__Origins__
The global positioning system is designed by the Department of Defense and
the Department of Transportation of the United States of America. On April 27,
1995 the system, containing 24 operational satellites, was formally declared as
able to meet the requirement of *Full Operation Capability*. Since then,
the system has been taken into full use.
__How it works__
The GPS satellite system is based on the concept of *trilateration*. The heart of a GPS
receiver has the ability to find the satellite's distance from four (or more) GPS
satellites. Once it determines its distance from the four satellites, the
receiver can calculate its exact location and altitude on Earth. If the receiver
can only find three satellites, then it can use an imaginary sphere to represent
the Earth and can give you location information but no altitude information.
For a GPS receiver to find your location, it has to determine two things:
- The location of at least three
satellites above you
- The distance between you and
each of those satellites
GPS satellites send out radio signals that your GPS receiver can detect. Then
the receiver measures the amount of time it takes for the signal to travel from
the satellite to the receiver. The receiver will receive signals at least from
four satellites. Since radio signals travel at the speed of light, we can figure
out how far they've traveled by figuring out how long it took for them to
arrive.
The receiver looks at all the signals that it receives and calculates exact
time and the exact location simultaneously using a normal quartz clock. When
measuring the distance to four located satellites, four spheres that are drawn
should intersect at one point. Since the receiver makes all of its time and
distance measurements, using the clock it is equipped with, the distances could
be proportionally incorrect. However, the receiver can easily calculate exactly what
distance adjustment will cause the four spheres to intersect at one point. This
allows it to adjust its clock to adjust its measure of distance. For this
reason, a GPS receiver actually measures extremely accurate time.
Information that GPS receivers can provide
**1)
**Coordinates information on electronic maps which is stored in memory
**2)
**Distance traveled
**3)
**Current speed and average speed
__Remarks__
One problem with this method is the measure of speed. The Earth is not a
vacuum, and its atmosphere slows down the transmission of the signal. A GPS receiver
estimates the actual speed of the signal, using complex mathematical models of a
wide range of atmospheric conditions. The satellites can also transmit
additional information to the receiver.
The satellites send radio signals to GPS receivers so that the receivers can
find out how far away each satellite is. Because the satellites are orbiting at
a distance of 12,660 miles (20,370 km) overhead, the signals are fairly weak by
the time they reach your receiver. This implies that receivers have to be in a
fairly open area in order to work. |