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Satellite Communications
Text book:
Satellite Communications, 4th ed.
Dennis RoddyMcGraw-Hill International Ed.
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1.1 Introduction
Features offered by satellite communications
� large areas of the earth are visible from the satellite, thus the
satellite can form the star point of a communications net linking
together many users simultaneously, users who may be widely
separated geographically
�Provide communications links to remote communities
�Remote sensing detection of pollution, weather conditions,
search and rescue operations.
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1.2 Frequency allocations
� International Telecommunication Union
(ITU) coordination and planning
� World divided into three regions:
± Region 1: Europe, Africa, formerly Soviet
Union, Mongolia
± Region 2: North and South America, Greenland
± Region 3: Asia (excluding region 1), Australia,
south west Pacific
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Frequency band designations in common use for
satellite service
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1.3 Intelsat
� International Telecommunications Satellite
�Created in 1964, now has 140 member countries, >40
investing entities
�Geostationary orbit orbits earth`s equitorial plane.
�Atlantic ocean Region (AOR), Indian Ocean Region
(IOR), Pacific Ocean Region.
�Latest INTELSAT IX satellites wider range of servicesuch as internet, Direct to home TV, telemedicine, tele-
education, interactive video and multimedia
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Satellite Coverage Maps
Source: http://www.intelsat.com
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Coverage maps: Footprints
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1.4 U.S DOMSAT (Domestic Satellites)�Provide various telecommunication service within a
country
�In U.S.A all domsats in geostationary orbit
�Direct-to-home TV service can be classified as high
power, medium power, low power
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1.5 Polar orbiting satellites
� Orbit the earth such a way as to cover the north and south polar regions
� A satellite in a polar orbit passes above or nearly above both poles of the planet (or other celestial body) on eachrevolution. It therefore has an inclination of (or very closeto) 90 degrees to the equator.
� Since the satellite has a fixed orbital plane perpendicular to
the planet's rotation, it will pass over a region with adifferent longitude on each of its orbits.
� Polar orbits are often used for earth-mapping-, earthobservation, as well as some weather satellites.
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� in U.S.A, the National Oceanic and Atmospheric
Administration (NOAA) operates a weather satellite system,
geostationary operational environmental satellites
(GEOS) and
polar operational environmental satellites (PEOS)
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2.0 Orbits and Launching Methods� Johannes Kepler (1571 ± 1630) derive empirically three
laws describing planetary motion.
� Kepler ¶s laws apply quite generally to any two bodies in
space which interact through gravitation.
� The more massive of the two bodies is referred to as the
primary, the other, the secondary, or satellite.
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The center of mass of the two-body system, termed the
barycenter, is always centered on one of the foci
2.2 Kepler ¶s first law
states that the path followed by a satellite around the
primary will be an ellipse. An ellipse has two focal pointsshown as F1 and F2
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� In our specific case, because of the enormous difference
between the masses of the earth and the satellite, the center of mass coincides with the center of the earth, which is therefore
always at one of the foci.
� The semimajor axis of the ellipse is denoted by a, and thesemiminor axis, by b. The eccentricity e is given by
a
bae
22 !
For an elliptical orbit, 0 < e < 1. When e = 0, the orbit
becomes circular .
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2.4 Kepler ¶s Third Law
states that the square of the periodic time of orbitis proportional to the cube of the mean distance between
the two bodies.
The mean distance is equal to the semimajor axis a.
For the artificial satellites orbiting the earth, Kepler ¶s third
law can be written in the form
2
3
n
aQ
!
a = semimajor axis (meters)
n = mean motion of the satellite (radians per second)
Q = earth¶s geocentric gravitational constant.
= 3.986005 v 1014 m3/sec2
« (2.2)
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Eqt (2.2) applies only to ideal situation satellite
orbiting a perfectly spherical earth of uniform mass,with no pertubing forces acting, such as atmospheric
drag.
Section 2.8 will take account of the earth`s oblateness
and atmospheric drag.
With n in radians per second, the orbital period in
seconds is given by
(2.4)n
P T 2!
This shows that there is a fixed relationship between
period and size
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Chapter 3: Radio Wave Propagation
3.1 Introduction
A signal traveling between an earth station and a
satellite must pass through the earth¶s atmosphere,
including the ionosphere.This introduce certain impairments, summarized in
Table 4.1. (Refer text book, page 93)
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3.2 Atmospheric Losses
Losses occur in the earth¶s atmosphere as a result of
energy absorption by the atmospheric gases. These
losses are treated quite separately from those which
result from adverse weather conditions, which of course are also atmospheric losses.
To distinguish between these, the weather-related
losses are referred to as atmospheric attenuation and
the absorption losses simply as atmospheric
absorption.The atmospheric absorption loss varies with
frequency, as shown in Fig. 4.2. (Refer text book,
page 94)
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Two absorption peaks will be observed:1. at a frequency of 22.3 GHz, resulting from
resonance absorption in water vapor (H2O), and
2. at 60 GHz, resulting from resonance absorption
in oxygen (O2).
At other frequencies, the absorption is quite low.
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All these effects decrease as frequency increases.
Only the polarization rotation and scintillationeffects are of major concern for satellite communications.
Ionospheric scintillations
� are variations in the amplitude, phase, polarization, or
angle of arrival of radio waves.
� Caused by irregularities in the ionosphere which changes
with time.
� Effect of scintillations is fading of the signal.
Severe fades may last up to several minutes.
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Polarization rotation:
� porduce rotation of the polarization of a signal
(F ar ad ay rot ation)
�When linearly polarized wave traverses in the
ionosphere, free electrons in the ionosphere are sets in
motion a force is experienced, which shift the
polarization of the wave.
�Inversely proportional to frequency squared.
� not a problem for frequencies above 10 GHz.
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3.4 Rain Attenuation
Rain attenuation is a function of rain rate.
Rain rate, R p = the rate at which rainwater would
accumulate in a rain gauge situated at the ground in
the region of interest (e. g., at an earth station).
The rain rate is measured in millimeters per hour.
Of interest is the percentage of time that specified
values are exceeded. The time percentage is usuallythat of 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 min during any one
year.
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The specific attenuation E is
kmdBaRb
p /!E « (4.2)
where a and b depend on frequency and polarization.
Values for a and b are available in tabular form in anumber of publications. (eg, Table 4.2, pg 95)
Once the specific attenuation is found, the total
attenuation is determined as:
dB L A E! « (4.3)
where,
L = effective path length of the signal through the rain.
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Because the rain density is unlikely to be uniform
over the actual path length, an effective path lengthmust be used rather than the actual (geometric)
length.
Figure 4.3 shows the geometry of the situation.
Figure 4.3: Path length through rain
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The geometric, or slant, path length is shown as LS . This
depends on the antenna angle of elevation U and the rainheight hR , which is the height at which freezing occurs.
Figure 4.4 shows curves for hR for different climatic
zones.
Method 1: maritime climates
Method 2: Tropical climates
Method 3: continental climates
Figure 4.4: Rain height as a function of earth station latitude for
different climatic zones
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For small angles of elevation (E l < 10°), the determination
of LS is complicated by earth curvature.For E l u 10°, a flat earth approximation may be used.
From Fig. 4.3 it is seen that
El
hh L
o R
S sin
!
« (4.4)
The effective path length is given in terms of the slant
length by
pS r L L ! « (4.5)
where r p is a reduction factor which is a function of the
percentage time p and LG, the horizontal projection of LS .
R efer Table 4.3, page 97, for values of reduction factors, r p
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This chapter describes how the link-power budget
calculations are made. These calculations basically
relate two quantities, the transmit power and the
receive power, and show in detail how the differencebetween these two powers is accounted for.
Link budget calculations are usually made using
decibel or decilog quantities. These are explained in
App. G.
Chapter 4: The Space Link
4.1 Introduction
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4.2 Equivalent Isotropic Radiated Power
A key parameter in link budget calculations is the
equivalent isotropic radiated power, conventionally
denoted as EIRP.
The Maximum power flux density at some distance r from a transmitting antenna of gain G is
24 r
GP S M
T ] ! « (12.1)
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An isotropic radiator with an input power equal to GPS
would produce the same flux density. Hence thisproduct is referred to as the equivalent isotropic
radiated power, or
S P EI R P ! « (12.2)
EIRP is often expressed in decibels relative to one
watt, or dBW. Let PS be in watts; then
? A ? A ? A dBW G P E I RP S ! « (12.3)
where [PS] is also in dBW and [G] is in dB.
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The isotropic gain for a paraboloidal antenna is
2472.10 fDG L! « (12.4)
Where,
f
L
is the carrier frequency
is the reflector diameter in m
is the aperture efficiency
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4.3 Transmission Losses
The [EIRP] is the power input to one end of the
transmission link, and the problem is to find the power
received at the other end.
Losses will occur along the way, some of which areconstant. Other losses can only be estimated from
statistical data, and some of these are dependent on
weather conditions, especially on rainfall.
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The first step in the calculations is to determine the
losses for clear weather, or clear-sky, conditions.These calculations take into account the losses,
including those calculated on a statistical basis, which
do not vary significantly with time. Losses which are
weather-related, and other losses which fluctuate with
time, are then allowed for by introducing appropriatefade margins into the transmission equation.
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4.3.1 Free Space Transmission
Spreading of the signal is space causes power loss.
The power flux density at the receiving antenna is
2
4 r
EI R P M
T
] ! « (12.6)
The power delivered to a matched receiver is this
power flux density multiplied by the effective aperture of
the receiving antenna. The received power is therefore:
e ff M R A P ] !
« (12.7)
T
P
T 44
2
2
RG
r
E I RP !
2
4))(( ¹
º
¸©ª
¨!
r G EI R P R
T
P
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where
r = distance, or range, between the transmit and receiveantennas
GR = isotropic power gain of the receiving antenna. The
subscript R is used to identify the receiving antenna.
In decibel notation, equation (12.7) becomes
? A ? A ? A2
4log10 ¹
º
¸©ª
¨!
P
T r G EI R P P R R
« (12.8)
Free space loss is given by
? A2
4log10 ¹
º
¸©ª
¨!
P
T r FS L « (12.9)
f r log20log204.32 ! « (12.10)
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Equation (12.8) can then be written as
? A ? A ? A ? A F S LG E I RP P R R ! « (12.8)
The received power [PR] will be in dBW when the[EIRP] is in dBW and [FSL] in dB.
Equation (12.9) is applicable to both the uplink and
the downlink of a satellite circuit
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4.3.2 Feeder Losses
Losses occur in the connection between the
receive antenna and the receiver proper, such as in
the connecting waveguides, filters, and couplers.
These will be denoted by RFL, or [RFL] dB, for
receiver feeder losses.The [RFL] values are added to [FSL] in Eq. (12.11).
Similar losses occur in the filters, couplers, and
waveguides connecting the transmit antenna to the
high-power amplifier (HPA) output. However,
provided that the EIRP is stated, Eq. (12.11)
can be used without knowing the transmitter feeder
losses. These are needed only when it is desired
to relate EIRP to the HPA output
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4.3.3 Antenna misalignment losses
When a satellite link is established, the ideal situationis to have the earth station and satellite antennas
aligned for maximum gain, as shown in Fig. 12.1 a.
Figure 12.1: (a) aligned for maximum gain, (b) earth-station
antenna misaligned
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There are two possible sources of off-axis loss, one
at the satellite and one at the earth station.
The off-axis loss at the earth station is referred to as the
antenna pointing loss, which are usually only a few
tenths of a decibel.
Losses may also result at the antenna from misalignment
of the polarization direction. The polarization
misalignment losses are usually small, and is assumed
that the antenna misalignment losses, denoted by [AML],include both pointing and polarization losses resulting
from antenna misalignment.
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Antenna misalignment losses have to be estimated
from statistical data, based on the errors actually
observed for a large number of earth stations.
Separate antenna misalignment losses for the uplink
and the downlink must be taken into account.
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4.3.4 Fixed atmospheric and ionospheric losses
Atmospheric gases result in losses by absorption.
These losses usually amount to a fraction of a
decibel, and decibel value will be denoted by [AA].
Table 12.1 shows values of atmospheric absorptionlosses and Satellite pointing loss in Canada.
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4.4 The Link-Power Budget Estimation
Losses for clear sky conditions are
? A ? A ? A ? A ? A ? A P L AA AM L R
F L
FS L LO
SS E S
!
« (12.12)
The decibel equation for the received power is
? A ? A ? A ? A LOSS E S G E I RP P
R R! « (12.13)
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where
[PR] = received power, dBW
[EIRP] = equivalent isotropic radiated power, dBW
[FSL] = free-space spreading loss, dB
[RFL] = receiver feeder loss, dB
[AML] = antenna misalignment loss, dB
[AA] = atmospheric absorption loss, dB
[PL] = polarization mismatch loss, dB