GPS (Global Positing Satellites)
There are at 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.
The accuracy of
a position determined with GPS depends on the type of receiver.
Most hand-held GPS units have 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
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.
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.
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.