GPS
FREQUENTLY ASKED QUESTIONS
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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What is GPS Rollover?
This document
contains information about GPS Week 1024 Rollover.
How do I report a GPS
Mapping Data error such as an incorrect address for a home
or a business?
If you wish to report an incorrect address, report or request
a change to the address for a business or home, or add an
address, please submit your corrections to the Tele
Atlas website.
If you are experiencing GPS issues relating to positioning,
navigation, timing or signals please proceed to and fill out
a GPS outage report.
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