IONOSPHERIC
SCINTILLATION
AND
RAIN INDUCED
ATTENUATION
BY HARI SABARISH J
311121106022
IONOSPHERIC SCINTILLATION
1. Radio waves traveling between satellites and earth stations must traverse the ionosphere,
the upper region of Earth's atmosphere ionized mainly by solar radiation.
2. The ionosphere contains free electrons that are not uniformly distributed but form layers,
and traveling ionospheric disturbances can cause signal fluctuations on a statistical basis.
3. Effects of ionospheric disturbances include scintillation, absorption, direction of arrival
variation, propagation delay, dispersion, frequency change, and polarization rotation.
4. Effects decrease as frequency increases, particularly in inverse proportion to the
frequency squared, with polarization rotation and scintillation being major concerns for
satellite communications.
5. Ionospheric scintillations result from irregularities in the ionosphere, causing variations in
amplitude, phase, polarization, or angle of arrival of radio waves.
6. Scintillations lead to signal fading, which can be severe and last several minutes,
necessitating inclusion of a fade margin in link power-budget calculations to compensate for
ionospheric scintillation.
What is rain attenuation?
The attenuation of a signal due to rain is the
extent to which the strength of a signal is
reduced when passing through rain from a
transmitter to a receiver, Also called rain fade.
FACTORS AFFECTING SATELLITE EARTH
LINK IN RAIN
● FREQUENCY OF OPERATION
● ELEVATION ANGLE OF SATELLITE
● SLANT PATH ANGLE
● RAIN RATE
● RAIN HEIGHT
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Rain attenuation is a function of rain rate.
Rain rate is the rate at which rainwater would
accumulate in a rain gauge situated at the
ground in the region of interest.
Rain rate is measured in millimeters per
hour.
The percentage of time that specified rain
rate values are exceeded is of interest.
Time percentage is usually measured over a
year.
For example, a rain rate of 0.001 percent
means that the rain rate would be exceeded
for 0.001 percent of a year, or about 5.3
minutes during any one year.
In this case, the rain rate would be denoted
by R0.001.
Rain GAUGE:
Uplink and Downlink Attenuation in Rain
1. Rain attenuation affects uplinks and downlinks differently.
2. Assumption: Rain attenuation occurs on either the uplink or the downlink, but
not on both simultaneously.
3. This assumption is generally valid for well-separated earth stations but not for
those within close proximity (<20 km).
4. Heavy rain occurs with a somewhat random geographic distribution for less
than 1% of the time.
5. Probability of significant attenuation on both uplink and downlink
simultaneously is small when earth stations are separated by more than 20 km.
6. In analysis, it is assumed that one link is attenuated while the other operates
in clear air.
Uplink Attenuation in Rain:
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Transponder receiver noise temperature remains relatively constant when rain is present in the
uplink path to the satellite.
Satellite receiving antenna beam covers a large area of the earth's surface, resulting in
insignificant local noise temperature variations due to phenomena like thunderstorm clouds.
Earth's noise temperature varies according to latitude, with the tropics being warmer than northern
latitudes.
System noise temperature for transponders on a GEO satellite ranges from 300 to 800 K, with
higher values for higher frequency bands.
There is no significant increase in uplink noise power when heavy rain is present due to averaging
of radiation temperature over the satellite antenna's footprint.
Rain attenuation on the uplink path reduces power at the satellite receiver input, leading to a
reduction in Carrier-to-Noise Ratio (CNRup) in direct proportion to the attenuation on the slant
path.
In linear mode operation, the output power is reduced by the same amount as the attenuation on
the uplink, causing CNR at the satellite receiver (CNRan) to fall by an equal amount.
When both CNRup and CNRan are reduced by Aup dB due to attenuation on the uplink, the value of
CNR is reduced by exactly the same amount, Aup dB.
Downlink Attenuation in Rain
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The earth station receiver noise temperature can change very significantly when
rain is present in the
downlink path from the satellite. The sky noise temperature can increase to a
value close to the physical
temperature of the individual raindrops, particularly in very heavy rain. A
reasonable temperature to assume
for temperate latitudes in a variety of rainfall rates is 270 K, although values
above 290 K have been
observed in the tropics. An increase in sky noise temperature to 270 K will
increase the receiving antenna
temperature markedly above its clear air value. See Section 4.5 for an illustration
of this effect. The result is
that the received power level, C, is reduced and the noise power, N, in the receiver
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