Viavi 87339, Operation Manual

Page: 120 / 198

Principals of Operation
6 -3
6.1.2.A Space Segment
The Space segment consists of the GPS space vehicles (SV’s) or Satellites, nominally
24 SV’s plus spares. The terms SV and satellite shall be interchangeable in this
document. Each vehicle has a 12 hour orbit at 20,200 km above the earth and repeats
same ground track daily. 5 to 12 SV’s are visible from anywhere on earth.
Fig. 6-2 GPS Satellite
Six orbital planes are used, each spaced equally around the earth, separated by
60 degrees (360 degrees/6 planes=60 degrees) and inclined 55 degrees from equatorial
plane. The planes are named A to F. Each orbital plane hosts four satellites. These
satellites are not spaced evenly on each plane. Spacing between adjacent satellites
varies from 31.13 degrees to 119.98 degrees. Each plane exhibits a different angular
spacing for the satellites resident to it.
A computer model determines the satellite spacing to accommodate a single satellite
failure and still maintain optimal satellite geometry.
Fig. 6-3 GPS Satellite Orbital Planes
Fig. 6-3 shows the motion of nine satellites. The ground tracks show the movement of
these satellites over a twelve hour period and the position of the satellites at one moment
in time.
The ground tracks show a number of features. Each satellite follows a unique path over
the ground. Also, every satellite operates between 55 degrees North and 55 degrees
south.
The primary mission of GPS satellites is the transmission of precisely timed GPS signals
and the data stream required to decode the signals to produce a position. The timing
signals are referenced to atomic clocks, either cesium or rubidium.