Page 230 - Satellite Communications, Fourth Edition

210 Chapter Seven

Orbital correction is carried out by command from the TT&C earth
station, which monitors the satellite position. East-west and north-
south station-keeping maneuvers are usually carried out using the same
thrusters as are used for attitude control. Figure 7.9 shows typical lat-
itude and longitude variations for the Canadian Anik-C3 satellite which
remain after station-keeping corrections are applied.
Satellite altitude also will show variations of about 0.1 percent of
the nominal geostationary height. If, for sake of argument, this is taken
as 36,000 km, the total variation in the height is 72 km. A C-band satel-
lite therefore can be anywhere within a box bound by this height and
the 0.1° tolerances on latitude and longitude. Approximating the geo-
stationary radius as 42,164 km (see Sec. 3.1), an angle of 0.2° subtends
an arc of approximately 147 km. Thus both the latitude and longitude
sides of the box are 147 km. The situation is sketched in Fig. 7.10, which
also shows the relative beamwidths of a 30-m and a 5-m antenna. As
shown by Eq. (6.33), the 3-dB beamwidth of a 30-m antenna is about
0.12°, and of a 5-m antenna, about 0.7° at 6 GHz. Assuming 38,000 km
(typical) for the slant range, the diameter of the 30-m beam at the
satellite will be about 80 km. This beam does not encompass the whole

Figure 7.9 Typical satellite motion. (Courtesy of Telesat, Canada, 1983.)