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 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. 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. 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. The Global Positioning System reached full Operational Capability (FOC) on
July 17, 1995. 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 accuracies 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.
Frequently Asked Questions About GPS
What is GPS?
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How is GPS used?
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Who uses GPS?
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What's the status of the GPS?
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What is the Standard Positioning Service?
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©Global Mapping Services 2002