Page 125 - Satellite Communications, Fourth Edition

Radio Wave Propagation 105

TABLE 4.1 Propagation Concerns for Satellite Communications Systems
Propagation impairment Physical cause Prime importance
Attenuation and sky Atmospheric gases, Frequencies above
noise increases cloud, rain about 10 GHz
Signal depolarization Rain, ice crystals Dual-polarization systems
at C and Ku bands
(depends on system
configuration)
Refraction, atmospheric Atmospheric gases Communication and tracking
multipath at low elevation angles
Signal scintillations Tropospheric and ionospheric Tropospheric at frequencies
refractivity fluctuations above 10 GHz and
low-elevation angles;
ionospheric at frequencies
below 10 GHz
Reflection multipath, Earth’s surface, objects Mobile satellite services
blockage on surface
Propagation delays, Troposphere, ionosphere Precise timing and location
variations systems; time division
multiple access (TDMA)
systems
Intersystem interference Ducting, scatter, diffraction Mainly C band at present;
rain scatter may be
significant at higher
frequencies
SOURCE: Brussard and Rogers, 1990.

free electrons in the ionosphere are not uniformly distributed but form
in layers. Furthermore, clouds of electrons (known as traveling ionos-
pheric disturbances) may travel through the ionosphere and give rise to
fluctuations in the signal that can only be determined on a statistical
basis. The effects include scintillation, absorption, variation in the direc-
tion of arrival, propagation delay, dispersion, frequency change, and
polarization rotation (CCIR Report 263-5, 1982). All these effects
decrease as frequency increases, most in inverse proportion to the fre-
quency squared, and only the polarization rotation and scintillation
effects are of major concern for satellite communications. Polarization
rotation is described in Sec. 5.5.
Ionospheric scintillations are variations in the amplitude, phase,
polarization, or angle of arrival of radio waves. They are caused by
irregularities in the ionosphere which change with time. The main effect
of scintillations is fading of the signal. The fades can be quite severe,
and they may last up to several minutes. As with fading caused by
atmospheric scintillations, it may be necessary to include a fade margin
in the link power-budget calculations to allow for ionospheric scintillation.