GPS (Global Positioning System)
GPS (Global Positioning System) is a satellite-based system that allows users to calculate their locations anywhere on Earth. There are currently two "public" GPS systems. The NAVSTAR system is owned by the United States and is managed by the US Department of Defense. The GLONASS system is owned by the Russian Federation. While both NAVSTAR and GLONASS systems are global positioning systems, the NAVSTAR system is often referred to (in the US anyway) as the GPS because it was generally available first. Nevertheless, both systems are GPS systems and work in essentially the same way.
The satellites of a global positioning system transmit timing information, satellite location information, and satellite "health" information. The Space Segment is the 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 satellites. 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. Future systems, known generically as GNSS (global navigation satellite system) will use either or both NAVSTAR and GLONASS to provide positioning, along with other components to improve accuracy and provide fast warning of any problems.