16-03-2010, 07:46 PM
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Satellite communications
Introduction
Before the lecture
Try to find out more by reading:
http://ctd.grc.nasa.gov/rleonard/regcontents.html
http://aticoursesiridium.htm
http://aticoursesglobal_positioning_system.htm
http://mlesatArticle9.html
http://mlesattutorial.html
Satellites
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
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
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 earthâ„¢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
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 be a problem
Ku band assignments
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
Chronology
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
Chronology (2)
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
Satellite communications
Introduction
Before the lecture
Try to find out more by reading:
http://ctd.grc.nasa.gov/rleonard/regcontents.html
http://aticoursesiridium.htm
http://aticoursesglobal_positioning_system.htm
http://mlesatArticle9.html
http://mlesattutorial.html
Satellites
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
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
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 earthâ„¢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
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 be a problem
Ku band assignments
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
Chronology
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
Chronology (2)
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
