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WHAT IS GPS?
The GPS System
The Global Positioning System (or GPS)
is a collection of satellites owned by the U.S. Government that
provides highly accurate, worldwide positioning and navigation
information, 24 hours a day. It is made up of twenty-four
NAVSTAR GPS satellites which orbit 12,000 miles above the
earth, constantly transmitting the precise time and their
position in space. GPS receivers on (or near) the earth's surface,
listen in on the information received from three to twelve
satellites and, from that, determine the precise location of the
receiver, as well as how fast and in what direction it is moving.
It's a question of timing
GPS uses the triangulation of signals
from the satellites to determine locations on earth. GPS
satellites know their location in space and receivers can
determine their distance from a satellite by using the travel time
of a radio message from the satellite to the receiver. After
calculating its relative position to at least 3 or 4
satellites, a GPS receiver can calculate its position using
triangulation. GPS satellites have four highly accurate atomic
clocks on board. They also have a database (or almanac)
of the current and expected positions for all of the satellites
that is frequently updated from earth. That way when a GPS
receiver locates one satellite, it can download all satellite
location information, and find the remaining needed satellites
much more quickly.
Accuracy
Even with highly accurate atomic clocks,
certain errors do creep into the process of determining your
position. Selective Availability (SA) is the program
implemented by the U.S. Department of Defense that makes GPS
less accurate for non-military users for security reasons.
With SA in effect, the accuracy of your position may be within
50 to 100 meters. Even without SA, other errors will be
encountered. The most significant of these errors is due to variations
in the earth's ionosphere, which effects the speed of GPS
radio signals. Another source of error is from water vapor
in the troposphere. Both of these errors are fairly small. The
accuracy of GPS can be improved with DGPS (differential
GPS) capabilities, which is the ability of your receiver to read
signals from a nearby DGPS beacon receiver. (Also see GPS RECEIVER
PERFORMANCE section on the HOW TO USE GPS page)
In May 2000, the Selective Availability affect was discontinued,
and now typical accuracy of most units is 20-50 ft. (up to 15
meters).
In 2001, manufacturers started
making GPS receivers that are capable of getting
WAAS (Wide
Area Augentation System) corrections.
Basically, it's a system of
satellites and ground stations that provide GPS signal
corrections, giving you even better position accuracy. If you are
in an open location and can get the WAAS signal, you can improve
your position accuracy to within 3 meters most of the time.
Civilian equipment and
consumer uses (Fun stuff)
Over the last several years, an increasing
array of affordable GPS receivers have been released for the
average consumer. As the technology has improved, many additional
features are added to these units, while the price and size
continue to decrease. What was state-of-the-art consumer GPS in
1996, is now relegated to the least expensive units, while today's
latest-and-greatest units have features only wished for years ago.
Primarily, these are handheld GPS receivers
that vary in price from $100 to $600. Almost all consumer
GPS receivers are 12 parallel channel, and data capable.
Most can even be connected to a laptop, and used with a
street-level mapping software, for real-time automobile
navigation.
Some specialized GPS receivers for consumers
include:
-- handheld GPS receivers that have background
moving maps (of highways) for car navigation (with and without
mini-cartridges),
-- GPS receivers that provide automatic routing and navigation for
drivers
-- GPS receivers that attach to PDAs (personal data assistants)
-- handheld and mounted GPS receivers for boating and fishing,
-- aviation GPS receivers with built-in Jeppensen airport
information,
-- GPS receivers that combine with radios, or e-mail, into
one unit.
Consumers have been using GPS technology for
business and for outdoor adventures. These activities include
hiking and biking, fishing and hunting, kayaking and boating, auto
travel on vacation and on business trips, research and data
collection. GPS is an excellent tool to help you locate a specific
position, and to help you and your family stay on track, so that
you don't get lost. (For those who are directionally- challenged.)
Military applications
The GPS system was initially set up for use by
the military
to provide precise position information. GPS products are
used by the military to coordinate and track the movement of
soldiers and equipment in the field, to guide military ships at
sea, and to provide position and navigation information to
military aircraft.
Commercial applications
GPS products have been developed for use for
many commercial applications. These include: surveying and
mapping,
aviation and marine navigation,
vehicle
tracking systems and mobile computer and cellular
platforms.
Surveying and mapping consist primarily of the
collection and processing of position information and usually
requires specialized GPS equipment. In the surveying market,
applications include construction
and engineering surveying, route surveying (roads, pipelines,
cable and utility lines) and geodetic research. Data can be
collected for evaluation later in the office, or used real-time,
in the field. Mapping applications use large amounts of position
data in the development of Geographic Information Systems (GIS)
databases, and natural resource mapping.
The FAA has adopted a policy in 1994 to
establish GPS as the future standard for aviation navigation.
Therefore many pilots are turning to GPS as a supplemental navigation aid for their aircraft. At sea, GPS receivers are used
on recreational and commercial vessels to provide real-time
latitude, longitude, time, course and speed information, and
assist with coast-line and harbor navigation.
Many GPS products are being used by businesses
and government agencies to track their vehicle locations
using wireless communications. Some GPS receivers have been
integrated into mobile radios, cellular phones and mobile data
terminals to meet the needs of vehicle fleet managers.
Most of the commercial applications for GPS
require equipment that exceeds the capability of the handheld GPS
units that Adventure GPS Products offers. However, if this is
something you might need professionally, and you cannot find it on
the Internet, contact me and I will try to send you in the right
direction.
More information about GPS on the Internet:
Univ. of Colorado GPS Department of Geography
GPS
Principles, from Aerospace Corporation
GPS World Magazine
US Coast Guard Navigation
Windows
based Tutorial on GPS
Usenet Newsgroup: sci.geo.satellite-nav |
