In Simple Terms, How Does GPS Work?
Dept. of Geodesy and Geomatics Engineering
24 March 2003
Updated 6 January 2006
Updated 20 October 2007
Updated 16 February 2008
The Navstar Global Positioning System, or GPS for short, consists of three segments or components:
• A constellation of satellites (currently 30*) orbiting about 20,000 km above the earth’s surface which transmit ranging signals on two frequencies in the microwave part of the radio spectrum. For an accurate depiction of the actual orbiting constellation, see the QuickTime movie.
• A control segment which maintains GPS through a system of ground monitor stations and satellite upload facilities.
• The user receivers – both civil and military.
Each satellite transmits a unique digital code sequence of 1’s and 0’s – precisely timed by an atomic clock – which is picked up by a GPS receiver’s antenna and matched with the same code sequence generated inside the receiver. By lining up or matching the signals, the receiver determines how long it takes the signals to travel from the satellite to the receiver. These timing measurements are converted to distances using the speed of light (about 300,000 kilometres per second), the same speed with which radio waves travel.
Measuring distances to four or more satellites simultaneously and knowing the exact locations of the satellites (included in the signals transmitted by the satellites), the receiver can determine its latitude, longitude, and height while at the same time synchronizing its clock with the GPS time standard which also makes the receiver a precise time piece.
Determining a position from measurements of distances is known as trilateration (not triangulation, which involves the measurement of angles; and the term multilateration is sometimes used when more than three distances are involved).
GPS receivers are just that: receivers. They receive satellite signals; they don’t transmit or bounce signals off the satellites. By being a passive, receive-only system, GPS can support an unlimited number of users, both military and civilian.
GPS provides 24 hour per day global coverage. It is an all-weather system and is not affected by rain, snow, fog, or sand storms.
GPS is a dual-use system – both civil and military – and is controlled by a joint civilian/military executive board of the U.S. government. The system is maintained by the U.S. Air Force on behalf of all users.
Most civil GPS receivers access the C/A-code (coarse/acquisition code) transmitted on the L1 frequency (1575.42 MHz). Military receivers, in addition, use the encrypted P-code (precise or precision code) which is transmitted on both L1 and the L2 frequency (1227.60 MHz). Some military receivers can access the P-code directly rather than acquiring the C/A-code first and then transferring to the P-code.
Starting with the GPS satellite launched on 16 December 2005, the newest satellites transmit a new civil signal, called L2C, on the L2 frequency. Although some specialized dual-frequency civil GPS receivers could suboptimally make measurements on the L2 frequency with the legacy signals (and by combining the measurements with L1 measurements remove most of the ionosphere's effect on the GPS signals), the new signal will make this task easier and more precise.
Depending on the quality of the receiver, the environment, the type of measurements made, and how the measurements are processed, the positioning accuracy of GPS (latitude and longitude within the true position, 19 times out of 20) can vary from a few metres to below 1 centimetre, permitting a wide range of positioning applications from vehicle navigation to studies of the motion of the Earth's tectonic plates.
* For current constellation information, see Latest GPS Constellation Status or GPS Status.