Page 105 - Satellite Communications, Fourth Edition

The Geostationary Orbit 85

to give the expected values for a satellite the longitude of which is close
to the earth-station longitude. In some cases, especially with direct
broadcast satellites (DBS), the home antenna is aligned to one particu-
lar cluster of satellites, as described in Chap. 16, and no further adjust-
ments are necessary.

3.3 The Polar Mount Antenna

Where the home antenna has to be steerable, expense usually precludes
the use of separate azimuth and elevation actuators. Instead, a single
actuator is used which moves the antenna in a circular arc. This is
known as a polar mount antenna. The antenna pointing can only be
accurate for one satellite, and some pointing error must be accepted for
satellites on either side of this. With the polar mount antenna, the dish
is mounted on an axis termed the polar axis such that the antenna
boresight is normal to this axis, as shown in Fig. 3.5a. The polar mount
is aligned along a true north line, as shown in Fig. 3.5, with the bore-
sight pointing due south. The angle between the polar mount and the
local horizontal plane is set equal to the earth-station latitude l ; simple
E
geometry shows that this makes the boresight lie parallel to the equa-
torial plane. Next, the dish is tilted at an angle relative to the polar
mount until the boresight is pointing at a satellite position due south
of the earth station. Note that there does not need to be an actual satel-
lite at this position. (The angle of tilt is often referred to as the decli-
nation, which must not be confused with the magnetic declination used
in correcting compass readings. The term angle of tilt will be used for
in this text.)
The required angle of tilt is found as follows: From the geometry of
Fig. 3.5b,
o

90 El l E (3.13)
0
where El is the angle of elevation required for the satellite position due
0
south of the earth station. But for the due south situation, angle B in
Eq. (3.8) is equal to zero; hence, from Eq. (3.9), b l . Hence, from Eq.
E
(3.12), or Fig 3.5c.
a GSO
cos El sin l E (3.14)
0
d
Combining Eqs. (3.13) and (3.14) gives the required angle of tilt as

90 arccosa a GSO b sin l l E (3.15)
E
d