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.



Each GPS satellite transmits
data that indicates its location and the current time. All GPS
satellites synchronize operations so that these repeating signals are
transmitted at the same instant. The signals, moving at the speed of
light, arrive at a GPS receiver at slightly different times because
some satellites are farther away than others. The distance to the GPS
satellites can be determined by estimating the amount of time it takes
for their signals to reach the receiver. When the receiver estimates
the distance to at least four GPS satellites, it can calculate its
position in three dimensions.

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.



Remember before GPS there was the
Sextant



May Your Next Trip Be Red Light Free And
Easily Found!

Thanks E & B Enterprises

For additional information on GPS history and other related topics to
GPS Visit

OUR links page