Satellite Communications
Introduction and Historical Background
What is a Satellite?
Satellite: In astronomical terms, a
satellite is a celestial body that orbits
around a planet.
Example: The moon is a satellite of Earth.
In aerospace terms, a satellite is a
space vehicle launched by humans and
orbits around Earth or another celestial
body.
What is a Satellite? (Contd.)
Communications Satellite: It is a microwave
repeater in the sky that consists of a diverse
combination of one or more components including
transmitter, receiver, amplifier, regenerator, filter
onboard computer, multiplexer, demultiplexer,
antenna, waveguide etc.
A satellite radio repeater is also called
transponder. This is usually a combination of
transmitter and receiver.
What is a satellite system?
A satellite system consists of one or
more satellites, a ground-based station
to control the operation of the system,
and a user network earth stations that
provides the interface facilities for the
transmission and reception of terrestrial
communications traffic.
How a satellite works?
A satellite stays in orbit because the gravitational
pull of the earth is balanced
by the centripetal force of the revolving
satellite.
One Earth station transmits the signals to the
satellite at Up link frequency. Up link frequency
is the frequency at which Earth station is
communicating with a satellite.
The satellite transponder process the signal and
sends it to the second Earth station at another
frequency called downlink frequency.
Basics: Advantages of
Satellites
The advantages of satellite
communication over terrestrial
communication are:
The coverage area of a satellite greatly
exceeds that of a terrestrial system.
Transmission cost of a satellite is
independent of the distance from the
center of the coverage area.
Satellite to Satellite communication is very
precise.
Higher Bandwidths are available for use.
Basics: Disadvantages of
Satellites
The disadvantages of satellite
communication:
Launching satellites into orbit is costly.
Satellite bandwidth is gradually becoming
used up.
There is a larger propagation delay in
satellite communication than in terrestrial
communication.
Basics: Factors in satellite
communication
Elevation Angle: The angle of the horizontal of the
earth surface to the center line of the satellite
transmission beam.
This effects the satellites coverage area. Ideally, you want a
elevation angle of 0 degrees, so the transmission beam
reaches the horizon visible to the satellite in all directions.
However, because of environmental factors like objects
blocking the transmission, atmospheric attenuation, and the
earth electrical background noise, there is a minimum
elevation angle of earth stations.
Basics: Factors in satellite
communication (cont.)
Coverage Angle: A measure of the
portion of the earth surface visible to a
satellite taking the minimum elevation
angle into account.
R/(R+h) = sin(π/2 - β - θ)/sin(θ + π/2)
= cos(β + θ)/cos(θ)
R = 6370 km (earth’s radius)
h = satellite orbit height
β = coverage angle
θ = minimum elevation angle
Basics: Factors in satellite
communication (cont.)
Other impairments to satellite communication:
The distance between an earth station and a satellite (free
space loss).
Satellite Footprint: The satellite transmission’s strength is
strongest in the center of the transmission, and decreases
farther from the center as free space loss increases.
Atmospheric Attenuation caused by air and water can impair
the transmission. It is particularly bad during rain and fog.
Basics: How Satellites are
used
Service Types
Fixed Service Satellites (FSS)
• Example: Point to Point Communication
Broadcast Service Satellites (BSS)
• Example: Satellite Television/Radio
• Also called Direct Broadcast Service (DBS).
Mobile Service Satellites (MSS)
• Example: Satellite Phones
Regions of Space
Space is defined as a place free from obstacles
It can be divided into three regions:
Air Space -> region below 100 km from earth’s surface
Outer Space -> also called cosmic space and ranges from
100 km up till 42, 000 km. It is mostly used by
communication satellites.
Deep Space -> Regions beyond 42,000 km fall in this
category
Active and Passive Satellites
Active satellites are used for linking and also for
processing the signals.
The linkage is known as bent pipe technology where processing
like frequency translation, power amplification etc take place.
Active satellites employ ‘Regenerative Technology’ which
consists of demodulation, processing, frequency translation,
switching and power amplification are carried out. Block used
for this purpose is called transponder.
Passive satellites do-not have on-board processing and
are just used to link two stations through space.
Low cost - Loss of power – not useful for communication
applications.
Historical Overview
1945 Theorist named Clarke studied that satellite orbiting in equatorial
orbit at radius of approx. 42,000 km would look as if stationary if moving at a
specific speed. 3 satellites at a space of 120 degree apart can cover the whole
world. Evolution of the concept of GEO
1950’s –Putting the pieces together:
1956 -Trans-Atlantic cable opened (about 12 telephone channels
per operator).
1957 First man-made satellite launched by former USSR (Sputnik-1,
LEO). It was used to identify atmospheric density of various orbital
layers. It provided data about radio signal distribution in ionosphere.
1958 First US satellite launched (SCORE). First voice communication
established via satellite (LEO, lasted 35
days in orbit).
Historical Overview
1960’s –First satellite communications:
1960 First passive communication satellite (Large
balloons, Echo I and II).
1962: First active communication satellite (Telstar I
, MEO).
1963: First satellite into geostationary (GEO) orbit
(Syncom1, communication failed).
1964: International Telecomm. Satellite
Organization (INTELSAT) created.
1965 First successful communications GEO (Early
Bird / INTELSAT 1).
Historical Overview
1970’s –GEO Applications Development,
DBS:
1972 First domestic satellite system
operational (Canada).
1975 First successful direct broadcast
experiment (USA-India).
1977 A plan for direct broadcast satellites
(DBS) assigned by the ITU
1979 International Mobile Satellite
Organization (Inmarsat) established.
Historical Overview
1980’s –GEO Applications Expanded, Mobile:
1981 First reusable launch vehicle flight.
1982 International maritime communications made
operational.
1984 First direct-to-home broadcast system
operational (Japan).
1987 Successful trials of land-mobile
communications (Inmarsat).
1989-90 Global mobile communication service
extended to land mobile and aeronautical use
(Inmarsat)
Historical Overview
1990+’s NGSO applications development and GEO
expansion
1990-95:
Proposals of non-geostationary (NGSO) systems for mobile
communications.
Continuing growth of VSATs around the world.
Spectrum allocation for non-GEO systems.
Continuing growth of DBS. DirectTV created.
1997:
Launch of first batch of LEO for hand-held terminals (Iridium).
Voice-service portables and paging-service pocket size mobile
terminals launched (Inmarsat).
1998-2000:
Mobile LEO systems initiate service and fail afterwards
(Iridium,Globalstar).
Historical Overview
1945 Arthur C. Clarke Article: "Extra-Terrestrial Relays"
1955 John R. Pierce Article: "Orbital Radio Relays"
1956 First Trans-Atlantic Telephone Cable: TAT-1
1957 Sputnik: Russia launches the first earth satellite.
1962 TELSTAR and RELAY launched
1962 Communications Satellite Act (U.S.)
1963 SYNCOM launched
1965 COMSAT's EARLY BIRD: 1st commercial communications
satellite
1969 INTELSAT-III series provides global coverage
Historical Overview
1972 ANIK: 1st Domestic Communications Satellite (Canada)
1974 WESTAR: 1st U.S. Domestic Communications Satellite
1975 RCA SATCOM: 1st operational body-stabilized comm.
satellite
1976 MARISAT: 1st mobile communications satellite
1988 TAT-8: 1st Fiber-Optic Trans-Atlantic telephone cable
1994 GPS system deployed by USAF
1998-2001 Iridium
Types of Satellites
Satellite Orbits
GEO
LEO
MEO
Molniya Orbit
HAPs
Frequency Bands
Geostationary Earth Orbit
(GEO)
These satellites are in orbit 35,863 km
above the earth’s surface along the
equator.
Objects in Geostationary orbit revolve
around the earth at the same speed as
the earth rotates. This means GEO
satellites remain in the same position
relative to the surface of earth.
GEOs
Originally proposed by Arthur C. Clarke
Circular orbits above the equator
Angular separation about 2 degrees -
allows 180 satellites
Orbital height above the earth about
23000 miles/35000km
Round trip time to satellite about 0.24
seconds
GEOs (2)
GEO satellites require more power for
communications
The signal to noise ratio for GEOs is
worse because of the distances involved
A few GEOs can cover most of the
surface of the earth
Note that polar regions cannot be
“seen” by GEOs
GEOs (3)
Since they appear stationary, GEOs do
not require tracking
GEOs are good for broadcasting to wide
areas
GEO (cont.)
Advantages
A GEO satellite’s distance from earth gives
it a large coverage area, almost a fourth of
the earth’s surface.
GEO satellites have a 24 hour view of a
particular area.
These factors make it ideal for satellite
broadcast and other multipoint
applications.
GEO (cont.)
Disadvantages
A GEO satellite’s distance also cause it to
have both a comparatively weak signal and
a time delay in the signal, which is bad for
point to point communication.
GEO satellites, centered above the
equator, have difficulty broadcasting
signals to near polar regions
Low Earth Orbit (LEO)
LEO satellites are much closer to the
earth than GEO satellites, ranging from
500 to 1,500 km above the surface.
LEO satellites don’t stay in fixed
position relative to the surface, and are
only visible for 15 to 20 minutes each
pass.
A network of LEO satellites is necessary
for LEO satellites to be useful
LEO (cont.)
Advantages
A LEO satellite’s proximity to earth
compared to a GEO satellite gives it a
better signal strength and less of a time
delay, which makes it better for point to
point communication.
A LEO satellite’s smaller area of coverage is
less of a waste of bandwidth.
LEO (cont.)
Disadvantages
A network of LEO satellites is needed,
which can be costly
LEO satellites have to compensate for
Doppler shifts cause by their relative
movement.
Atmospheric drag effects LEO satellites,
causing gradual orbital deterioration.
Medium Earth Orbit (MEO)
A MEO satellite is in orbit somewhere between 8,000
km and 18,000 km above the earth’s surface.
MEO satellites are similar to LEO satellites in
functionality.
MEO satellites are visible for much longer periods of
time than LEO satellites, usually between 2 to 8
hours.
MEO satellites have a larger coverage area than LEO
satellites.
MEO (cont.)
Advantage
A MEO satellite’s longer duration of
visibility and wider footprint means fewer
satellites are needed in a MEO network
than a LEO network.
Disadvantage
A MEO satellite’s distance gives it a longer
time delay and weaker signal than a LEO
satellite, though not as bad as a GEO
satellite.
Other Orbits
Molniya Orbit Satellites
Used by Russia for decades.
Molniya Orbit is an elliptical orbit. The
satellite remains in a nearly fixed position
relative to earth for eight hours.
A series of three Molniya satellites can act
like a GEO satellite.
Useful in near polar regions.
Other Orbits (cont.)
High Altitude Platform (HAP)
One of the newest ideas in satellite
communication.
A blimp or plane around 20 km above the
earth’s surface is used as a satellite.
HAPs would have very small coverage
area, but would have a comparatively
strong signal.
Cheaper to put in position, but would
require a lot of them in a network.
Early experiments
US Navy bounced messages off the
moon
ECHO 1 “balloon” satellite - passive
ECHO 2 - 2nd passive satellite
All subsequent satellites used active
communications
ECHO 1
Photo from NASA
Early satellites
Relay
4000 miles orbit
Telstar
Allowed live transmission across the
Atlantic
Syncom 2
First Geosynchronous satellite
TELSTAR
Picture from NASA
SYNCOM 2
Picture from NASA
Major problems for satellites
Positioning in orbit
Stability
Power
Communications
Harsh environment
Positioning
This can be achieved by several methods
One method is to use small rocket motors
These use fuel - over half of the weight of
most satellites is made up of fuel
Often it is the fuel availability which
determines the lifetime of a satellite
Commercial life of a satellite typically 10-15
years
Stability
It is vital that satellites are stabilised
to ensure that solar panels are aligned properly
to ensure that communications antennae are
aligned properly
Early satellites used spin stabilisation
Either this required an inefficient omni-directional
aerial
Or antennae were precisely counter-rotated in
order to provide stable communications
Stability (2)
Modern satellites use reaction wheel
stabilisation - a form of gyroscopic
stabilisation Other methods of
stabilisation are also possible
including:
eddy currrent stabilisation
(forces act on the satellite as it moves
through the earth’s magnetic field)
Reaction wheel stabilisation
Heavy wheels which rotate at high speed - often in
groups of 4.
3 are orthogonal, and the 4th (spare) is a backup at
an angle to the others
Driven by electric motors - as they speed up or slow
down the satellite rotates
If the speed of the wheels is inappropriate, rocket
motors must be used to stabilise the satellite - which
uses fuel
Power
Modern satellites use a variety of power means
Solar panels are now quite efficient, so solar power is
used to generate electricity
Batteries are needed as sometimes the satellites are
behind the earth - this happens about half the time
for a LEO satellite
Nuclear power has been used - but not
recommended
Harsh Environment
Satellite components need to be specially “hardened”
Circuits which work on the ground will fail very
rapidly in space
Temperature is also a problem - so satellites use
electric heaters to keep circuits and other vital parts
warmed up - they also need to control the
temperature carefully
Alignment
There are a number of components
which need alignment
Solar panels
Antennae
These have to point at different parts of
the sky at different times, so the
problem is not trivial
Antenna alignment
A parabolic dish can be used which is
pointing in the correct general direction
Different feeder “horns” can be used to
direct outgoing beams more precisely
Similarly for incoming beams
A modern satellite should be capable of
at least 50 differently directed beams
Satellite - satellite
communication
It is also possible for satellites to
communicate with other satellites
Communication can be by microwave or
by optical laser
LEOs
Low earth orbit satellites - say between
100 - 1500 miles
Signal to noise should be better with
LEOs
Shorter delays - between 1 - 10 ms
typical
Because LEOs move relative to the
earth, they require tracking
Orbits
Circular orbits are simplest
Inclined orbits are useful for coverage
of equatorial regions
Elliptical orbits can be used to give
quasi stationary behaviour viewed from
earth
using 3 or 4 satellites
Orbit changes can be used to extend
the life of satellites
Communication frequencies
Microwave band terminology
L band 800 MHz - 2 GHz
S band 2-3 GHz
C band 3-6 GHz
X band 7-9 GHz
Ku band 10-17 GHz
Ka band 18-22 GHz
Early satellite communications
Used C band in the range 3.7-4.2 GHz
Could interfere with terrestrial
communications
Beamwidth is narrower with higher
frequencies
More recent communications
Greater use made of Ku band
Use is now being made of Ka band
Rain fade
Above 10 GHz rain and other
disturbances can have a severe effect
on reception
This can be countered by using larger
receiver dishes so moderate rain will
have less effect
In severe rainstorms reception can be
lost
In some countries sandstorms can also
Ku band assignments
© copyright 1996 MLE INC.
Satellite management
Satellites do not just “stay” in their
orbits
They are pushed around by various
forces
They require active management
Systems of satellites
Example - Iridium
Deploy many satellites to give world
wide coverage - including polar regions
So far have not proved commercially
viable
Other systems “coming along” -
Teldesic
The future
Because Iridium has not been a
commercial success the future of
satellites is uncertain
Satellites still have major advantages
for wide area distribution of data
Motivation to use the Sky
Orbital Types
Frequency Bands
Different kinds of satellites use different frequency
bands.
L–Band: 1 to 2 GHz, used by MSS
S-Band: 2 to 4 GHz, used by MSS, NASA, deep space research
C-Band: 4 to 8 GHz, used by FSS
X-Band: 8 to 12.5 GHz, used by FSS and in terrestrial imaging, ex:
military and meteorological satellites
Ku-Band: 12.5 to 18 GHz: used by FSS and BSS (DBS)
K-Band: 18 to 26.5 GHz: used by FSS and BSS
Ka-Band: 26.5 to 40 GHz: used by FSS
Capacity Allocation
FDMA
FAMA-FDMA
DAMA-FDMA
TDMA
Advantages over FDMA
FDMA
Satellite frequency is already broken
into bands, and is broken in to smaller
channels in Frequency Division Multiple
Access (FDMA).
Overall bandwidth within a frequency
band is increased due to frequency
reuse (a frequency is used by two
carriers with orthogonal polarization).
FDMA (cont.)
The number of sub-channels is limited
by three factors:
Thermal noise (too weak a signal will be
effected by background noise).
Intermodulation noise (too strong a signal
will cause noise).
Crosstalk (cause by excessive frequency
reusing).
FDMA (cont.)
FDMA can be performed in two ways:
Fixed-assignment multiple access (FAMA):
The sub-channel assignments are of a
fixed allotment. Ideal for broadcast
satellite communication.
Demand-assignment multiple access
(DAMA): The sub-channel allotment
changes based on demand. Ideal for point
to point communication.
TDMA
TDMA (Time Division Multiple Access)
breaks a transmission into multiple time
slots, each one dedicated to a different
transmitter.
TDMA is increasingly becoming more
widespread in satellite communication.
TDMA uses the same techniques (FAMA
and DAMA) as FDMA does.
TDMA (cont.)
Advantages of TDMA over FDMA.
Digital equipment used in time division
multiplexing is increasingly becoming
cheaper.
There are advantages in digital
transmission techniques. Ex: error
correction.
Lack of intermodulation noise means
increased efficiency.
