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GPS is managed and developed by the United States and was originally intended for military applications. However, the diffusion of technology and the mass market for GPS have today made it increasingly important to guarantee civilian applications as well.
GPS is designed to guarantee access to at least four satellites no matter where you are on earth. At least four satellites are also the minimum requirement for a GPS receiver to be able to determine its four basic parameters: three-dimensional coordinates in the GPS reference frame WGS 84, and the receiver clock error.
American satellite system
The Global Positioning System (GPS) was the first GNSS to be developed, beginning in the 1970s.
The ordinary GPS satellite constellation means that at least 24 satellites are in operation, evenly distributed over six separate orbital planes about 20,000 kilometres above the earth's surface. The GPS satellites transmit so-called navigation messages on two separate frequencies, L1 and L2, both of which are in the L-band within the ultra-high frequency spectrum (approximately 1-2 GHz). The message consists of a radio signal which is modulated with satellite-specific codes, allowing a GPS receiver to quickly identify which satellite the signal is coming from.
Control of the system
The GPS satellites are controlled and adjusted through a number of ground-based control stations. The control stations follow the orbits of the satellites and can correct them if necessary.The control stations are also the base for what information the GPS satellites are to send out in the navigation message, i.e. the coded signals.
Since the turn of the millennium, GPS has been modernised, which means, among other things, improved models of the satellites. The improvements also include the transmission of additional signals (L2C and L5) which replace the older ones.