| |
The
Global Positioning System is a network of twenty-four satellites, and
five ground stations, designed to provide anyone carrying a portable satellite
receiver with a highly specific determination of his or her location,
anywhere, anytime, and in any weather. It promises that people and their
vehicles will never get lost ... missiles and bombs, as well as airplanes,
will land exactly where they ought to ... and a world of stationary objects,
from telephone poles to wetlands to private homes, will be fixed once
and for all in their proper places.
The
satellites, launched and operated by the U.S. military, are arranged in
orbits that make it possible for at least four of them to be "seen" at
one time by a receiver anywhere on earth, and constantly emit signals
specifying the time and their own positions. The GPS receiver measures
the time the different signals take to reach it, and by comparing that
with what it learns about where the satellite is, can calculate its own
position. Because the signals can become degraded in the atmosphere, and
because the military purposely distorts them, these position readings
are only accurate to within about 100 meters. But if the receiver is connected
to another receiver at a known location or "base station" within roughly
300 miles (the area covered by one group of four satellites), the errors
currently in effect can be measured very exactly and the readings corrected
accordingly. This is called "differential correction," and can ordinarily
bring the accuracy of the readings to within 2 to 5 meters. With very
advanced signal processing equipment, and a real-time link to the base
station, it is said, instant point locations accurate to about a centimeter
can be obtained.
How
to locate a point with GPS? Stand somewhere open to the sky, with a GPS
receiver, for ten minutes, and collect a stream of position readings.
[Scatter] Atmospheric interference, military scrambling, and the
"multipathing" bounce of the signals in a built environment, combine to
represent that stationary position as a complex scattering of points.
Download the data to a computer from the receiver, and then download from
a local base station its (scattered) readings for the same time period
[Reference]. Correct your readings and reduce the drift [Average],
average the points, and learn where you were ... within a few meters [Average].
In the computer, the satellites draw the points for you, and as the readings
become more precise the points grow to fill the screen.
|
