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What is GPS?
GPS is a satellite-based radionavigation system developed and operated
by the U.S. Department of Defense (DOD). GPS permits land, sea,
and airborne users to determine their three-dimensional position,
velocity, and time 24 hours a day, in all weather, anywhere in the
world with a precision and accuracy far better than other radionavigation
systems available today or in the foreseeable future.
GPS consists of three segments: space, control, and user.
- The Space Segment, consists of a minimum of 24 operational
satellites in six circular orbits 20,200 km (10,900 NM) above
the earth at an inclination angle of 55 degrees with a 12 hour
period. The satellites are spaced in orbit so that at any time
a minimum of 6 satellites will be in view to users anywhere in
the world. The satellites continuously broadcast position and
time data to users throughout the world.
- The Control Segment consists of a master control station
in Colorado Springs, with five monitor stations and three ground
antennas located throughout the world. The monitor stations track
all GPS satellites in view and collect ranging information from
the satellite broadcasts. The monitor stations send the information
they collect from each of the satellites back to the master control
station, which computes extremely precise satellite orbits. The
information is then formatted into updated navigation messages
for each satellite. The updated information is transmitted to
each satellite via the ground antennas, which also transmit and
receive satellite control and monitoring signals.
- The User Segment consists of the receivers, processors,
and antennas that allow land, sea, or airborne operators to receive
the GPS satellite broadcasts and compute their precise position,
velocity and time.
The GPS concept of operation is based upon satellite ranging. Users
figure their position on the earth by measuring their distance from
the group of satellites in space. The satellites act as precise
reference points.
Each GPS satellite transmits an accurate position and time signal.
The user's receiver measures the time delay for the signal to reach
the receiver, which is the direct measure of the apparent range
to the satellite. Measurements collected simultaneously from four
satellites are processed to solve for the three dimensions of position,
velocity and time.
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How is GPS used?
GPS receivers collect signals from satellites in view. They display
the user's position, velocity, and time, as needed for their marine,
terrestrial, or aeronautical applications. Some display additional
data, such as distance and bearing to selected waypoints or digital
charts.
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Who uses GPS?
GPS is used to support land, sea, and airborne navigation, surveying,
Geophysical exploration, mapping and geodesy, vehicle location systems,
and a wide variety of additional applications.
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Will GPS be
free in the future?
GPS will be free as stated in the
Presidential Decision Document (29
March 1996) and by Congress in the 1998
Public Law (105-85)
Both state that the U.S. "will
continue to provide the GPS Standard Positioning Service for peaceful
civil, commercial and
scientific use on a continuous, worldwide basis, free of direct
user fees.
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What's the status of the GPS?
The Global Positioning System reached full
Operational Capability (FOC) on July 17, 1995.
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What is the Standard Positioning Service?
GPS provides two levels of service -- a Standard Positioning Service
(SPS) for general public use and an encoded Precise Positioning
Service (PPS) primarily intended for use by the Department of Defense.
SPS signal accuracy is intentionally degraded to protect U.S. national
security interests. This process, called Selective Availability
(SA), controls the availability of the system's full capabilities.
The SPS accuracy specifications, given below, include the effects
of SA.
SPS provides accuracy's of (for position, the accuracy with respect
to geographic, or geodetic coordinates of the Earth) within:
100 meters (2 drms) horizontal 156 meters (2 Sigma) vertical 300
meters (99.99% prob.) horizontal 340 nanoseconds time (95% prob.)
SPS Coverage is continuous and worldwide, with a position dilution
of precision (PDOP) of 6 or less.
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What is the status of Selective Availability (SA)?
Effective as of Midnight 01 May 2000,
Selective Availability
has been set to zero.
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