Page 109 - Satellite Communications, Fourth Edition

The Geostationary Orbit 89

Example 3.4 Determine the limits of visibility for an earth station situated at
mean sea level, at latitude 48.42° north, and longitude 89.26 degrees west. Assume
a minimum angle of elevation of 5°.
Solution Given data:
l E 48.42°; E 89.26°; El min 5°; a GSO 42164 km; R 6371 km

min 90° El min
Equation (3.17) gives:

S arcsina 6371 sin 95 b
42164
8.66°
Equation (3.18) gives:
b 180 95° 8.66°
76.34°

Equation (3.19) gives:

B arccosa cos 76.34 b
cos 48.42
69.15°
The satellite limit east of the earth station is at

E B 20° approx.
and west of the earth station at

E B 158° approx.

3.5 Near Geostationary Orbits
As mentioned in Sec. 2.8, there are a number of perturbing forces that
cause an orbit to depart from the ideal keplerian orbit. For the geo-
stationary case, the most important of these are the gravitational
fields of the moon and the sun, and the nonspherical shape of the
earth. Other significant forces are solar radiation pressure and reaction
of the satellite itself to motor movement within the satellite. As a result,
station-keeping maneuvers must be carried out to maintain the satel-
lite within set limits of its nominal geostationary position. Station keep-
ing is discussed in Sec. 7.4.
An exact geostationary orbit therefore is not attainable in practice, and
the orbital parameters vary with time. The two-line orbital elements