Satellite Navigation is based
on a global network of satellites that transmit radio
signals from medium earth orbit. Users of Satellite
Navigation are most familiar with the 24 Global Positioning
System (GPS) satellites. The United States, which
developed and operates GPS, and Russia, which developed
a similar system known as GLONASS, have offered free
use of their respective systems to the international
community. The International Civil Aviation Organization
(ICAO), as well as other international user groups,
has accepted GPS and GLONASS as the core for an international
civil satellite navigation capability known as the
Global Navigation Satellite System (GNSS).
The basic GPS service provides
users with approximately 100-foot accuracy, 95% of
the time, anywhere on or near the surface of the earth.
To accomplish this, each of the 24 satellites emits
signals to receivers that determine their location
by computing the difference between the time that
a signal is sent and the time it is received. GPS
satellites carry atomic clocks that provide extremely
accurate time. The time information is placed in the
codes broadcast by the satellite so that a receiver
can continuously determine the time the signal was
broadcast. The signal contains data that a receiver
uses to compute the locations of the satellites and
to make other adjustments needed for accurate positioning.
The receiver uses the time difference between the
time of signal reception and the broadcast time to
compute the distance, or range, from the receiver
to the satellite. The receiver must account for propagation
delays, or decreases in the signal's speed caused
by the ionosphere and the troposphere. With information
about the ranges to three satellites and the location
of the satellite when the signal was sent, the receiver
can compute its own three-dimensional position. An
atomic clock synchronized to GPS is required in order
to compute ranges from these three signals. However,
by taking a measurement from a fourth satellite, the
receiver avoids the need for an atomic clock. Thus,
the receiver uses four satellites to compute latitude,
longitude, altitude, and time.
GPS is comprised of three segments:
the control, space,
and user segments.
Next:
Control Segment
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