timing corrections are uploaded from the ground on a periodic basis.
Both NAVSTAR and GLONASS provide two sets of positioning signals. The higher accuracy system is reserved for each country's military use. The lower accuracy system is freely available to civilian users.