In the 1950s and early 1960s, people tried to set up communication systems
by bouncing signals off metallized weather balloons. Unfortunately, the received
signals were too weak to be of any practical use. Then the U.S. Navy noticed a
kind of permanent weather balloon in the sky—the moon—and built an operational
system for ship-to-shore communication by bouncing signals off it.
Further progress in the celestial communication field had to wait until the first communication satellite was launched. The key difference between an artificial satellite and a real one is that the artificial one can amplify the signals before sending them back, turning a strange curiosity into a powerful communication system.
Communication satellites have some interesting properties that make them attractive for many applications. In its simplest form, a communication satellite can be thought of as a big microwave repeater in the sky. It contains several transponders, each of 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. This mode of operation is known as a bent pipe. Digital processing can be added to separately manipulate or redirect data streams in the overall band, or digital information can even be received by the satellite and rebroadcast. Regenerating signals in this way improves performance compared to a bent pipe because the satellite does not amplify noise in the upward 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.
According to Kepler’s law, the orbital period of a satellite varies as the radius of the orbit to the 3/2 power. The higher the satellite, the longer the period. Near the surface of the earth, the period is about 90 minutes. Consequently, low-orbit satellites pass out of view fairly quickly, so many of them are needed to provide continuous coverage and ground antennas must track them. 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, as anyone who has observed the moon regularly can testify.
A satellite’s period is important, but it is not the only issue in determining where to place it. 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 particles. These factors lead to three regions in which satellites can be placed safely. These regions and some of their properties are illustrated in figure. Below we will briefly describe the satellites that inhabit each of these regions.
With current technology, it is unwise to have geostationary satellites spaced
much closer than 2 degrees in the 360-degree equatorial plane, to avoid interference.
With a spacing of 2 degrees, there can only be 360/2 = 180 of these satellites
in the sky at once. However, each transponder can use multiple frequencies
and polarizations to increase the available bandwidth.
To prevent total chaos in the sky, orbit slot allocation is done by ITU. This process is highly political, with countries barely out of the stone age demanding ‘‘their’’ orbit slots (for the purpose of leasing them to the highest bidder). Other countries, however, maintain that national property rights do not extend up to the moon and that no country has a legal right to the orbit slots above its territory. To add to the fight, commercial telecommunication is not the only application. Television broadcasters, governments, and the military also want a piece of the orbiting pie.
Modern satellites can be quite large, weighing over 5000 kg and consuming 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. This fine-tuning activity is called station keeping. However, when the fuel for the motors has been exhausted (typically after about 10 years) the satellite drifts and tumbles helplessly, so it has to be turned off. Eventually, the orbit decays and the satellite reenters the atmosphere and burns up (or very rarely crashes to earth).
Orbit slots are not the only bone of contention. Frequencies are an issue, too, because the downlink transmissions interfere with existing microwave users. Consequently, ITU has allocated certain frequency bands to satellite users. The main ones are listed in figure. The C band was the first to be designated for commercial satellite traffic. Two frequency ranges are assigned in it, the lower one for downlink traffic (from the satellite) and the upper one for uplink traffic (to the satellite). To allow traffic to go both ways at the same time, two channels are required. These channels are already overcrowded because they are also used by the common carriers for terrestrial microwave links. The L and S bands were added by international agreement in 2000. However, they are narrow and also crowded.
telephone. Stringing telephone wires to thousands of small villages is far beyond
the budgets of most Third World governments, but installing 1-meter VSAT
dishes powered by solar cells is often feasible. VSATs provide the technology
that will wire the world.
Communication satellites have several properties that are radically different from terrestrial point-to-point links. To begin with, even though signals to and from a satellite travel at the speed of light (nearly 300,000 km/sec), 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. A typical value is 270 msec (540 msec for a VSAT system with a hub).
For comparison purposes, 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. The latter are slower than the former because electromagnetic signals travel faster in air than in solid materials.
Another important property of satellites is that they 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. For some applications, this property is very useful. For example, one could imagine a satellite broadcasting popular Web pages to the caches of a large number of computers spread over a wide area. Even when broadcasting can be simulated with point-to-point lines,satellite broadcasting may be much cheaper. On the other hand, from a privacy point of view, satellites are a complete disaster: everybody can hear everything. Encryption is essential when security is required.
Satellites also have the property that the cost of transmitting a message is independent of the distance traversed. 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, a major consideration for disaster response and military communication.
Further progress in the celestial communication field had to wait until the first communication satellite was launched. The key difference between an artificial satellite and a real one is that the artificial one can amplify the signals before sending them back, turning a strange curiosity into a powerful communication system.
Communication satellites have some interesting properties that make them attractive for many applications. In its simplest form, a communication satellite can be thought of as a big microwave repeater in the sky. It contains several transponders, each of 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. This mode of operation is known as a bent pipe. Digital processing can be added to separately manipulate or redirect data streams in the overall band, or digital information can even be received by the satellite and rebroadcast. Regenerating signals in this way improves performance compared to a bent pipe because the satellite does not amplify noise in the upward 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.
According to Kepler’s law, the orbital period of a satellite varies as the radius of the orbit to the 3/2 power. The higher the satellite, the longer the period. Near the surface of the earth, the period is about 90 minutes. Consequently, low-orbit satellites pass out of view fairly quickly, so many of them are needed to provide continuous coverage and ground antennas must track them. 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, as anyone who has observed the moon regularly can testify.
A satellite’s period is important, but it is not the only issue in determining where to place it. 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 particles. These factors lead to three regions in which satellites can be placed safely. These regions and some of their properties are illustrated in figure. Below we will briefly describe the satellites that inhabit each of these regions.
- 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, 1945). He went
on to describe a complete communication system that used these (manned) geostationary
satellites, including the orbits, solar panels, radio frequencies, and
launch procedures. Unfortunately, he concluded that satellites were impractical
due to the impossibility of putting power-hungry, fragile vacuum tube amplifiers
into orbit, so he never pursued this idea further, although he wrote some science
fiction stories about it.
tion stories about it.
The invention of the transistor changed all that, and 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. These high-flying satellites are often
called GEO (Geostationary Earth Orbit) satellites.
To prevent total chaos in the sky, orbit slot allocation is done by ITU. This process is highly political, with countries barely out of the stone age demanding ‘‘their’’ orbit slots (for the purpose of leasing them to the highest bidder). Other countries, however, maintain that national property rights do not extend up to the moon and that no country has a legal right to the orbit slots above its territory. To add to the fight, commercial telecommunication is not the only application. Television broadcasters, governments, and the military also want a piece of the orbiting pie.
Modern satellites can be quite large, weighing over 5000 kg and consuming 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. This fine-tuning activity is called station keeping. However, when the fuel for the motors has been exhausted (typically after about 10 years) the satellite drifts and tumbles helplessly, so it has to be turned off. Eventually, the orbit decays and the satellite reenters the atmosphere and burns up (or very rarely crashes to earth).
Orbit slots are not the only bone of contention. Frequencies are an issue, too, because the downlink transmissions interfere with existing microwave users. Consequently, ITU has allocated certain frequency bands to satellite users. The main ones are listed in figure. The C band was the first to be designated for commercial satellite traffic. Two frequency ranges are assigned in it, the lower one for downlink traffic (from the satellite) and the upper one for uplink traffic (to the satellite). To allow traffic to go both ways at the same time, two channels are required. These channels are already overcrowded because they are also used by the common carriers for terrestrial microwave links. The L and S bands were added by international agreement in 2000. However, they are narrow and also crowded.
The next-highest band available to commercial telecommunication carriers is
the Ku (K under) band. This band is not (yet) congested, and at its higher frequencies,
satellites can be spaced as close as 1 degree. However, another problem
exists: rain. Water absorbs these short microwaves well. Fortunately, heavy
storms are usually localized, so using several widely separated ground stations instead
of just one circumvents the problem, but at the price of extra antennas, extra
cables, and extra electronics to enable rapid switching between stations. Bandwidth
has also been allocated in the Ka (K above) band for commercial satellite
traffic, but the equipment needed to use it is expensive. In addition to these commercial
bands, many government and military bands also exist.
A modern satellite has around 40 transponders, most often with a 36-MHz
bandwidth. Usually, each transponder operates as a bent pipe, but recent satellites
have some on-board processing capacity, allowing more sophisticated operation.
In the earliest satellites, the division of the transponders into channels was static:
the bandwidth was simply split up into fixed frequency bands. Nowadays, each
transponder beam is divided into time slots, with various users taking turns. We
will study these two techniques (frequency division multiplexing and time division
multiplexing).
The first geostationary satellites had a single spatial beam that illuminated
about 1/3 of the earth’s surface, called its footprint. With the enormous decline
in the price, size, and power requirements of microelectronics, a much more
sophisticated broadcasting strategy has become possible. Each satellite is
equipped with multiple antennas and multiple transponders. Each downward
beam can be focused on a small geographical area, so multiple upward and downward
transmissions can take place simultaneously. Typically, these so-called 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.
A recent development in the communication satellite world is the development
of low-cost microstations, sometimes called VSATs (Very Small Aperture
Terminals) (Abramson, 2000). These 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. The uplink is generally good for up to 1 Mbps, but the downlink is often
up to several megabits/sec. Direct broadcast satellite television uses this technology
for one-way transmission.
In many VSAT systems, the microstations do not have enough power to communicate
directly with one another (via the satellite, of course). Instead, a special
ground station, the hub, with a large, high-gain antenna is needed to relay traffic
between VSATs, as shown in figure. In this mode of operation, either the
sender or the receiver has a large antenna and a powerful amplifier. The trade-off
is a longer delay in return for having cheaper end-user stations.
VSATs have great potential in rural areas. It is not widely appreciated, but
over half the world’s population lives more than hour’s walk from the nearest
Communication satellites have several properties that are radically different from terrestrial point-to-point links. To begin with, even though signals to and from a satellite travel at the speed of light (nearly 300,000 km/sec), 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. A typical value is 270 msec (540 msec for a VSAT system with a hub).
For comparison purposes, 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. The latter are slower than the former because electromagnetic signals travel faster in air than in solid materials.
Another important property of satellites is that they 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. For some applications, this property is very useful. For example, one could imagine a satellite broadcasting popular Web pages to the caches of a large number of computers spread over a wide area. Even when broadcasting can be simulated with point-to-point lines,satellite broadcasting may be much cheaper. On the other hand, from a privacy point of view, satellites are a complete disaster: everybody can hear everything. Encryption is essential when security is required.
Satellites also have the property that the cost of transmitting a message is independent of the distance traversed. 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, a major consideration for disaster response and military communication.
No comments:
Post a Comment