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GPS is a space-based system of 24 satellites launched by the US
military since 1978 for a cost of approximately $10 billion. Each
Navstar satellite, 12,400 miles above the Earth with a twelve hour
period, transmits a precise radio wave synchronized by ultraprecise
clocks. Ground stations in Colorado Springs, Hawaii, Ascension
Island, Diego Garcia, and Kwajalein monitor the satellites and make
continuous corrections as needed after coordination through the
master control center at Falcon AFB in Colorado. A hand-held
receiver, such as those manufactured by Trimble, consists of an
antenna, microprocessor, display and a power supply. It can
compare the signals from at least three of the possible eight
satellites built by Rockwell that are in range at any instant. The
signal is used to determine the exact distance to each satellite.
From the distance information the current location is determined. As
microprocessor technology, miniaturization and power cell technology has advanced, the size of the receivers have decreased
dramatically. They soon will be able to reside on a PCMCIA card for
a laptop or any digital assistant.
Accuracy - the military uses the most precise, encrypted (P/A)
signals that come from the satellites. Although no published
criteria are available, the accuracy is reported to reach that of inches. The military allows civilian use of similar (C/A)
signals that are not encoded. These signals are randomly scrambled
to decrease the accuracy to 100 meters.
In response to the decrease in accuracy, several companies have
developed "differential GPS". This system broadcasts a correction
signal from a known location to help triangulation and results in an
accuracy of 20-30 cm. Several ongoing efforts to further increase the
accuracy are being undertaken. At NASA's Goddard Space Flight Center,
scientists are bouncing lasers off each satellite to measure and
correct for tiny irregularities in the Earth's gravitational field.
Accuracy down to five millimeters is expected.
Downside - GPS signals are weak and susceptible to jamming. They
require an unobstructed path to the satellite and hence may not be
suitable for all urban applications without further improvements.
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