Global Positioning System Reference
In-Depth Information
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[71
Figure 3.6
Biannual eclipse periods.
Lin
—
-
——
No
PgE
satellite travels through the earth's shadow region twice per year. This occurs when-
ever the sun is in or near the orbital plane. See Figures 3.6 and 3.7 for a graphical
presentation. The umbra is that portion of the shadow cone that no light from the sun
can reach. The penumbra is the region of partial shadowing; it surrounds the umbra
cone. While the satellite transits through the shadow regions, the solar radiation force
acting on the satellite is either zero (umbra) or changing (penumbra). These changes
in force must be taken into consideration in precise orbital computations. In addition,
the thermal reradiation forces change as the temperature of the satellite drops. GPS
satellites move through the shadow regions in less than 60 minutes, twice per day.
The shadow regions cause an additional problem for precise orbit determination.
The solar panels are orientated toward the sun by the attitude control system (ACS)
[71
Figure 3.7
Earth shadow regions.
