2. Content
History
How Satellites are used
How Satellites Work
Satellites Orbits
o LEOs - Low earth orbit
o MEOs - Medium earth orbit
o GEOs - Geostationary earth orbit
Satellite Communications Needs
Major problems for satellites
Advantages of Satellite
Disadvantages of Satellite
Conclusion
3. History of Satellites
The First Satellites
The theory of satellites was simple enough - shoot something out
into space at the right speed and on the correct trajectory and it will
stay up there, orbiting Earth, for years - if not forever.
If the orbit is the right distance in space the satellite will keep pace
with the rotation of the Earth.
Pioneer Satellites (1957)
Early in October 1957 communications stations started picking up a
regular beeping noise coming from space.
The signals were coming from Russia's Sputnik 1, the world's first
man-made satellite.
It was January 1958, before a Jupiter rocket successfully launched
Explorer 1, the first American satellite.
4. History of Satellites
NASA's Syncom programme (1963) GEOs
In July 1963 the Hughes Aircraft Corporation launched the
experimental Syncom 2 for NASA, the world's first
geosynchronous communications satellite. Its earlier sister,
Syncom 1, had been blown up on launch earlier that year, but
the second version was a huge success.
It carried the first live two-way satellite call between heads of
state when President John F. Kennedy in Washington, D.C.,
telephoned Nigerian Prime Minister Abubaker Balewa in
Africa.
The third Syncom satellite transmitted live television coverage
of the 1964 Olympic Games from Tokyo.
5. History of Satellites
Early Bird (1965)
The world's first commercial communications satellite was
Early Bird, built for the Communications Satellite Corporation
(COMSAT) by Hughes.
The satellite was launched on April 6, 1965, and placed in
commercial service after moving into geosynchronous orbit
22,300 miles above the equator. That meant it was always on
station to provide line of sight communications between
Europe and North America.
Early Bird didn't have a battery - and worked only when its
solar panels were exposed to the sun.
6. 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
7. How Satellites Work
A Earth Station sends
message in GHz
range. (Uplink)
Satellite Receive and
retransmit signals
back. (Downlink)
Other Earth Stations
receive message in
useful strength area.
(Footprint)
9. 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.
10. 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.
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
11. 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
12. 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.
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.
13. 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.
14. 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.
15. 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.
16. 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.
17. Satellite Frequency Bands and
Antennas (Dishes)
The size of Satellite Dishes (antennas) are related to the
transmission frequency.
There is a inverse relationship between frequency and
wavelength.
As wavelength increases (and frequency decreases), larger
antennas (satellite dishes) are necessary to gather the signal.
18. Satellite Frequency Bands and
Antennas (Dishes)
C-Band Ku-Band
Most commonly used bands: C-band (4 to 8 GHz) , Ku-
band (11 to 17 GHz) , and Ka-band (20 to 30 GHz ).
19. Application Example
Telecommunications
Military communications
Navigation systems
Remote sensing and surveillance
Radio / Television Broadcasting
Astronomical research
Weather observation
20. Satellite Communications
Needs
Space vehicle used as communications platform
(Earth-Space-Earth, Space-Earth, Space-Space)
Space vehicle used as sensor platform with communications
Ground station(s) (Tx/Rx)
Ground receivers (Rx only)
21. 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
22. Major problems for satellites
Positioning in orbit
Stability
Power
Communications
Harsh environment
23. Advantages of Satellite
High channel capacity (>100 Mb/s)
Low error rates (Pe ~ 10-6)
Stable cost environment (no long-distance cables
or national boundaries)
Wide area coverage (whole North America, for
instance)
Coverage can be shaped by antenna patterns
24. Disadvantages of Satellite
Expensive to launch
Expensive ground stations required
Cannot be maintained
Limited frequency spectrum
Limited orbital space (geosynchronous)
Constant ground monitoring required for positioning and
operational control
25. Conclusions
Limited satellite transmitter power
Significant path losses
High gain antennas needed
Antenna patterns can be shaped as desired
Location and tracking necessary
Atmospheric effects can be significant
