Global Positioning System satellites transmit signals to equipment on the ground. GPS receivers passively receive satellite signals; they do not transmit. GPS receivers require an unobstructed view of the sky, so they are used only outdoors and they often do not perform well within forested areas or near tall buildings. GPS operations depend on a very accurate time reference, which is provided by atomic clocks at the U.S. Naval Observatory. Each GPS satellite has atomic clocks on board.
There are at least 24 operational GPS satellites at all times. The satellites, operated by the U.S. Air Force, orbit with a period of 12 hours. Ground stations are used to precisely track each satellite's orbit.
Determining Position
A GPS receiver "knows" the location of the satellites, because that
information is included in satellite transmissions. By estimating how far
away a satellite is, the receiver also "knows" it is located somewhere
on the surface of an imaginary sphere centered at the satellite. It then determines
the sizes of several 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-held GPS units have about 10-20 meter accuracy. Other types of receivers
use a method called Differential GPS (DGPS) to obtain much higher accuracy.
DGPS requires an additional receiver fixed at a known location nearby. Observations
made by the stationary receiver are used to correct positions recorded by
the roving units, producing an accuracy greater than 1 meter.
When the system was created, timing errors were inserted into GPS transmissions to limit the accuracy of non-military GPS receivers to about 100 meters. This part of GPS operations, called Selective Availability, was eliminated in May 2000.