Page 231 - Satellite Communications, Fourth Edition

The Space Segment 211

Figure 7.10 The rectangular box shows the positional
limits for a satellite in geostationary orbit in relation to
beams from a 30-m and a 5-m antenna.

of the box and therefore could miss the satellite. Such narrow-beam
antennas therefore must track the satellite.
The diameter of the 5-m antenna beam at the satellite will be about
464 km, and this does encompass the box, so tracking is not required.
The positional uncertainty of the satellite also introduces an uncer-
tainty in propagation time, which can be a significant factor in certain
types of communications networks.
By placing the satellite in an inclined orbit, the north-south station-
keeping maneuvers may be dispensed with. The savings in weight
achieved by not having to carry fuel for these maneuvers allows the com-
munications payload to be increased. The satellite is placed in an
inclined orbit of about 2.5° to 3°, in the opposite sense to that produced
by drift. Over a period of about half the predicted lifetime of the mis-
sion, the orbit will change to equatorial and then continue to increase
in inclination. However, this arrangement requires the use of tracking
antennas at certain ground stations.

7.5 Thermal Control
Satellites are subject to large thermal gradients, receiving the sun’s
radiation on one side while the other side faces into space. In addition,
thermal radiation from the earth and the earth’s albedo, which is the