Page 422 - Satellite Communications, Fourth Edition

402 Chapter Thirteen

large-diameter reflectors can be used with earth-station antennas, and
hence narrow beamwidths can be achieved. For example, a 10-m
antenna at 14 GHz has a 3-dB beamwidth of about 0.15°. This is very
much narrower than the 2° to 4° orbital spacing allocated to satellites.
To relate the C/I ratio to the antenna radiation pattern, it is necessary
first to define the geometry involved.
Figure 13.2 shows the angles subtended by two satellites in geosta-
tionary orbit. The orbital separation is defined as the angle subtended
at the center of the earth, known as the geocentric angle. However, from
an earth station at point P the satellites would appear to subtend an
angle . Angle is referred to as the topocentric angle. In all practical
situations relating to satellite interference, the topocentric and geo-
centric angles may be assumed equal, and in fact, making this assump-
tion leads to an overestimate of the interference (Sharp, 1983).
Consider now S as the wanted satellite and S as the interfering
2
1
satellite. An antenna at P will have its main beam directed at S and
1
an off-axis component at angle
directed at S . Angle
is the same as
2
the topocentric angle, which as already shown may be assumed equal
to the geocentric or orbital spacing angle. Therefore, when calculating
the antenna sidelobe pattern, the orbital spacing angle may be used, as
described in Sec. 13.2.4. Orbital spacing angles range from 2° to 4° in
0.5° intervals in the C band.
In Fig. 13.3 the satellite circuit being interfered with is that from
earth station A via satellite S to receiving station B.The B mode of
1
1
interference can occur from satellite S into earth station B,and the B 2
2
mode of interference can occur from earth station C into satellite S .
1
The total single-entry interference is the combined effect of these two
modes. Because the satellites cannot carry very large antenna reflectors,
the beamwidth is relatively wide, even for the so-called spot beams. For
example, a 3.5-m antenna at 12 GHz has a beamwidth of about 0.5°,

Figure 13.2 Geocentric angle and the topocentric angle .