How GPS works:

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*When people talk about "a GPS,"

they usually mean a GPS receiver.


**The Global Positioning System
(GPS) is actually a constellation of
27 Earth-orbiting satellites (24 in
operation and three extras in case
one fails).


** The U.S. military developed and
implemented this satellite
network as a military navigation
system, but soon opened it up to
everybody else.


**Each of these 3,000- to 4,000-
pound solar-powered satellites
circles the globe at about 12,000
miles (19,300 km), making two
complete rotations every day.


** The orbits are arranged so that
at any time, anywhere on Earth,
there are at least four satellites
"visible" in the sky.


**A GPS receiver's job is to locate
four or more of these satellites,
figure out the distance to each,
and use this information to
deduce its own location.
This operation is based on a
simple mathematical principle
called TRILATERATION.
GPS receiver has to know two
things


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**The location of at least three
satellites above you


*The distance between you and
each of those satellites


*The GPS receiver figures both of
these things out by analyzing
high-frequency, low-power radio
signals from the GPS satellites.
Better units have multiple
receivers, so they can pick up
signals from several satellites
simultaneously.


**You can use maps stored in the
receiver's memory, connect the
receiver to a computer that can
hold more detailed maps in its
memory, or simply buy a detailed
map of your area and find your
way using the receiver's latitude
and longitude readouts.


** Some receivers let you
download detailed maps into
memory or supply detailed maps
with plug-in map cartridges.


**A standard GPS receiver will not
only place you on a map at any
particular location, but will also
trace your path across a map as
you move.


* If you leave your receiver on, it
can stay in constant
communication with GPS satellites
to see how your location is