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INTRODUCTION
The Global Positioning System, usually called GPS (the US military refers to it
as NAVSTAR), is an intermediate circular orbit (ICO) satellite navigation system
used for determining one's precise location and providing a highly accurate time
reference almost anywhere on Earth or in Earth orbit.
The first of 24 satellites that form the current GPS constellation (Block II)
was placed into orbit on February 14, 1989. The 50th GPS satellite since the beginning
in 1978 was launched March 21, 2004 aboard a Delta II rocket
GPS HISTORY
The initial concept of GPS began to take form soon after the launch of Sputnik
in 1957. ".... Some scientists and engineers realized that radio transmissions
from a satellite in a well-defined orbit could indicate the position of a receiver
on the ground" This knowledge resulted in the U.S. Navy's development and
use of the "transit" system in the 1960's. This system, however, proved
to be cumbersome to use and limited in terms of positioning accuracy.
Starting
in the mid-1970s the U.S. Department of Defense (DOD) began the construction of
today's GPS and has funded, operated, and maintained control of the system it
developed. Eventually $12 billion dollars would take GPS from concept to completion.
Full Operational Capacity (FOC) of GPS was reached on July 17, 1995 (U.S.C.G.,
1996, www). At one point GPS was renamed NAVSTAR. This name, however, seems to
be regularly ignored by system users and others. Although the primary use of GPS
was thought to be for classified military operations, provisions were made for
civilian use of the system. National security reasons, however, would require
that civilian access to accurate positioning be intentionally degraded.
GPS ELEMENTS
GPS was designed as a system of radio navigation that utilizes "ranging"
-- the measurement of distances to several satellites -- for determining location
on ground, sea, or in the air. The system basically works by using radio frequencies
for the broadcast of satellite positions and time. With an antenna and receiver
a user can access these radio signals and process the information contained within
to determine the "range", or distance, to the satellites. Such distances
represent the radius of an imaginary sphere surrounding each satellite. With four
or more known satellite positions the users' processor can determine a single
intersection of these spheres and thus the positions of the receiver . The system
is generally comprised of three segments:
1. The space segment
2. The
control segment
3. The user segment
SPACE SEGMENT
The space segment consists of 24 satellites, each in its own orbit 11,000 nautical
miles above the Earth. The user segment consists of receivers, which you can hold
in users' hands or mount in users' vehicle. The control segment consists of ground
stations located around the world that make sure the satellites are working properly. The
GPS space segment uses a total of 24 satellites in a constellation of six orbiting
planes. This configuration provides for at least four equally- spaced satellites
within each of the six orbital planes. The orbital path is continuous in relation
to the earth, meaning that a satellite's orbit will follow the same path on the
earth with each orbit. At 10,900nm (20,200km) GPS satellites are able to complete
one orbit around the earth every 12 hours. GPS satellites orbit at a 55-degree
inclination to the equatorial plane. This space segment configuration provides
for a minimum of 5 satellites to be in view from any place on earth, fulfilling
the necessary four needed for three-dimensional positioning.
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