Page 148 - Satellite Communications, Fourth Edition

128 Chapter Five

The magnitude of h is
9 2
7 2
6 2
ZhZ 2s7.2894 ? 10 d s 6.3214 ? 10 d s1.3812 ? 10 d
1.383 ? 10 9
From Eq. (5.13):
h
p
ZhZ
a x 0.0527 a y 0.0046 a z 0.9986

The dot product of p and f is
7 8
p ? f 0.0527 0 ( 0.0046) 8.3 10 0.9986 1.35 10
8 2
1.356 10 km
The magnitude of f is
8 2
7 2
ZfZ 2( 8.3 10 ) (1.35 10 )
8
1.588 10 km 2
and from Eq. (5.10)
1.356
arcsin
1.588
58.64°

5.4 Cross-Polarization Discrimination
The propagation path between a satellite and earth station passes
through the ionosphere, and possibly through layers of ice crystals in the
upper atmosphere and rain, all of which are capable of altering the
polarization of the wave being transmitted. An orthogonal component
may be generated from the transmitted polarization, an effect referred
to as depolarization. This can cause interference where orthogonal
polarization is used to provide isolation between signals, as in the case
of frequency reuse.
Two measures are in use to quantify the effects of polarization inter-
ference. The most widely used measure is called cross-polarization
discrimination (XPD). Figure 5.11a shows how this is defined. The trans-
mitted electric field is shown having a magnitude E before it enters the
1
medium which causes depolarization. At the receiving antenna the elec-
tric field may have two components, a copolar component, having mag-
nitude E , and a cross-polar component, having magnitude E . The
12
11
cross-polarization discrimination in decibels is defined as
E 11
XPD 20 log (5.17)
E 12