Abstract :
The GPS (Global Positioning System) is a “constellation” of
24 well-spaced satellites that orbit the Earth and make it possible for people
with ground receivers to pinpoint their geographic location. The location
accuracy is anywhere from 100 to 10 meters for most equipment. Accuracy can be
pinpointed to within one (1) meter with special military-approved equipment.
GPS equipment is widely used in science and has now become sufficiently
low-cost so that almost anyone can own a GPS receiver.
GPS receivers are
becoming consumer products. In addition to their outdoor use (hiking,
cross-country skiing, ballooning, flying, and sailing), receivers can be used
in cars to relate the driver’s location with traffic and weather information.
Here are some Web locations that describe GPS receiver products.The GPS is
owned and operated by the U.S. Department of Defense but is available for
general use around the world. Briefly, here’s how it works:
21 GPS satellites and three spare satellites are in orbit at
10,600 miles above the Earth.
The satellites are spaced so that from any point on Earth, four satellites will be above the horizon.
Each satellite contains a
computer, an atomic clock, and a radio. With an understanding of its own orbit
and the clock, it continually broadcasts its changing position and time. (Once
a day, each satellite checks its own sense of time and position with a ground
station and makes any minor correction.) On the ground, any GPS receiver
contains a computer that “triangulates” its own position by getting bearings
from three of the four satellites. The result is provided in the form of a
geographic position - longitude and latitude - to, for most receivers, within
100 meters. If the receiver is also equipped with a display screen that shows a
map, the position can be shown on the map. If a fourth satellite can be
received, the receiver/computer can figure out the altitude as well as the
geographic position.The satellites are spaced so that from any point on Earth, four satellites will be above the horizon.
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