Global Positioning System (GPS) satellites transmit signals to equipment on the ground. GPS receivers passively receive satellite signals, which do not transmit. GPS receivers require a clear vision of sky, so they are used only outdoors and often do not perform well in wooded areas or near tall buildings. GPS operations depend on a very precise time reference, which is provided by atomic clocks of the U.S. Naval Observatory. Each GPS satellite has atomic clocks on board.
Each GPS satellite transmits data that indicates its location and current time. All GPS satellites synchronize operations so that these signals are transmitted to repeat once. The signals, moving at the speed of light, arrive at a GPS receiver at slightly different times because some satellites farther than others. The distance of the GPS satellites can be determined by estimating the amount of time it takes for their signals to reach the receiver.
When the receiver calculates the distance of at least four GPS satellites, you can calculate its position in three dimensions. There are at least 24 operational GPS satellites at any time. The satellites, operated by U.S. Air Force, orbit with a period of 12 hours. Earth stations are used to accurately track each orbit satellite.
Determine the location of a GPS receiver "knows" the position of satellites, as such information is included in broadcasts via satellite. Investigating to what extent is a satellite, the receiver also "knows" that is located somewhere on the surface of an imaginary sphere centered on the satellite. Then determines the size of different spheres, one for each satellite. The receiver is located where these spheres intersect.
GPS Accuracy The accuracy of a position determined with GPS depends on the type of receiver. Most hand-GPS has an accuracy of about 10-20 meters. Other types of receivers use a method called Differential GPS (DGPS) to obtain much greater accuracy. DGPS requires an additional receiver fixed at a known location nearby. Comments from the stationary receiver are used to correct positions recorded by the mobile units, producing an accuracy of better than 1 meter. When the system was created in timing errors were inserted into GPS transmissions to limit the accuracy of receptors for non-military GPS around 100 meters. This part of GPS operations, called Selective Availability, was eliminated in May 2000.
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