Communication Satellites
Geostationary Satellites ; MediumEarth Orbit Satellites; Low-Earth
Orbit Satellites; Satellites versus Fiber
Communication Satellites
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In the 1950s and early 1960s –
attempts for communication systems
by bouncing signals off metalized
weather balloons
• received signals - too weak to be of
any practical use
Communication satellites can be
regarded as big microwave repeaters in
the sky
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Communication Satellites
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Contain several transponders – device which
listens to some portion of the spectrum,
amplifies the incoming signal, and then
rebroadcasts it at another frequency to avoid
interference with the incoming signal
The downward beams can be
- broad, covering a substantial fraction of the
earth's surface, or
- narrow, covering an area only hundreds of
kilometers in diameter - bent pipe mode
Communication Satellites
• The higher the satellite, the longer the orbital
period
• Near the surface of the earth, the period is about
90 minutes
• low-orbit satellites pass out of view fairly quickly,
so many of them are needed to provide
continuous coverage
• At an altitude of about 35,800 km, the period is
24 hours
• At an altitude of 384,000 km, the period is about
one month, proof - the moon
Communication Satellites
• Another issue is the presence of the Van Allen
belts - layers of highly charged particles trapped
by the earth's magnetic field
• Any satellite flying within them would be
destroyed fairly quickly by the highly-energetic
charged particles trapped there by the earth's
magnetic field
• Hence there are three regions in which satellites
can be placed safely - illustrated in the following
figure
Communication Satellites
Communication satellites and some of their properties, including
altitude above the earth, round-trip delay time and number of
satellites needed for global coverage.
Communication Satellites
Geostationary Satellites
• In 1945, the science fiction writer Arthur C. Clarke
calculated that a satellite at an altitude of 35,800
km in a circular equatorial orbit would appear to
remain motionless in the sky, so it would not
need to be tracked
• Clarke described a complete communication
system that used these (manned) geostationary
satellites, including the orbits, solar panels, radio
frequencies, and launch procedures
Geostationary Satellites
• The first artificial communication satellite,
Telstar, was launched in July 1962
• Since then, communication satellites have
become a multibillion dollar business and the
only aspect of outer space that has become
highly profitable
• Often called GEO (Geostationary Earth Orbit)
satellites
Geostationary Satellites
• Satellites in geostationary orbit must all occupy a single
ring above the Equator and must be spaced apart to avoid
harmful radio-frequency interference during operations
• Limited number of orbital "slots" available, thus only a
limited number of satellites can be operated in
geostationary orbit
• Conflict between different countries wishing access to the
same orbital slots - countries near the same longitude
(east-west position of a point on the Earth's surface) but
differing latitudes and also conflict on radio frequencies (L,
S bands – crowded, C, Ku, Ka bands – with problems-rain…)
• Disputes are addressed through the International
Telecommunication Union's allocation mechanism
Geostationary Satellites
• Modern satellites can weigh up to 4000 kg and consume
several kilowatts of electric power produced by the solar
panels
• The effects of solar, lunar, and planetary gravity tend to
move them away from their assigned orbit slots and
orientations, an effect countered by on-board rocket
motors – with them fine-tuning activity is achieved - station
keeping.
• When the fuel for the motors runs out (in about 10 years),
the satellite drifts and tumbles helplessly - must be turned
off
• Eventually, the orbit decays and the satellite reenters the
atmosphere and burns up or occasionally crashes to earth
Geostationary Satellites
• A modern satellite has around 40 transponders,
each with an 80-MHz bandwidth
• Each transponder operates as a bent pipe
(narrow down link)
• Earliest satellites had static division of channels the bandwidth was split up into fixed frequency
bands
• With modern satellites each transponder beam is
divided into time slots, with various users taking
turns
Geostationary Satellites
• Each downward beam can be focused on a small
geographical area, so multiple upward and
downward transmissions can take place
simultaneously
• Typically, the spot beams are elliptically shaped,
and can be as small as a few hundred km in
diameter
• A communication satellite for the United States
typically has one wide beam for the contiguous
48 states, plus spot beams for Alaska and Hawaii
VSAT systems
• VSAT (Very Small Aperture Terminal) – a satellite
communication system that serves home and
business users for data, voice, and video signals
• Tiny terminals have 1-meter or smaller antennas
(versus 10 m for a standard GEO antenna) and
can put out about 1 watt of power.
• A VSAT end user has a box that interfaces
between the user's computer and an outside
antenna with a transceiver.
• The tranceiver receives /sends a signal to a
satellite transponder
VSAT systems
• The satellite sends and receives signals from
an earth station computer that acts as
a hub for the system.
• Each end user is interconnected with the hub
station via the satellite in a star topology. For
one end user to communicate with another,
each transmission has to first go to the hub
station which retransmits it via the satellite to
the other end user's VSAT
VSAT systems
• VSAT is used both by home users (who sign up
with a large service) and by private companies
that operate or lease their own VSAT systems
• VSAT offers a number of advantages over
terrestrial alternatives - companies can have
total control of their own communication system
without dependence on other companies
• Business and home users also get higher speed
reception than if using ordinary telephone service
or ISDN
• VSATs have great potential in rural areas
VSATs using a hub
Satellites vs terrestrial
• The long round-trip distance introduces a
substantial delay for GEO satellites
• Depending on the distance between the user and
the ground station, and the elevation of the
satellite above the horizon, the end-to-end transit
time is between 250 and 300 msec - typically 270
msec (540 msec for a VSAT system with a hub)
• Terrestrial microwave links have a propagation
delay of roughly 3 µsec/km, and coaxial cable or
fiber optic links have a delay of approximately 5
µsec/km
Satellites vs terrestrial
• Satellites are inherently broadcast media - it does
not cost more to send a message to thousands of
stations within a transponder's footprint than it
does to send to one
• Useful property in some cases - a satellite can
broadcast popular Web pages to the caches of a
large number of computers spread over a wide
area
• Security and privacy – poor - everybody can hear
everything
• When security is required, encryption is needed
Satellites vs terrestrial
• The cost of transmitting a message is
independent of the distance traversed for
satellites
• A call across the ocean costs no more to
service than a call across the street
• Satellites also have excellent error rates and
can be deployed almost instantly
Medium-Earth Orbit Satellites
• The MEO (Medium-Earth Orbit) satellites are
between the two Van Allen belts
• Takes something like 6 hours to circle the earth
• As MEO are lower than the GEOs (typically
16,000 km above Earth), they have a smaller
footprint on the ground and require less
powerful transmitters to reach them
• Currently they are not used for
telecommunications but in navigation systems
Medium-Earth Orbit Satellites
• The 24 GPS (Global Positioning System) satellites
orbiting at about 18,000 km are American (DoD)
but free to use by anyone
• Russia is using Glonass – 24 satellites orbiting at
19,100 km
• European Union works on Galileo – 30 satellite
MEO navigation system orbiting at 23200 km global navigation satellite system (GNSS) (2019)
• China is building Compass – 35 satellites – 5 GEO
and 30 MEO at 21,100 km (10 in use and offering
services now) (2020)
Low-Earth Orbit Satellites
• A Low Earth Orbit (LEO) is generally defined as an orbit below
an altitude of approximately 2,000 kilometers and above 160
kilometers – almost all human spaceflights have taken place in
LEO
• (-) large numbers of LEO are needed for a complete system
due to their rapid motion (26,000 to 27,000 km per hour)
• (-) most spacecraft and artificial satellites have rapidly
decaying orbits due to the drag created by air molecules in
the thermosphere, the last layer of the Earth’s atmosphere
• (+) satellites are very close to the earth, so the ground
stations do not need much power
• (+) the round-trip delay is only a few milliseconds
Low-Earth Orbit Satellites
• Because these LEO orbits are not
geostationary, a network (or "constellation")
of satellites is required to provide continuous
coverage
• LEOs are used for a variety of civil, scientific
and military roles including Earth observation,
radar, optical, telecoms and demonstrator
• Some use as many as 66 satellites (Iridium –
satellite phones)
Low-Earth Orbit Satellites
• The Iridium satellite constellation - group of satellites
providing voice and data coverage to satellite phones,
pagers and integrated transceivers over Earth's entire
surface
• Originally developed in 1992, services started in 1998,
bankrupted in August 1999 and subsequently restarted
in March 2001 (513 000 subscribers in 2011)
• Satellites are at a height of approximately 781 km
• Customers include the maritime, aviation, and oil
exploration industries, as well as people traveling in
parts of the world lacking a telecommunications
infrastructure (e.g., deserts, mountains, jungles, and
some Third World countries)
Iridium
• The Iridium satellites are in circular polar
orbits
• They are arranged in north-south necklaces,
with one satellite every 32 degrees of latitude
• With six satellite necklaces, the entire earth is
covered
• Each satellite can support up to 1100
concurrent phone calls and weighs about
680 kg
Low-Earth Orbit Satellites
Iridium
(a) The Iridium satellites from six necklaces around the
earth.
(b) 1628 moving cells cover the earth.
Iridium
• Each satellite has a maximum of 48 cells (spot beams), with
a total of 1628 cells over the surface of the earth (figure in
previous slide)
• Each satellite has a capacity of 3840 channels, or 253,440 in
all
• Some of these are used for paging and navigation, while
others are used for data and voice
• An interesting property of Iridium is that communication
between distant customers takes place in space, with one
satellite relaying data to the next one
• A caller at the North Pole contacts a satellite directly
overhead - the call is relayed via other satellites and finally
sent down to the callee at the South Pole.
Iridium - Globalstar
(a) Relaying in space.
(b) Relaying on the ground.
Globalstar
• Globalstar - based on 48 LEO satellites but uses a
different switching scheme than that of Iridium (relays
calls from satellite to satellite, which requires
sophisticated switching equipment in the satellites)
• Globalstar uses a traditional bent-pipe design - a call
originating at the North Pole is sent back to earth and
picked up by the large ground station
• The call is then routed via a terrestrial network to the
ground station nearest the callee and delivered by a
bent-pipe connection as shown
• Advantage - puts much of the complexity on the
ground, where it is easier to manage (412300
subscribers in 2010)
Teledesic
• Teledesic was as high-speed, wireless, switched global
network -an IP-based system designed to support data,
voice and video at the same quality level as fiber-based
terrestrial systems - a global "Internet-in-the-Sky“ providing
broadband and Internet access
• Originally (1995) planning 840 active satellites with in-orbit
spares at an altitude of 700 km,
• Scaled (1997) to 288 active satellites at 1400 km
• Officially suspended its satellite construction work on
October 1, 2002
• Funding from Microsoft (investing US$30 million for an
8.5% stake), Craig McCaw, Bill Gates, Paul Allen, and Saudi
prince Alwaleed bin Talal, and for achieving allocation on
the Ka-band frequency spectrum for non-geostationary
services
Highly elliptical orbit
• HEO – an elliptic orbit with a low-altitude (about
1,000 kilometres) perigee and a high-altitude
(over 35,786 kilometres) apogee
• Advantage - long dwell times at a point in the sky
during the approach to, and descent from,
apogee - phenomenon known as apogee dwell
(acts like GEO) (3-4 hours can receive signal with
0.6 m antenna)
• Used by Sirius Satellite Radio (operating in North
America), the Molniya communication satelites
(at least 3) (Russia),
Satellites versus Fiber
• A single fiber has more potential bandwidth than
all the satellites ever launched but this bandwidth
is not available to most users
• The fibers that are installed are used within the
telephone system to handle many long distance
calls at once and not provide individual users with
high bandwidth
• With satellites, a user has an antenna on the roof
of the building and completely bypasses the
telephone system to get high bandwidth
Satellites versus Fiber
• Many people nowadays want to communicate while
jogging, driving, sailing, and flying
• Terrestrial fiber optic links are of no use to them, but
satellite links potentially are
• A combination of cellular radio and fiber will do an
adequate job for most users (except for those airborne
or at sea)
• A message sent by satellite can be received by
thousands of ground stations at once – i.e. an
organization transmits a stream of stock, bond, or
commodity prices to thousands of dealers - a satellite
system may be cheaper than simulating broadcasting
on the ground
Satellites versus Fiber
• Places with hostile terrain or a poorly developed
terrestrial infrastructure - Indonesia, for example, has
its own satellite for domestic telephone traffic
• In areas where obtaining the right of way for laying
fiber is difficult or unduly expensive
• Where rapid deployment is critical, as in military
communication systems in time of war, satellites win
easily
• Seems that mainstream communication of the future
will be terrestrial fiber optics combined with cellular
radio, but for some specialized uses, satellites are
better