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Global Positioning System (GPS)
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History of GPS
Until a few years ago,
the most accurate way of calculating your position in the middle of an
ocean involved using a device called a
sextant to calculate the bearing
and height above the horizon of the sun or certain stars. Combined with
an accurate timepiece, this method carried millions of people all over
the world, but was not accurate enough to enable precise navigation in
fog, rain or poor visibility.
In the late 1960s, during the height of the Cold War, the US Department of Defense began to
develop satellite-based navigation support for
ballistic missiles to enable their precise launch and delivery. In
1973, the US Navy and Air Force systems were combined in order to
provide accurate data on position and time (and therefore velocity) to
military and civilian users alike. This system was called NAVSTAR, and gradually became known
as the Global Positioning System (GPS).
Over time, the cost and size of GPS-based navigation equipment has
dropped, and the lifting of Selective Availability (S/A) in 2000 means
that even the cheapest civilian equipment can now calculate position to
within a few metres. Previously, this accuracy was only available to US
military and governmental organisations.
How
GPS Works
The fundamental way
GPS works is in fact relatively simple. GPS is made up of a
constellation of 28 satellites
orbiting 20,110 kilometres (12,500 miles) above the Earth. At any one
time, at least four of these satellites should be "visible" from any
point on the Earth's surface.
Each satellite follows an extremely accurately-controlled fixed orbit
and must remain in exactly the same position at all times. The GPS Master Control Station in
Colorado, USA, monitors all GPS satellites to make sure that they
remain in the right orbit and that the atomic clocks they have on board
are extremely accurate.
Each satellite transmits a constant stream of information towards the
Earth's surface - this stream's transmission time is synchronised with the streams of all
the other satellites.
Once a GPS device is receiving a signal from at least 3 of the GPS
satellites, because the GPS device knows the positions of all the
satellites in the constellation, that the signals were transmitted at
the same time, and that signals always travel at a constant speed (the
Speed of Light) from the satellite, it is possible to calculate
the relative distance of each
GPS satellite from each satellite by comparing the difference in time
of arrival of each signal. From this the GPS device can calculate its
exact position on the surface of the Earth.
New technology to improve the accuracy of GPS is beginning to appear on
the market. The Wide Area Augmentation System (WAAS) is a land-based
system across North American which gives WAAS-equipped devices 1-2
metres horizontal and 2-3 metres vertical accuracy, sufficient to allow
aircraft to land automatically on small runways in zero visibility.
Mobile telephony networks are beginning to use Assisted GPS (A-GPS) to
allow mobile telephones to use GPS to determine their positions both
inside and outside buildings.
GPS
Devices
When GPS devices first
became available to the civilian market, they cost as much as a small
car, used a great deal of power, and were as big as an average
suitcase. Over time, the cost, size and power consumption have dropped
so that devices the size of a pack of cards are now available, which
run for many hours on two small batteries, can communicate wirelessly
with a computer or PDA, and cost less than US$100 (~€90 or £75).
The following are links to well-known manufacturers of GPS devices:
Benefon
Garmin
Magellan
Navman
Navtrak-CoPilot
Thales
Trimble
Each of these manufacturers produces GPS devices which will work well
with A2B.
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