Page 143 - Satellite Communications, Fourth Edition

Polarization 123

5.3 Polarization of Satellite Signals
As mentioned above, the directions “horizontal” and “vertical” are
easily visualized with reference to the earth. Consider, however, the
situation where a geostationary satellite is transmitting a linear
polarized wave. In this situation, the usual definition of horizontal
polarization is where the electric field vector is parallel to the equa-
torial plane, and vertical polarization is where the electric field vector
is parallel to the earth’s polar axis. It will be seen that at the sub-
satellite point on the equator, both polarizations will result in elec-
tric fields that are parallel to the local horizontal plane, and care
must be taken therefore not to use “horizontal” as defined for terres-
trial systems. For other points on the earth’s surface within the foot-
print of the satellite beam, the polarization vector (the unit vector in
the direction of the electric field) will be at some angle relative to a
reference plane. Following the work of Hogg and Chu (1975), the ref-
erence plane will be taken to be that which contains the direction of
propagation and the local gravity direction (a “plumb line”). This is
shown in Fig. 5.8.
With the propagation direction denoted by k and the local gravity
direction at the ground station by r, the direction of the normal to the
reference plane is given by the vector cross-product:

f k r (5.8)

k
r

Local horizontal plane
Figure 5.8 The reference plane for the direction of propagation and
the local gravity direction.