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Figure 4.14
Free-space loss,
A
0
, as a function of orbit altitude and frequency band.
Figure 4.15
Typical relationship between atmospheric absorption and the microwave frequency
of transmission, expressed in decibels.
elevation angle toward the zenith (
⌰
=
90
°
). The absorption essentially is constant
and will be under 1 dB at C- and Ku-bands. The only place on the ground where
⌰
=
90
°
is at the subsatellite point, on the equator directly below the satellite. At
other locations the elevation angle takes values down to perhaps a practical low
of 5
°
. At a particular frequency shown along the X-axis, the atmospheric absorption
for elevation angles less than 90
°
would increase due to the greater thickness of
atmosphere. Below 10
°
, absorption is highly sensitive to multipath due to ground
reflections and atmospheric ducting. The following formula indicates how atmo-
spheric absorption changes for elevation angles greater than 10
°
:
A
a
≈
8
a
O
+
2
a
W
sin
4.2
Propagation on the Earth-Space Link
127
The two parameters,
a
O
and
a
W
, represent constituent absorptions due to
oxygen and water vapor, respectively. We can view the numerator as being the
total attenuation indicated on the Y-axis of Figure 4.15.
As frequency increases above 15 GHz, the constituents of the atmosphere reach
individual points of resonance, and absorption can become very high, even total.
The bands of fequencies around 22 and 66 GHz correspond to resonances for
water vapor and oxygen, respectively, and are not employed for either uplinks
or downlinks. Direct links between satellites, called intersatellite links (ISLs) or,
alternatively, cross-links, bypass the atmosphere and hence may utilize the absorp-
tive bands.
4.2.5.3
Rain Attenuation
After free-space loss, the most detrimental effect on commercial satellite links
above C-band is rain attenuation, which results from absorption and scattering of
microwave energy by rain drops. That loss, which increases with frequency, was
discussed in Chapter 1 in the comparison of frequency bands. Rain attenuation is
not predictable on an instantaneous basis, but statistical estimates can be made
that allow links to be designed. Obviously, dry seasons and regions of the world
with low rainfall would not suffer greatly from this phenomenon. However, links in
regions with heavy thunderstorm activity—and hence rainfall—should be provided
with greater link margin, or service might not be maintained with sufficient avail-
ability to satisfy commercial requirements.
Intense rain is contained in rain cells, which have somewhat limited geographic
size. The statistical relationship between local rainfall and the resulting attenuation
caused at a particular microwave frequency is a complex combination of several
factors. The dimensions of the rain cell vary based on the rain rate, measured
typically in millimeters per hour. At a particular rain rate and cell size, the attenua-
tion increases with path length through the cell. Because a cell is shaped like an
oblate spheroid (i.e., flat like a hamburger), the path length is inversely related to
the elevation angle. Thus, rain attenuation varies in much the same manner as does
atmospheric attention in the absence of rain. Other factors include gaseous water
vapor, clouds, and ice particles. The work of combining all of these factors is
largely done and available to us in the form of rain propagation models. The
ITU has adopted the DAH model, which was developed by the research team of
Dissanayake, Allnut, and Haidara. Figure 4.16 provides in summary form some
rain attenuation data as a function of frequency and for a temperate climate such
as that of Western Europe or the northeastern United States. Comparable data for
a tropical region would be two to three times greater in decibels. The best approach
is to use a software tool like SatMaster (discussed later in this chapter) that uses
the DAH model to calculate rain attenuation for a specific location.
As indicated in the figure, the lower the elevation angle from the ground, the
greater is the amount of attenuation from a given amount of rainfall. Heavy rainfall
also alters the polarization of the signal because atmospheric drag causes raindrops
to flatten and not be perfectly spherical. That reduces cross-polarization isolation
between linear polarized transmissions. For example, the maximum isolation in
Figure 4.11 decreases at Ku-band in heavy rain from 40 dB to approximately
128
Microwave Link Engineering
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