weather (but typically will not work inside a building), and the system is accessible
at all times.
The applications of GPS technology are virtually limitless. Inexpensive GPS
receivers can pinpoint the user’s location within a few meters, and provide a map
showing the location. In addition, the location can be stored, so that the user can return
to virtually the exact same place in the future. More expensive GPS receivers can pro-
vide the location of the user, or an object being mapped, to within a few centimeters.
GPS is a highly useful tool for outdoor recreation. Those engaged in hiking,
fishing, and hunting can find their location if lost, record a favorite spot and the
path to reach it, or even map an area using a GPS receiver. Business applications
are also numerous. Law enforcement can track the movements of suspected crimi-
nals by attaching a GPS unit to a vehicle. Surveyors can use GPS to survey a piece
of property much more quickly than when using older techniques. Farmers can use
high-precision planting techniques that employ GPS technology, increasing yields
and conserving costs of production. Aircraft can be precisely tracked by ground
controllers using GPS, and the aircraft itself can use GPS for navigational pur-
poses. There are also applications for forestry, wildlife management, and many
other fields. Of course, perhaps the broadest application of GPS technology is to
commercial mapping, which can now be done more quickly and more accurately.
There are three components to the GPS system. First, there is the ground control
component. This consists of a system of control stations located on the surface of
the Earth that regulates the signals being sent from the satellites. There is a master
control station, currently located in Colorado, a backup control station used only if
the master control station ceases to function, and several monitoring stations
located around the world. The master controlstationisconstantlyinoperation
and regularly receives data from the monitoring stations that has been downloaded
from the satellites and corrects any errors in the signals. The second component is
the network of satellites. The satellites are not in stationary orbit but revolve
around the planet at an altitude of about 12,000 miles, and are arranged in space
so that at least four are above the horizon at all times, as seen from any point on
the Earth’s surface. The final component of the GPS system is the user, who
accesses the signal from the satellite using a GPS receiver. The receiving unit must
acquire signals from four satellites (this is why the satellite system is configured to
have four satellites above the horizon at all times), and can accurately locate the
position of the user by comparing the timerequired for each signal to reach the
unit. Any errors that may be in the signal must be corrected by the master control
station, or the readings provided may be inaccurate. Errors may be generated by
disturbances in the atmosphere, which delay the time the signal reaches the surface
of the Earth. If the time measurement is inaccurate, the location provided by the
receiver will also be inaccurate.156 Global Positioning System