GPS satellites provide service to civilian and military users. The civilian service is freely available to all users on a continuous, worldwide basis. The military service is available to U.S. and allied armed forces as well as approved Government agencies.
The Global Positioning System is a U.S.-owned utility that provides users with positioning, navigation, and timing services. When people talk about “a GPS,” they usually mean a GPS receiver. The U.S. military developed and implemented this satellite network as a military navigation system, but soon opened it up to everybody else. They develops, maintains, and operates the space and control segments.
This system consists of three segments: the space segment, the control segment, and the user segment.
Space Segment :
The Global Positioning System (GPS) is actually a constellation of 27 Earth-orbiting satellite (24 in operation and three extras in case one fails). Active satellite transmit one way signal that give the current satellite position and time. Each of these 3,000- to 4,000-pound solar-powered satellites circles the globe at about 12,000 miles (19,300 km), making two complete rotations every day.
The satellites in the GPS constellation are arranged into six equally-spaced orbital planes surrounding the Earth. Each plane contains four “slots” occupied by baseline satellites. This 24-slot arrangement ensures there are at least four satellites in view from virtually any point on the planet.
Control Segment :
The control segment consists of worldwide monitor and control stations that maintain the satellites in their proper orbit through occasional command and adjust the satellite clocks, It tracks the GPS satellites, upload updated navigational data and maintains health and status of the satellite.
User Segment :
The user segment consists of the GPS receiver equipment, which receives the signals from the GPS satellites and uses the transmitted information to calculate the user’s three dimensional position and time.
A GPS receiver’s job is to locate four or more of these satellites, figure out the distance to each, and use this information to deduce its own location. This operation is based on a simple mathematical principle called trilateration.
In order to make this simple calculation, then, the GPS receiver has to know two things:
- The location of at least three satellites above you
- The distance between you and each of those satellites
The GPS receiver figures both of these things out by analyzing high-frequency, low-power radio signals from the GPS satellites. Better units have multiple receivers, so they can pick up signals from several satellites simultaneously.
Here is a visualization how a GPS receiver find out his location.
With 2 satellite :
With 3 Satellite.
Question for test your knowledge :
GPS satellites continuously track and record the movements of every GPS user.
True or false?