• A satellite is an object that orbits another large object like planet.
• A communication satellite is a staion in space that is used for telecommuncation, radio and television signals.
• The first satellite with radio transmitter was in 1957.
Geostationary orbits
• What are them? Geostationary orbits is fixed position to an earth-based observer.
• When was the first use? The first truly geostationary sateliite was the SYNCOM3 in 1964.
• Why they are important in communications? - The antennas in the ground don’t need equipment to track the satellite. - Lower cost & complixity.
• Disadvantages? - Not always suitable for providing services at high latitudes. - Molniya satellite was introduced as a solution.
Lunching Satellites
How does a satellite stay in it’s orbit?
Frequency Bands
• Three common bands:
1) C-Band.
2) KU-Band.
3) KA-Band.
• Most common are C-Band & KU-Band.
• C-Band occupy 4 to 8 GHz frequency:- Low frequency.- Large antenna (2-3 meters).
• KU-Band occupy 11 to 17 GHz:- Large frequency.- Small antenna (18-inches!)
How Does a Satellite Work?
Consider the light bulb example
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Applications
• Telephony - Fixed points<> Satellite> earth station> fixed points.
• Televesion & Radio - e.g. Direct broadcast satellite (DBS) & Fixed service satellite (FFS).
• Mobile satellite technology - Special antenna called mobile satellite antenna. - No matter where or how this antenna is mounted on.
Amateur radio - Access to OSCAR satellite. - Low earth orbits.
• Internet - High Speed. - Useful for far away places.
• Military - Uses geostationary satellites. - Example: The Defense Satellite Communications System (DSCS).
Disadvantages
• The antenna noise due to energy - Unwanted radiation sources (stars – galaxies - …etc). - Worsen S/N ratio.
• Atmosphere behaves as a resistive medium - Supplies noise power to the antenna.
• Meteors - Have to be programmed to avoid any rock or any harmful thing. - Rules of orbits.
• Expensive - only for governments or large organizations.
Satellites remain the best utilization used for communications due to their speed and other advantages
Orbits
• Several types
• LEOs - Low earth orbit
• MEOs - Medium earth orbit
• GEOs - Geostationary earth orbit
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
• 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
• Since they appear stationary, GEOs do not require tracking
• GEOs are good for broadcasting to wide areas
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
Early satellites
• Relay
– 4000 miles orbit
• Telstar
– Allowed live transmission across the Atlantic
• Syncom 2
– First Geosynchronous satellite
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TELSTAR
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SYNCOM 2.bmp)
Major problems for satellites
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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
• 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
Antennae 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
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 be a problem
Ku band assignments
• 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
• 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
Antennae 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
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 be a problem
Ku band assignments
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