Page 57 - Electronic Navigation Cyber Book

SPACE SEGMENT

A visual example of a 24 satellite GPS constellation in motion with the
Earth rotating. Notice how the number of satellites in view from a given
point on the Earth's surface, in this example at 45°N, changes with
time.The space segment (SS) is composed of the orbiting GPS satellites,
or Space Vehicles (SV) in GPS parlance. The GPS design originally
called for 24 SVs, eight each in three approximately circular orbits, but
this was modified to six orbital planes with four satellites each. The six
orbit planes have approximately 55° inclination (tilt relative to
Earth's equator) and are separated by 60°right ascension of
the ascending node (angle along the equator from a reference point to
the orbit's intersection). The orbital period is one-half a sidereal day, i.e.,
11 hours and 58 minutes so that the satellites pass over the same
locations or almost the same locations every day. The orbits are arranged
so that at least six satellites are always within line of sight from almost
everywhere on Earth's surface. The result of this objective is that the four
satellites are not evenly spaced (90 degrees) apart within each orbit. In
general terms, the angular difference between satellites in each orbit is
30, 105, 120, and 105 degrees apart which sum to 360 degrees.

Orbiting at an altitude of approximately 20,200 km (12,600 mi); orbital
radius of approximately 26,600 km (16,500 mi), each SV makes two
complete orbits each sidereal day, repeating the same ground track each
day. This was very helpful during development because even with only
four satellites, correct alignment means all four are visible from one spot

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