GPS Satellite Constellation

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