| There currently are 24 operational
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| | lifespan. The GPS satellites have been
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| satellites as part of the GPS satellite
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| | designed to provide a lifespan of ~8
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| constellation, that orbit the earth with
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| | years. New satellites are being launched
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| a orbital radius of ~16,000 miles (from
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| | to space at regular intervals of time to
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| the center of the earth). The satellite
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| | make up for satellites that have lived
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| orbits are spaced out in 6 orbital planes
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| | out their lifespan.
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| (orbital plane is the plane surface
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| | When the satellites are launched into
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| formed by the path traced by the orbit of
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| | space, they are inserted to
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| the satellite - it is nearly circular).
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| | pre-calculated positions in their orbital
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| The six orbital planes are separated by
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| | planes. The pre-calculations ascertain
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| an angle of 60 degrees. Refer to the
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| | that a GPS receiver located anywhere on
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| constellation figure below. Four to six
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| | earth and at all times, can receive
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| satellites occupy each orbital plane.
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| | signals from at least four satellites.
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| In addition to the 24 operational
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| | This is required because earth blocks the
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| satellites, there currently are 6
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| | GPS signals from passing through it. For
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| satellites in orbit that are maintained
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| | a GPS receiver to be able to receive a
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| as spares, should any of the 24
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| | signal from a satellite, the straight
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| operational satellites malfunction. The
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| | line-of-sight line should not have any
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| six spare satellites are kept operational
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| | obstructions in between. Since the
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| for users to receive signals from them
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| | satellites are flying around the earth,
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| just like from the 24 satellites.
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| | not all satellites are visible to a GPS
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| However, the U.S. Government does not
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| | receiver located somewhere on earth.
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| guarantee their availability at all
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| | Thus, though there are 30 operational
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| times.
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| | satellites, not all of them can be
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| Remember that the GPS satellites are
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| | visible at a location on earth, at any
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| being launched since 1974. Satellites
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| | given time.
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| like everything else have limited
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|