سه شنبه ۲۰ شهریور ۰۳ | ۲۲:۲۷ ۷ بازديد
REN requirements for development of seamless nomadic networks necessitates that
NREN staff have a working knowledge of basic satellite technology. This paper
applications, technology trends, orbits, and spectrum, and hopefully will afford the reader
an end-to-end picture of this important technology.
Satellites are objects in orbits about the Earth. An orbit is a trajectory able to maintain
gravitational equilibrium to circle the Earth without power assist. Physical laws that were
comeptualized by Newton and Kepler govern orbital mechanics.
The first satellite was the Moon, of course, but the idea of communications satellites
came from Sir Arthur C. Clark in 1945. The Soviets launched the first manmade satellite,
Sputnik I , in 1957. The first communications satellite (a simple reflector) was the US.
Echo I in 1960. The first “geosynchronous” (explained later) satellite, Syncom, went up
in 1962. There are now over 5000 operational satellites in orbit, 232 of which are large
commercial (mostly communications) satellites.
Satellites have become essential for modern life. Among the important applications of
satellite technology are video, voice, IP data, radio, Earth and space observation, global
resource monitoring, military, positioning (GPS), micro-gravity science and many others.
From direct-to-home TV to the Hubble telescope, satellites are one of the defining
technologies of the modem age. Video is the most successful commercial application for
satellites, and direct-to-home distribution is the most promising application for the
technology at this time. “Spot” images of places on Earth, GPS, and Internet
access-both for providers and direct-to-home or o f i c e h a v e been most successful,
while cell phone systems based on fleets of low flying satellites have been a flop. Mobile
phone-like connections for marine and mobile services have been with us for some time,
however.
Satellite services have some big advantages, such as being available almost everywhere
on Earth without wires, being mobile, being the perfect broadcast medium, and being
protocol agnostic. The downside to satellite technology is that satellites have either a
limited visibility over a spot on Earth, or a long round-trip time, and they broadcast data
that can be received by anyone under them. Satellite transmissions are also affected by
both terrestrial and space weather. They are subject to a higher error rate than fiber, and
they are complex from both a physical and regulatory point of view.
Satellites are launched from Earth by the shuttle, from high-flying airplanes, or from
ground-based rockets. Once launched, payloads must reach proper elevation and escape
velocity to be boosted into orbit. In order to maintain proper orbit, satellites are
i A A t o c c n c th- c n m n n n e n t o v e n i i ; ~ a A fnr 9 c m t a l l ; t o - h i c o A r n m m z r n ; , - m t ; n n c c x r c t o m
UU~”““”” ..LA” w”Luy”-”l*ru ‘”‘IU” v u L V I Y “ Y b Y I I L . ” “ I U I U “V.A.A*I-*I“..c*VILy “ J UCVL..,
Introduction to Satellite Communication Technology for NREN
controlled from a ground station on Earth that sends commands and receives status and
telemetry from the satellite.
2. ORBITS
Satellites are classified by the distance of their orbits above the Earth. Low Earth
Orbiting (LEO) satellites are located at an altitude of from 100 to 1200 miles, and
Medium Eartl Orbits (lVEO)are located at m althtde of from 4,000-!2,000 riles. Geo-
stationary orbits are located exactly 23.4 miles high. There are two Van Allen radiation
belts around the Earth. These areas of intense radiation are to be avoided in deploying
satellites. The two zones separate LEO and ME0 orbits.
LEG and i v E 0 (sub-geosynchronousj orbiring sareiiires are visitxe for oniy a period of
time from the point of view of an observer on Earth. Satellites in these orbits can
communicate with the Earth by “pass of€” to a fleet of like satellites providing full Earth
coverage. They can also pass off to a geosynchronous relay satellite, or they can dump
data to the ground as they pass over a ground station. Most of these “near Earth” satellites
are in orbits that go over or near the poles; some go over the same place on Earth at least
several times a day. The coordination and placement of Earth stations to “operate” and
take data from these satellites is a major consideration in the life cycle of a satellite. The
advantage of sub-geo satellites over those in geosynchronous orbit is that lower orbits are
less expensive to launch, it takes less power to send and receive data, and the round trip
time from Earth is faster.
. ...
0
THE0 (Twelve Hour Eccentric Orbits, or Molniya orbits) are not quite polar orbits that
stay in a hemisphere for half of a day. This approach allows for two M E 0 satellites to
cover an area of the Earth, thus giving continuous service as a geosynchonous satellite
would. This was developed and used extensively in Russia.
Geosynchronous orbits are the most desirable and the most expensive to obtain.
“Geosynch” satellites appear stationary to observers on the Earth. The orbit is always at
the equator at a height of 23,400 miles; the satellite thus travels in the same direction and
I
Introduction to Satellite Communication Technology for NREN
speed as the rotation of the Earth. These satellites, because they provide continuous
coverage, are the workhorses of the commercial satellite industry. Geosynch satellites
must be spaced at least 1/2 a degree apart to prevent signal interference. “Slots” are
closely regulated by the International Telecommunications Union (T.C.U.). They are
highly sought after and expensive to acquire, and since geosynch orbits are also
expensive to launch and maintain, most satellites in this orbit are big and complex.
Geosynchronous orbits require at least a quarter second round-trip time because of the
23,400-mile distance above the Earth plus the distance to the equator from the ground
station. More power is also needed because of the distance (although a larger antenna on
some satellites mitigates some of this). Since geosynch satellites are always in
approximately the same place with regard to the Earth, it is possible to precisely map a
“footprint” so that users can calibrate ground station equipment to minimize power and
bandwidth.
Geosynch satellites are kept tightly in their orbits by expending fuel. Older satellites are
often let to drift north and south while keeping in their east-west slots. Satellites allowed
to dnfi are said to be inclined orbits. This might require some tracking by ground
equipment, but when the drift is large enough, it allows the satellites to be seen in the
polar regions.
3. SPECTRUM
Satellites use the microwave band for communications (some older ones used the radio
ffequencies UHF, VHF). For a description of the electromagnetic spectrum check
http://imagine.gsfc.nasa.gov/docs/scienceAnow-I I/emspectrum.htm1.
I
~ ~~~~~~ ~~~
We modify sound waves to make speech. Dynamics such as loudness and tone are
changed to convey meaning. Telecommunication modifies electromechanical signals
such as microwaves to carry information in the same ways.
Microwave bandwidth is regulated by the FCC in the U.S. and by the ITU internationally.
Obtaining parts of the regulated bandwidth is expensive. Satellite operators must apply
Introduction to Satellite Communication Technology for NREN
years in advance to get spectrum for launch support, tracking, telemetery and operations,
and for data transfer.
Parts of the microwave spectrum are designated and used as follows (for a detailed
description see: http://www.NTIA.doc.gov/osmhome/nebbie.html):
VHFA-JHF 0.1-0.3 GHz - Milsat- amateur radio, “little” LEO
L Band - 1-2 GHz - Mobile Sat/manne, “big” LEO
S Band 2-4 GHz - Satellite commandcontrol
C Band 4-8 GHz - Data, voice, video distribution
Ku 12-18 GHz - Direct TV, Data, Voice, SNG, IP services
K 18-27 GHz -N/A (22.3-HzO absorption)
Ka 27-40 GHz - The next wave
* V 40-75 GHz - Released in the future (60 GHz - 0 2 absorption)
Z& v n - - A o 1 OTT- n s:i:~-- PACI
A YOllU O - I L U I i L - lvLlllLilly-cwa
Uplink (from the Earth to the satellite) uses the upper part of a kequency range while
downlink uses the lower fi-equencies. This is because a satellite is less capable of dealing
with the overtones derived from transmission.
The higher the fi-equencythe more bandwidth it takes to send a bit. That is because there
that are absorbed by water and free Oxygen. Also, the higher the frequency the smaller is
the antenna needed for the same bandwidth, as antenna size is related to the size of the
waveforms. The exception to this is L-Band where the signal is omni-directional. K-
Band is not used for satellite transmission as these frequencies are absorbed completely
by water vapor. Traditional C-Band, the first spectrum opened to satellites for
communications, requires a BUD (big ugly dish) at least 3 meters in diameter for even a
small bandwidth V-Sat, and up to 20 meters for a proper Earth station. Ku and Ka use
small dishes that can be portable.
4. EARTH STATIONS
Teleports, gateways, and “flyaways” are names for the terrestrial interface where the
signal from satellites meets the ground and where data is uplinked to the satellite. The
traditional architecture for satellite systems is to have a central complex with an antenna
or antennas controlling transmission to and &om satellites. Smaller antennas
communicate to other ground stations or terrestrial networks via.the central teleport.
Teleports act as a gateway between terrestrial networks such as the PSTN, the Internet or
cable TV infrastructure, 2nd the srtellite.
is gicztei atterriattioii of the sipa! “vj: the amospheie i i ~i;v‘e get c!osei to &e f r e q u e i ~ c i ~ ~
4
Introduction to Satellite Communication Technology for NREN
Aside from the interface equipment to connect to ground networks, the Earth station will
have an assortment of hardware to receive and send the microwave signals to and from
the satellite:
Reflector
Low Noise Amplifier
(for Receive)
Solid State Power
Amplifier (for Send)
1I Up/Down Converter
Demodulate
1-1 Modulate Satellite Modem
t
DigitaPSignal
1. A reflector and feed: This is the ''dish" that is the most visible component of the
system. The dish focuses the signal on the feed which sends it on. The size and shape
of the dish are a function of the frequencies used and the strength of the signal. The
larger the dish, the smaller the area of the sky it can see. This seems counter-intuitive
but it is true.
2. Outdoor equipment: This equipment isolates the signals to those we are interested in
and amplifies the signals (both send and receive). The equipment to change the
ffequency range to those the satellite expects (up and down converters) from L-Band
can be either indoor or outdoor.
3. Cable: Cable connects the antenna to the inside equipment.
4. A satellite modem that operates in L-Band (this could be separate modulators and
demodulators): These modems do more than the common terrestrial versions in doing
1
Introduction to Satellite Communication Technology for NREN
A/D and D/A conversion. They also define the signal method, do error correction
using redundancy coding, do time and frequency multiplexing, and perform link
monitoring. Satellite links are often asymmetric, having the ability to receive more
dzta than they can send.
5. Tracking gears: Gears move the antenna, if needed.
6 . A mtc~owove~cnsporcntbt;i!dirzg (radome): A radome is required if the reflectm,
feed and outdoor equipment requires covering from wind and snow.
7. The terrestrial interconnect: This could be a router, or a video codex or a multiplexer
to combine several input streams.
In order for an Earth station to become operational it must go through a series of tests and
adjustments. The tests are designed to test whether proper frequencies and time slots (for
TDM access) are used. Measurements are made and documented to insure that the power
levels are correct. The power is measured in terms of Effective Isotropic Radiated Power
(EIRE’). The units are decibels (dBW). Too much power can cause the signal to be
distorted and could disrupt other communications. Under-powered transmission can be
error prone.
The noise level in both directions is measured. This is G/T (the ratio of the gain fi-om the
antenna and outdoor equipment to the signal noise). The higher the ratio the better. The
definitive test is of course the actual error statistics measured by sending test data in both
db3CtiOES.
5. COMMUNICATIONS ONBOARD A SATELLITE
Earth orbiting satellites are a wonder of modern technology. Everything must withstand
the rigors of space in addition to the strain of the launch. In space there are extreme cold
and heat, radiation and micrometeorites, and storms on the Sun. Orbits must be obtained
and maintained and critical systems must be monitored. There are as many “floor plans”
to satellites as there are applications for the technology
NREN staff have a working knowledge of basic satellite technology. This paper
applications, technology trends, orbits, and spectrum, and hopefully will afford the reader
an end-to-end picture of this important technology.
Satellites are objects in orbits about the Earth. An orbit is a trajectory able to maintain
gravitational equilibrium to circle the Earth without power assist. Physical laws that were
comeptualized by Newton and Kepler govern orbital mechanics.
The first satellite was the Moon, of course, but the idea of communications satellites
came from Sir Arthur C. Clark in 1945. The Soviets launched the first manmade satellite,
Sputnik I , in 1957. The first communications satellite (a simple reflector) was the US.
Echo I in 1960. The first “geosynchronous” (explained later) satellite, Syncom, went up
in 1962. There are now over 5000 operational satellites in orbit, 232 of which are large
commercial (mostly communications) satellites.
Satellites have become essential for modern life. Among the important applications of
satellite technology are video, voice, IP data, radio, Earth and space observation, global
resource monitoring, military, positioning (GPS), micro-gravity science and many others.
From direct-to-home TV to the Hubble telescope, satellites are one of the defining
technologies of the modem age. Video is the most successful commercial application for
satellites, and direct-to-home distribution is the most promising application for the
technology at this time. “Spot” images of places on Earth, GPS, and Internet
access-both for providers and direct-to-home or o f i c e h a v e been most successful,
while cell phone systems based on fleets of low flying satellites have been a flop. Mobile
phone-like connections for marine and mobile services have been with us for some time,
however.
Satellite services have some big advantages, such as being available almost everywhere
on Earth without wires, being mobile, being the perfect broadcast medium, and being
protocol agnostic. The downside to satellite technology is that satellites have either a
limited visibility over a spot on Earth, or a long round-trip time, and they broadcast data
that can be received by anyone under them. Satellite transmissions are also affected by
both terrestrial and space weather. They are subject to a higher error rate than fiber, and
they are complex from both a physical and regulatory point of view.
Satellites are launched from Earth by the shuttle, from high-flying airplanes, or from
ground-based rockets. Once launched, payloads must reach proper elevation and escape
velocity to be boosted into orbit. In order to maintain proper orbit, satellites are
i A A t o c c n c th- c n m n n n e n t o v e n i i ; ~ a A fnr 9 c m t a l l ; t o - h i c o A r n m m z r n ; , - m t ; n n c c x r c t o m
UU~”““”” ..LA” w”Luy”-”l*ru ‘”‘IU” v u L V I Y “ Y b Y I I L . ” “ I U I U “V.A.A*I-*I“..c*VILy “ J UCVL..,
controlled from a ground station on Earth that sends commands and receives status and
telemetry from the satellite.
2. ORBITS
Satellites are classified by the distance of their orbits above the Earth. Low Earth
Orbiting (LEO) satellites are located at an altitude of from 100 to 1200 miles, and
Medium Eartl Orbits (lVEO)are located at m althtde of from 4,000-!2,000 riles. Geo-
stationary orbits are located exactly 23.4 miles high. There are two Van Allen radiation
belts around the Earth. These areas of intense radiation are to be avoided in deploying
satellites. The two zones separate LEO and ME0 orbits.
LEG and i v E 0 (sub-geosynchronousj orbiring sareiiires are visitxe for oniy a period of
time from the point of view of an observer on Earth. Satellites in these orbits can
communicate with the Earth by “pass of€” to a fleet of like satellites providing full Earth
coverage. They can also pass off to a geosynchronous relay satellite, or they can dump
data to the ground as they pass over a ground station. Most of these “near Earth” satellites
are in orbits that go over or near the poles; some go over the same place on Earth at least
several times a day. The coordination and placement of Earth stations to “operate” and
take data from these satellites is a major consideration in the life cycle of a satellite. The
advantage of sub-geo satellites over those in geosynchronous orbit is that lower orbits are
less expensive to launch, it takes less power to send and receive data, and the round trip
time from Earth is faster.
. ...
0
THE0 (Twelve Hour Eccentric Orbits, or Molniya orbits) are not quite polar orbits that
stay in a hemisphere for half of a day. This approach allows for two M E 0 satellites to
cover an area of the Earth, thus giving continuous service as a geosynchonous satellite
would. This was developed and used extensively in Russia.
Geosynchronous orbits are the most desirable and the most expensive to obtain.
“Geosynch” satellites appear stationary to observers on the Earth. The orbit is always at
the equator at a height of 23,400 miles; the satellite thus travels in the same direction and
I
speed as the rotation of the Earth. These satellites, because they provide continuous
coverage, are the workhorses of the commercial satellite industry. Geosynch satellites
must be spaced at least 1/2 a degree apart to prevent signal interference. “Slots” are
closely regulated by the International Telecommunications Union (T.C.U.). They are
highly sought after and expensive to acquire, and since geosynch orbits are also
expensive to launch and maintain, most satellites in this orbit are big and complex.
Geosynchronous orbits require at least a quarter second round-trip time because of the
23,400-mile distance above the Earth plus the distance to the equator from the ground
station. More power is also needed because of the distance (although a larger antenna on
some satellites mitigates some of this). Since geosynch satellites are always in
approximately the same place with regard to the Earth, it is possible to precisely map a
“footprint” so that users can calibrate ground station equipment to minimize power and
bandwidth.
Geosynch satellites are kept tightly in their orbits by expending fuel. Older satellites are
often let to drift north and south while keeping in their east-west slots. Satellites allowed
to dnfi are said to be inclined orbits. This might require some tracking by ground
equipment, but when the drift is large enough, it allows the satellites to be seen in the
polar regions.
3. SPECTRUM
Satellites use the microwave band for communications (some older ones used the radio
ffequencies UHF, VHF). For a description of the electromagnetic spectrum check
http://imagine.gsfc.nasa.gov/docs/scienceAnow-I I/emspectrum.htm1.
I
~ ~~~~~~ ~~~
We modify sound waves to make speech. Dynamics such as loudness and tone are
changed to convey meaning. Telecommunication modifies electromechanical signals
such as microwaves to carry information in the same ways.
Microwave bandwidth is regulated by the FCC in the U.S. and by the ITU internationally.
Obtaining parts of the regulated bandwidth is expensive. Satellite operators must apply
years in advance to get spectrum for launch support, tracking, telemetery and operations,
and for data transfer.
Parts of the microwave spectrum are designated and used as follows (for a detailed
description see: http://www.NTIA.doc.gov/osmhome/nebbie.html):
VHFA-JHF 0.1-0.3 GHz - Milsat- amateur radio, “little” LEO
L Band - 1-2 GHz - Mobile Sat/manne, “big” LEO
S Band 2-4 GHz - Satellite commandcontrol
C Band 4-8 GHz - Data, voice, video distribution
Ku 12-18 GHz - Direct TV, Data, Voice, SNG, IP services
K 18-27 GHz -N/A (22.3-HzO absorption)
Ka 27-40 GHz - The next wave
* V 40-75 GHz - Released in the future (60 GHz - 0 2 absorption)
Z& v n - - A o 1 OTT- n s:i:~-- PACI
A YOllU O - I L U I i L - lvLlllLilly-cwa
Uplink (from the Earth to the satellite) uses the upper part of a kequency range while
downlink uses the lower fi-equencies. This is because a satellite is less capable of dealing
with the overtones derived from transmission.
The higher the fi-equencythe more bandwidth it takes to send a bit. That is because there
that are absorbed by water and free Oxygen. Also, the higher the frequency the smaller is
the antenna needed for the same bandwidth, as antenna size is related to the size of the
waveforms. The exception to this is L-Band where the signal is omni-directional. K-
Band is not used for satellite transmission as these frequencies are absorbed completely
by water vapor. Traditional C-Band, the first spectrum opened to satellites for
communications, requires a BUD (big ugly dish) at least 3 meters in diameter for even a
small bandwidth V-Sat, and up to 20 meters for a proper Earth station. Ku and Ka use
small dishes that can be portable.
4. EARTH STATIONS
Teleports, gateways, and “flyaways” are names for the terrestrial interface where the
signal from satellites meets the ground and where data is uplinked to the satellite. The
traditional architecture for satellite systems is to have a central complex with an antenna
or antennas controlling transmission to and &om satellites. Smaller antennas
communicate to other ground stations or terrestrial networks via.the central teleport.
Teleports act as a gateway between terrestrial networks such as the PSTN, the Internet or
cable TV infrastructure, 2nd the srtellite.
is gicztei atterriattioii of the sipa! “vj: the amospheie i i ~i;v‘e get c!osei to &e f r e q u e i ~ c i ~ ~
4
Aside from the interface equipment to connect to ground networks, the Earth station will
have an assortment of hardware to receive and send the microwave signals to and from
the satellite:
Reflector
Low Noise Amplifier
(for Receive)
Solid State Power
Amplifier (for Send)
1I Up/Down Converter
Demodulate
1-1 Modulate Satellite Modem
t
DigitaPSignal
1. A reflector and feed: This is the ''dish" that is the most visible component of the
system. The dish focuses the signal on the feed which sends it on. The size and shape
of the dish are a function of the frequencies used and the strength of the signal. The
larger the dish, the smaller the area of the sky it can see. This seems counter-intuitive
but it is true.
2. Outdoor equipment: This equipment isolates the signals to those we are interested in
and amplifies the signals (both send and receive). The equipment to change the
ffequency range to those the satellite expects (up and down converters) from L-Band
can be either indoor or outdoor.
3. Cable: Cable connects the antenna to the inside equipment.
4. A satellite modem that operates in L-Band (this could be separate modulators and
demodulators): These modems do more than the common terrestrial versions in doing
Introduction to Satellite Communication Technology for NREN
A/D and D/A conversion. They also define the signal method, do error correction
using redundancy coding, do time and frequency multiplexing, and perform link
monitoring. Satellite links are often asymmetric, having the ability to receive more
dzta than they can send.
5. Tracking gears: Gears move the antenna, if needed.
6 . A mtc~owove~cnsporcntbt;i!dirzg (radome): A radome is required if the reflectm,
feed and outdoor equipment requires covering from wind and snow.
7. The terrestrial interconnect: This could be a router, or a video codex or a multiplexer
to combine several input streams.
In order for an Earth station to become operational it must go through a series of tests and
adjustments. The tests are designed to test whether proper frequencies and time slots (for
TDM access) are used. Measurements are made and documented to insure that the power
levels are correct. The power is measured in terms of Effective Isotropic Radiated Power
(EIRE’). The units are decibels (dBW). Too much power can cause the signal to be
distorted and could disrupt other communications. Under-powered transmission can be
error prone.
The noise level in both directions is measured. This is G/T (the ratio of the gain fi-om the
antenna and outdoor equipment to the signal noise). The higher the ratio the better. The
definitive test is of course the actual error statistics measured by sending test data in both
db3CtiOES.
5. COMMUNICATIONS ONBOARD A SATELLITE
Earth orbiting satellites are a wonder of modern technology. Everything must withstand
the rigors of space in addition to the strain of the launch. In space there are extreme cold
and heat, radiation and micrometeorites, and storms on the Sun. Orbits must be obtained
and maintained and critical systems must be monitored. There are as many “floor plans”
to satellites as there are applications for the technology
dfs3434