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Satellite TVRO Part 1
The first TV signal sent via satellite happened in 1962 using the Telstar satellite. Since then, the number of satellites used for TV transmission has risen to 324. That’s not counting satellites used for things such as GPS, earth science, NOAA as well as other applications. In total, there are more than 15,000 satellites orbiting the Earth today. Fortunately for the broadcaster, only about 428 of those are in geosynchronous orbit, and out of that number, only 72 of those are used for TV transmission over North America. (See Figure 1.)
Figure 1: Satellites in geostationary orbit
Satellite basics
Communication satellites, which cover wide areas (or footprints) of the Earth’s surface, are specialized space vehicles outfitted as repeaters to receive uplink signals and resend them back to Earth. To send signals across the oceans, satellites positioned over the oceans receive signals from one continent and retransmit them to the other continent. (See Figure 2.)
Figure 2: Satellite orbiting above North America
Satellites used for TV signal transmission as well as other types of data are distributed around the equator at an altitude of 22,300mi. This is called the Clarke Belt, named after Arthur C. Clark, who first proposed the idea back in the 1945. At this altitude, the satellites can maintain what is called a geostationary orbit, in which they do not move in relation to the Earth’s surface. Their position is measured as being either west or east from the Prime Meridian, or zero longitude from 0 degrees to 180 degrees. This means that if you are at 121 degrees west latitude on Earth and a satellite is positioned at 121 degrees west, such as Galaxy 23, then the satellite is exactly due south of your position, or 180 degrees on a compass. Satellites used for TV signals have been deployed over North America in an 87-degree arc from 61 degrees west to 148 degrees west. (See Figure 3.)
Figure 3: 87-degree arc covering North America
Communication satellites maintain their position within a very tight box of ±0.05 degrees to ±0.10 degrees around their assigned position. This works out to a box about 90mi on each side. Due to gravitational pull from the sun and moon, satellites tend to drift away from their assigned orbital slots. To correct this, thrusters on the satellites are used to maintain their position west and east (longitude) as well as north and south (inclination). One of the main reasons a satellite reaches its end of life is that it runs low on fuel for the thrusters.
Some satellites, as they near the end of their operational lifetime, are allowed to enter into an inclined geosynchronous (not geostationary) orbit to save on fuel. When an operator places a satellite in an inclined orbit, it maintains its normal east and west position, but uses very little fuel for north and south corrections. This lets it oscillate, within a 24-hour window, above and below the equator during its orbit, making a figure eight. Depending on the amount of a satellite’s inclination, it must be tracked by the transmitting and receiving antennas, which not all antennas can do. Satellites in inclined orbits can move far enough that the signal will be lost in less than an hour, depending on receive dish size. In some situations, satellites with large inclined orbits can be used for intermittent use by non-tracking dishes, depending on their size.
Controlling satellites
The responsibility of keeping satellites within their orbital slots falls on the satellite operators such as SES Global, PanAmSat and Loral Space Systems. These companies actually monitor and control the communication satellites in space. First, the satellite is built and then launched into space near its orbital slot. It is run through tests to ensure it is working as expected, and then it’s maneuvered into its assigned slot using thrusters.
It may sound easy to keep a satellite in place, but it becomes much more challenging when you have to keep two or more satellites within the same orbital slot and not hit each other. This is what is happening when you see two or more satellite names with the same latitude, such as 101 degrees west where AMC 2 and AMC 4 both reside, along with DirecTV 4S. All three satellites perform a sort of dance that keeps them within the box but not too close as to collide or block each other’s signals. The satellites can share an orbital slot but not the same frequencies. (See Figure 4.)
These days, most satellites sent up are replacing existing ones that are nearing the end of their operational lifetimes. When this happens, the retiring satellite is moved into an orbit about 180mi higher into what is called a space graveyard, where the batteries are forced to run down and all fuel is released to reduce the chance of any of them exploding.
To extend the lifetime and/or increase the payload of a satellite, the amount of fuel used needs to be reduced so it can stay in its orbital position longer. Ion propulsion can be 10 times more efficient than conventional thrusters and has been under development for several decades, with NASA and Boeing doing much of the work. Several satellites have now used this propulsion system, mostly in conjunction with standard thrusters. One of the few communications satellites using ion propulsion is PAS-5, which was launched in 1997.
The ion propulsion works by extracting ions from xenon gas then passing them through electrically charged electrodes, which causes the ions to be accelerated to speeds of 62,000mph, 10 times that of normal thrusters. Although the speed is high, the mass is very low, and this lower force from the ion thrusters does not disturb or shake the spacecraft when they are used, compared with conventional thrusters.
Faults
Not all satellites make it up to their orbital slot without a hitch. Sometimes the rockets malfunction causing the satellite to not make it all the way to its geostationary orbit altitude. This is what happened to satellite AMC 14 in March 2008 when one of its rocket stages shut down early. SES Americom considers the satellite a total loss.
Then there’s Astra 5A, which had been working since November 1997 and should have had a few more years of service. But in January of this year (2009), it suffered a sudden failure that could not be corrected from the ground. Another satellite was moved into its place to take over its communication duties until a new satellite can be launched.
To control the satellite, its antennas must point toward Earth. If the satellite moves out of alignment and it loses communications with its control center, there is the possibility that the satellite could collide with another. When the satellite’s antenna is not pointed at Earth, NASA’s Jet Propulsion Laboratory has a very large dish antenna can be aimed at the satellite, and, with enough power, a signal can be sent to the satellite to get it back under control.
Transponders
Communication satellites are capable of receiving and retransmitting several channels at once. The most common number of channels or transponders is 24, and they can use different frequency segments or bands to expand that number. The signal sent up to the satellite (uplink) is always higher in frequency than the signal sent back down (downlink). This keeps the two signals from interfering with one another.
Each communications satellite uses a band or bands of frequencies to receive and send signals to and from Earth. The bands used are:
- L–band: 1Ghz-2GHz, used by Mobile Service Satellites (MSS) and after downconversion from the LNB to the satellite receiver
- S-band: 2GHz-4GHz, used by MSS, NASA and deep space research
- C-band: 4GHz-8GHz, used by Fixed Service Satellites (FSS)
- X-band: 8GHz-12.5GHz, used by FSS and in terrestrial imaging, e.g. military and meteorological satellites
- Ku-band: 12GHz-18GHz, video satellite service use 11.7GHz-12.7GHz and is also called FSS, while DBS uses 12.2GHz-12.7GHz
- K-band: 18GHz-26.5GHz, used by FSS and Broadcast Service Satellites (BSS)
- Ka-band: 26.5GHz-40GHz, used by FSS and DBS
The most common bands used by broadcasters are the C-band and Ku-band. The C-band uses frequencies employed for microwave communications on Earth, so its power is limited to prevent interference. The Ku-band uses higher frequencies that will not cause interference, but can be blocked by heavy rain at times.
A signal uplinked to a transponder is received at the main receive antenna and then connected to a filter, for a particular transponder channel, followed by a preamplifier and then to a downconverter (to place it on the correct downlink frequency). The signal is fed to a high-power amplifier, a filter and then to the transmit antenna. All communication satellites carry spare amplifiers that can be switched in to replace any failed ones to extend the life of the satellite. (See Figure 5.)
Transponder frequencies are listed by their center frequency; most C-band transponders have 36Mhz of bandwidth, while Ku-band transponders have bandwidths of 24Mhz and some transponders have bandwidths of up to 54Mhz. C-band frequencies for U.S. domestic satellites are fairly standard, but Ku-band frequencies can vary from satellite to satellite. Satellites covering different parts of the world use different uplink and downlink frequencies, as does transoceanic satellite service.
A modern communications satellite will normally carry 24 C-band transponders and 24 Ku-band transponders. When only analog transmissions were used, that limited a satellite to a maximum of 48 channels that could be broadcast. Today, all of that has changed.
Satellite TVRO Part 2
Satellite transponders are simple repeaters that downconvert the incoming signal before rebroadcasting it. There are a limited number of satellites and a limited amount of RF spectrum in which to send and receive these signals, so to overcome these limitations, several methods have been implemented to expand the number of signals that a communication satellite can handle.
Polarization
Each transponder’s frequency band is allowed to overlap the next one, because each transponder has a different polarity than the transponders on either side of it. Using crosspolarity reduces the amount of crosstalk between adjacent transponders and reduces the amount of frequency spectrum required.
Broadcast communication satellites use linear polarity, i.e. horizontal and vertical. But in direct-to-home satellite transmissions, as well as others, circular polarity is the standard where there is right-hand circular polarity (RHCP) and left-hand circular polarity (LHCP). (See Figure 1.)
Switching between polarities provides 20db-40dB of isolation, which allows overlapping frequency bands between crosspolarized transponders. Receiving antennas are aligned to receive signals of one or the other polarity, whether linear or circular. The physical design of the antenna determines which polarity is passed through. For linearly polarized antennas, a motor can move the probe of the antenna and shift it from horizontal to vertical and back again, so one output can present either polarity’s signal. Other linear antennas can supply both polarities at the same time to two different ports. The polarity of a linear antenna must be set precisely to be able to receive the intended polarity.
Circular polarity antennas can be designed for LHCP, RHCP or both, but only one polarity can be delivered to one port; to receive both polarities, two ports are required.
Circular polarization has two main advantages over linear polarization. First, when setting up a receive antenna, there is no alignment of the polarity because the antenna is designed to accept the correct polarity. Second, circularly polarized signals cannot be depolarized. When polarized signals pass through the atmosphere, they can become depolarized, where the off-axis shift of the polarity of the signals (in linearly polarized signals) causes a reduction in the desired polarity and an increase in the opposite polarity, resulting in interference. This effect is most noticeable in the lower-frequency C-band signals. Due to the nature of circularly polarized waves, a rotation of the signal will not affect its reception.
Fractionalized transponders
Normally, a single uplink facility sends a signal up to (illuminates) a satellite transponder, but with precise control, several uplink facilities can transmit to a single transponder. This cuts down on costs and conserves transponder usage. To do this, all the uplink facilities must coordinate with the satellite’s control center and keep the power levels very close. This is very specialized and is not used for normal TV transmissions. (See Figure 2.)
Frequencies
For C-band satellites, the uplink frequencies are 5.925GHz–6.425GHz. Once the satellite’s transponder downconverts the signals, they are downlinked on 3.7MHz–4.2Mhz. Ku-band satellite uplink frequencies are 14GHz–14.5GHz, and the satellite’s transponder downlinks them on 11.7GHz–12.7 GHz. (See Figure 3.)
These signals are received at the antenna and fed into a low-noise block converter where the signal is amplified and mixed with a local oscillator (LO) to produce a lower set of frequencies that will travel a longer distance over coax cable to the receiver or integrated receiver-decoder.
Different LOs are used for different frequency bands, so the resulting frequencies fall into a common band enabling a receiver to use a single intermediate-frequency front-end to receive several different types of satellite signals. That common band for C and Ku is 950MHz-1450Mhz. The LO used for C-band is 5150MHz, and the resulting signals are above the LO. For the Ku-band, the LO is 10750MHz, and the resulting output is below the LO. The stability of the LO can have a great effect on the quality of the signal you receive, or even if you get a signal.
Just as with today’s over-the-air DTV signal that can carry more than one program, so, too, can satellite transmissions. If your station transmits more than one program, then you have two or more encoders whose outputs are combined in a multiplexer and sent on to your digital transmitter. For DTV, we are limited to one 6Mhz-wide channel and the 8-VSB modulation method, which give every station a data rate of 19.4Mb/s. For satellites, a transponder’s bandwidth can be as wide as 70MHz, and the modulation methods have evolved over the years from BPSK and QPSK to SP-QPSK. All these modulation methods provide a higher data rate with which to pass more information within a smaller bandwidth. The “PSK” of all these methods stands for phase shift keying. Binary PSK is where there are only two states of the carrier’s phase, so it can only represent one bit of data, one or zero. Quadrature PSK is where there are four states of the carrier’s phase, which means that it can represent two bits at every state.
Just as in 8-VSB, where there are eight different amplitude levels, each amplitude level represents a single symbol. A symbol corresponds to one of eight possible combinations of three bits, so each symbol sent transmits three bits of data, an eightfold increase of data flow. (See Figure 4.)
8PSK is similar to 8-VSB in that it also has eight different states. One of the newest modulation methods is SP-QPSK (sinusoidal-shaped π/4 QPSK), which is a more efficient way to modulate the satellite carrier and something The Associated Press has just started using.
The symbol rate is the number of symbols sent per second. It contains the data rate and the forward error correction (FEC) data. The symbol and data rates correspond to each other, and a formula can be used to convert between the two. (See Figure 5.)
FEC is an important part of digital broadcasting. In satellite transmissions, the amount of error correction is stated as follows: 1/2, 2/3, 3/4, 7/8. The first number is the number of actual data bits, followed by the total number of bits transmitted. The difference is the amount of error correction bits. So for an FEC of 1/2, there is one error correction bit for data every bit. At the other end, an FEC of 7/8 means there is one error correction bit for every seven data bits.
An FEC of 1/2 equals a great deal of error correction, which will tolerate smaller antennas and lower-quality receive equipment, while an FEC of 7/8 means the signal provider is trying to squeeze as much data through as possible and may require a bigger antenna and higher-quality receive equipment.
Conclusion
All of the methods described allow for more data to be sent through fewer frequencies. Satellite transmission requires a great deal of high-tech equipment to be able to reliably send and receive the programs that TV stations use every day.
Satellite TVRO Part 3
Back in the 1980s, the way to find a satellite with your 5m satellite dish was to move its azimuth over a degree or two and them move elevation up and down while looking for any sign of a sync bar from the analog receiver. This would go on until you found a signal, and then you fine-tuned the azimuth, elevation and polarity and hoped it was the satellite you wanted. These days, it’s a little different.
There are very few analog signals to look for, which means you need specialized equipment to monitor the satellite signals and to identify them. This tutorial will cover today’s satellite equipment and how to use them to locate the desired satellite.
Antenna basics
There are three types of satellite dishes in use today: Cassegrain, parabolic and offset. (See Figure 1.) Each has advantages for its particular application. It’s common to refer to the dish itself as the “antenna,” but it is only the reflector. The actual antenna is contained within the feed horn, where the RF is received and turned into electrical signals.
Satellite dishes range in size from 18in to 70m. The most common for broadcast use is a parabolic dish in the 2m to 4m range, which is measured across the dish’s width. Cassegrain dishes are more commonly found in very large dishes, 5m and larger, and the LNBs are located within the hub of the dish where it’s more easily accessible. Small offset dishes are used for direct-to-home satellite reception and can be seen in most neighborhoods. But larger offset dishes of up to 6ft work very well for Ku-band reception.
Satellite dishes come in one of three basic configurations: solid, mesh and petal. A solid dish is just that — solid, made of a single piece of metal — which makes for higher delivery costs but will assure a longer trouble-free lifetime. (See Figure 2.) Mesh dishes are very common in backyards for “free-to-air” satellite programming. The largest is usually 10ft to 12ft. Mesh dishes suffer from a shorter lifetime, but this can be offset by their lower cost and less wind resistance. The petal configuration is easier to ship because it comes in pieces, each shaped like a triangle or petal. These are almost always made out of fiberglass with a metal structure to support the pieces and attach them to the mounting post. While they do work very well, petal dishes do not age well, and if you need to move them, there is the definite risk of unintentional damage. The only variable for dishes is their gain, which is directly related to their size. The bigger the dish, the more gain it has, so it can pickup weaker signals with less noise.
There is one more type of dish: the very wide-angle dish that can see the entire arc of satellites, or over 100 degrees, all at once. These are massive structures that weigh several tons and require very precise installation. Their advantage is the ability to see all satellites at once; to receive a new satellite, only a low-noise block converter (LNB) must be installed at the correct location. The downside is their cost and amount of space they require, but it’s the only dish you ever need to install.
The components of a satellite dish include the mounting pole, the mounting frame, the dish itself, the feed horn and the LNBs. The mounting pole is sunk into the ground and cemented in place, attached to a stand that sits on the ground or, in the case of a roof, a “nonpenetrating” mount is used. These last two must use weights to hold the mount, and the pole, in place. How much weight depends on the size of the dish, the type of dish and where it is being mounted. Without enough weight, the dish can move in high winds, throwing off the satellite dish’s aim. The pole must be plum for a steerable dish to be able to track the satellite arc correctly; fixed dish imitations only require the pole to be close to plum because the adjustments will compensate for errors. The mounting frame is either Az-El or polar. Az-El stands for azimuth-elevation, where each parameter can be adjusted independently, most common on fixed dish mounts. A polar mount allows the dish to track the arc of the satellites in the Clarke Belt with a single movement from east to west. The polar mount causes the dish to change elevation during the east-west movement. This is almost exclusively used on motorized, steerable dishes. The feed horn is the antenna of the satellite dish. Its position above the center of the dish sets the focus point for the reflected satellite signals from the dish. This focal length adjustment is a critical part of setting up a dish. It is also where the polarity of the received signal is selected. Feed horns also come in a number of configurations. The LNBs attach to the feed horn to receive the satellite signal and downconvert it to a frequency band that the receiver will accept. For broadcasters, LNBs are either for C-band or Ku-band signals. Some feed horns will accept one C and one Ku on the same polarity or two C or two Ku each on a separate polarity, so all transponders on a satellite can be received at the same time. (See Figure 3.)
Aligning a fixed dish
The three parameters for satellite dish positioning are elevation, azimuth and polarity. Elevation is the angle of the dish above the horizon; Azimuth is the angle the dish is facing on the compass scale; and Polarity is the angle of the receive antenna within the feed horn in relation to the polarity of the signal sent by the desired satellite. Properly adjusting all three of these parameters will allow you to pick up the satellite you want. (See Figure 4.)
If your satellite dish was located on the equator, the dish would be pointed straight up at a 90-degree angle, and then swung from east to west and all the satellites would be picked up — this is not the case for most of us. That straight line (east to west) at the equator is the line of geostationary satellites; as our dish moves north of the equator, that line becomes elliptical. The further north we move (higher latitudes), the more pronounced the elliptical curve of the geostationary satellites becomes. So we not only move the satellite dish from east to west, but also raise and lower its elevation to be able to track all the satellites in the Clarke Belt.
For fixed or stationary satellite dishes, you simply raise it to a fixed elevation above the horizon and aim it at a particular point on the compass, and the satellite you are looking for should be there (or close to it). There are many Web sites that allow you to enter your satellite dish’s location and which satellite you are looking for, and it will provide you with the elevation and azimuth readings to align your dish.
To find your dish’s elevation, it’s easiest to use an inclinometer or digital level, which measures the tilt or incline of an object. There are mechanical inclinometers with a large dial and the electronic variety that look like a carpenter’s level and have a digital readout — either one should work fine. The hardest part of setting a dish’s elevation is to find a flat space on the back of the dish where you can place the inclinometer that is perpendicular to the dish’s line of sight to the satellite. The back of all dishes is rounded, so that makes it harder to use; a part of the mounting frame can be used if it is parallel to the dish. The most assured way to measure elevation on a parabolic or Cassegrain dish is to place a straight board across the face of the dish, make sure it’s resting on the edges and then measure the elevation on the board.
Offset dishes pose their own set of problems in setting elevation, because the line of sight to the satellite is not perpendicular to the face of the dish. To align an offset dish, you must contact the manufacturer for the correct placement of the inclinometer.
Satellite TVRO Part 1
The first TV signal sent via satellite happened in 1962 using the Telstar satellite. Since then, the number of satellites used for TV transmission has risen to 324. That’s not counting satellites used for things such as GPS, earth science, NOAA as well as other applications. In total, there are more than 15,000 satellites orbiting the Earth today. Fortunately for the broadcaster, only about 428 of those are in geosynchronous orbit, and out of that number, only 72 of those are used for TV transmission over North America. (See Figure 1.)
Figure 1: Satellites in geostationary orbit
Satellite basics
Communication satellites, which cover wide areas (or footprints) of the Earth’s surface, are specialized space vehicles outfitted as repeaters to receive uplink signals and resend them back to Earth. To send signals across the oceans, satellites positioned over the oceans receive signals from one continent and retransmit them to the other continent. (See Figure 2.)
Figure 2: Satellite orbiting above North America
Satellites used for TV signal transmission as well as other types of data are distributed around the equator at an altitude of 22,300mi. This is called the Clarke Belt, named after Arthur C. Clark, who first proposed the idea back in the 1945. At this altitude, the satellites can maintain what is called a geostationary orbit, in which they do not move in relation to the Earth’s surface. Their position is measured as being either west or east from the Prime Meridian, or zero longitude from 0 degrees to 180 degrees. This means that if you are at 121 degrees west latitude on Earth and a satellite is positioned at 121 degrees west, such as Galaxy 23, then the satellite is exactly due south of your position, or 180 degrees on a compass. Satellites used for TV signals have been deployed over North America in an 87-degree arc from 61 degrees west to 148 degrees west. (See Figure 3.)
Figure 3: 87-degree arc covering North America
Communication satellites maintain their position within a very tight box of ±0.05 degrees to ±0.10 degrees around their assigned position. This works out to a box about 90mi on each side. Due to gravitational pull from the sun and moon, satellites tend to drift away from their assigned orbital slots. To correct this, thrusters on the satellites are used to maintain their position west and east (longitude) as well as north and south (inclination). One of the main reasons a satellite reaches its end of life is that it runs low on fuel for the thrusters.
Some satellites, as they near the end of their operational lifetime, are allowed to enter into an inclined geosynchronous (not geostationary) orbit to save on fuel. When an operator places a satellite in an inclined orbit, it maintains its normal east and west position, but uses very little fuel for north and south corrections. This lets it oscillate, within a 24-hour window, above and below the equator during its orbit, making a figure eight. Depending on the amount of a satellite’s inclination, it must be tracked by the transmitting and receiving antennas, which not all antennas can do. Satellites in inclined orbits can move far enough that the signal will be lost in less than an hour, depending on receive dish size. In some situations, satellites with large inclined orbits can be used for intermittent use by non-tracking dishes, depending on their size.
Controlling satellites
The responsibility of keeping satellites within their orbital slots falls on the satellite operators such as SES Global, PanAmSat and Loral Space Systems. These companies actually monitor and control the communication satellites in space. First, the satellite is built and then launched into space near its orbital slot. It is run through tests to ensure it is working as expected, and then it’s maneuvered into its assigned slot using thrusters.
It may sound easy to keep a satellite in place, but it becomes much more challenging when you have to keep two or more satellites within the same orbital slot and not hit each other. This is what is happening when you see two or more satellite names with the same latitude, such as 101 degrees west where AMC 2 and AMC 4 both reside, along with DirecTV 4S. All three satellites perform a sort of dance that keeps them within the box but not too close as to collide or block each other’s signals. The satellites can share an orbital slot but not the same frequencies. (See Figure 4.)
These days, most satellites sent up are replacing existing ones that are nearing the end of their operational lifetimes. When this happens, the retiring satellite is moved into an orbit about 180mi higher into what is called a space graveyard, where the batteries are forced to run down and all fuel is released to reduce the chance of any of them exploding.
To extend the lifetime and/or increase the payload of a satellite, the amount of fuel used needs to be reduced so it can stay in its orbital position longer. Ion propulsion can be 10 times more efficient than conventional thrusters and has been under development for several decades, with NASA and Boeing doing much of the work. Several satellites have now used this propulsion system, mostly in conjunction with standard thrusters. One of the few communications satellites using ion propulsion is PAS-5, which was launched in 1997.
The ion propulsion works by extracting ions from xenon gas then passing them through electrically charged electrodes, which causes the ions to be accelerated to speeds of 62,000mph, 10 times that of normal thrusters. Although the speed is high, the mass is very low, and this lower force from the ion thrusters does not disturb or shake the spacecraft when they are used, compared with conventional thrusters.
Faults
Not all satellites make it up to their orbital slot without a hitch. Sometimes the rockets malfunction causing the satellite to not make it all the way to its geostationary orbit altitude. This is what happened to satellite AMC 14 in March 2008 when one of its rocket stages shut down early. SES Americom considers the satellite a total loss.
Then there’s Astra 5A, which had been working since November 1997 and should have had a few more years of service. But in January of this year (2009), it suffered a sudden failure that could not be corrected from the ground. Another satellite was moved into its place to take over its communication duties until a new satellite can be launched.
To control the satellite, its antennas must point toward Earth. If the satellite moves out of alignment and it loses communications with its control center, there is the possibility that the satellite could collide with another. When the satellite’s antenna is not pointed at Earth, NASA’s Jet Propulsion Laboratory has a very large dish antenna can be aimed at the satellite, and, with enough power, a signal can be sent to the satellite to get it back under control.
Transponders
Communication satellites are capable of receiving and retransmitting several channels at once. The most common number of channels or transponders is 24, and they can use different frequency segments or bands to expand that number. The signal sent up to the satellite (uplink) is always higher in frequency than the signal sent back down (downlink). This keeps the two signals from interfering with one another.
Each communications satellite uses a band or bands of frequencies to receive and send signals to and from Earth. The bands used are:
- L–band: 1Ghz-2GHz, used by Mobile Service Satellites (MSS) and after downconversion from the LNB to the satellite receiver
- S-band: 2GHz-4GHz, used by MSS, NASA and deep space research
- C-band: 4GHz-8GHz, used by Fixed Service Satellites (FSS)
- X-band: 8GHz-12.5GHz, used by FSS and in terrestrial imaging, e.g. military and meteorological satellites
- Ku-band: 12GHz-18GHz, video satellite service use 11.7GHz-12.7GHz and is also called FSS, while DBS uses 12.2GHz-12.7GHz
- K-band: 18GHz-26.5GHz, used by FSS and Broadcast Service Satellites (BSS)
- Ka-band: 26.5GHz-40GHz, used by FSS and DBS
The most common bands used by broadcasters are the C-band and Ku-band. The C-band uses frequencies employed for microwave communications on Earth, so its power is limited to prevent interference. The Ku-band uses higher frequencies that will not cause interference, but can be blocked by heavy rain at times.
A signal uplinked to a transponder is received at the main receive antenna and then connected to a filter, for a particular transponder channel, followed by a preamplifier and then to a downconverter (to place it on the correct downlink frequency). The signal is fed to a high-power amplifier, a filter and then to the transmit antenna. All communication satellites carry spare amplifiers that can be switched in to replace any failed ones to extend the life of the satellite. (See Figure 5.)
Transponder frequencies are listed by their center frequency; most C-band transponders have 36Mhz of bandwidth, while Ku-band transponders have bandwidths of 24Mhz and some transponders have bandwidths of up to 54Mhz. C-band frequencies for U.S. domestic satellites are fairly standard, but Ku-band frequencies can vary from satellite to satellite. Satellites covering different parts of the world use different uplink and downlink frequencies, as does transoceanic satellite service.
A modern communications satellite will normally carry 24 C-band transponders and 24 Ku-band transponders. When only analog transmissions were used, that limited a satellite to a maximum of 48 channels that could be broadcast. Today, all of that has changed.
Satellite TVRO Part 2
Satellite transponders are simple repeaters that downconvert the incoming signal before rebroadcasting it. There are a limited number of satellites and a limited amount of RF spectrum in which to send and receive these signals, so to overcome these limitations, several methods have been implemented to expand the number of signals that a communication satellite can handle.
Polarization
Each transponder’s frequency band is allowed to overlap the next one, because each transponder has a different polarity than the transponders on either side of it. Using crosspolarity reduces the amount of crosstalk between adjacent transponders and reduces the amount of frequency spectrum required.
Broadcast communication satellites use linear polarity, i.e. horizontal and vertical. But in direct-to-home satellite transmissions, as well as others, circular polarity is the standard where there is right-hand circular polarity (RHCP) and left-hand circular polarity (LHCP). (See Figure 1.)
Switching between polarities provides 20db-40dB of isolation, which allows overlapping frequency bands between crosspolarized transponders. Receiving antennas are aligned to receive signals of one or the other polarity, whether linear or circular. The physical design of the antenna determines which polarity is passed through. For linearly polarized antennas, a motor can move the probe of the antenna and shift it from horizontal to vertical and back again, so one output can present either polarity’s signal. Other linear antennas can supply both polarities at the same time to two different ports. The polarity of a linear antenna must be set precisely to be able to receive the intended polarity.
Circular polarity antennas can be designed for LHCP, RHCP or both, but only one polarity can be delivered to one port; to receive both polarities, two ports are required.
Circular polarization has two main advantages over linear polarization. First, when setting up a receive antenna, there is no alignment of the polarity because the antenna is designed to accept the correct polarity. Second, circularly polarized signals cannot be depolarized. When polarized signals pass through the atmosphere, they can become depolarized, where the off-axis shift of the polarity of the signals (in linearly polarized signals) causes a reduction in the desired polarity and an increase in the opposite polarity, resulting in interference. This effect is most noticeable in the lower-frequency C-band signals. Due to the nature of circularly polarized waves, a rotation of the signal will not affect its reception.
Fractionalized transponders
Normally, a single uplink facility sends a signal up to (illuminates) a satellite transponder, but with precise control, several uplink facilities can transmit to a single transponder. This cuts down on costs and conserves transponder usage. To do this, all the uplink facilities must coordinate with the satellite’s control center and keep the power levels very close. This is very specialized and is not used for normal TV transmissions. (See Figure 2.)
Frequencies
For C-band satellites, the uplink frequencies are 5.925GHz–6.425GHz. Once the satellite’s transponder downconverts the signals, they are downlinked on 3.7MHz–4.2Mhz. Ku-band satellite uplink frequencies are 14GHz–14.5GHz, and the satellite’s transponder downlinks them on 11.7GHz–12.7 GHz. (See Figure 3.)
These signals are received at the antenna and fed into a low-noise block converter where the signal is amplified and mixed with a local oscillator (LO) to produce a lower set of frequencies that will travel a longer distance over coax cable to the receiver or integrated receiver-decoder.
Different LOs are used for different frequency bands, so the resulting frequencies fall into a common band enabling a receiver to use a single intermediate-frequency front-end to receive several different types of satellite signals. That common band for C and Ku is 950MHz-1450Mhz. The LO used for C-band is 5150MHz, and the resulting signals are above the LO. For the Ku-band, the LO is 10750MHz, and the resulting output is below the LO. The stability of the LO can have a great effect on the quality of the signal you receive, or even if you get a signal.
Just as with today’s over-the-air DTV signal that can carry more than one program, so, too, can satellite transmissions. If your station transmits more than one program, then you have two or more encoders whose outputs are combined in a multiplexer and sent on to your digital transmitter. For DTV, we are limited to one 6Mhz-wide channel and the 8-VSB modulation method, which give every station a data rate of 19.4Mb/s. For satellites, a transponder’s bandwidth can be as wide as 70MHz, and the modulation methods have evolved over the years from BPSK and QPSK to SP-QPSK. All these modulation methods provide a higher data rate with which to pass more information within a smaller bandwidth. The “PSK” of all these methods stands for phase shift keying. Binary PSK is where there are only two states of the carrier’s phase, so it can only represent one bit of data, one or zero. Quadrature PSK is where there are four states of the carrier’s phase, which means that it can represent two bits at every state.
Just as in 8-VSB, where there are eight different amplitude levels, each amplitude level represents a single symbol. A symbol corresponds to one of eight possible combinations of three bits, so each symbol sent transmits three bits of data, an eightfold increase of data flow. (See Figure 4.)
8PSK is similar to 8-VSB in that it also has eight different states. One of the newest modulation methods is SP-QPSK (sinusoidal-shaped π/4 QPSK), which is a more efficient way to modulate the satellite carrier and something The Associated Press has just started using.
The symbol rate is the number of symbols sent per second. It contains the data rate and the forward error correction (FEC) data. The symbol and data rates correspond to each other, and a formula can be used to convert between the two. (See Figure 5.)
FEC is an important part of digital broadcasting. In satellite transmissions, the amount of error correction is stated as follows: 1/2, 2/3, 3/4, 7/8. The first number is the number of actual data bits, followed by the total number of bits transmitted. The difference is the amount of error correction bits. So for an FEC of 1/2, there is one error correction bit for data every bit. At the other end, an FEC of 7/8 means there is one error correction bit for every seven data bits.
An FEC of 1/2 equals a great deal of error correction, which will tolerate smaller antennas and lower-quality receive equipment, while an FEC of 7/8 means the signal provider is trying to squeeze as much data through as possible and may require a bigger antenna and higher-quality receive equipment.
Conclusion
All of the methods described allow for more data to be sent through fewer frequencies. Satellite transmission requires a great deal of high-tech equipment to be able to reliably send and receive the programs that TV stations use every day.
Satellite TVRO Part 3
Back in the 1980s, the way to find a satellite with your 5m satellite dish was to move its azimuth over a degree or two and them move elevation up and down while looking for any sign of a sync bar from the analog receiver. This would go on until you found a signal, and then you fine-tuned the azimuth, elevation and polarity and hoped it was the satellite you wanted. These days, it’s a little different.
There are very few analog signals to look for, which means you need specialized equipment to monitor the satellite signals and to identify them. This tutorial will cover today’s satellite equipment and how to use them to locate the desired satellite.
Antenna basics
There are three types of satellite dishes in use today: Cassegrain, parabolic and offset. (See Figure 1.) Each has advantages for its particular application. It’s common to refer to the dish itself as the “antenna,” but it is only the reflector. The actual antenna is contained within the feed horn, where the RF is received and turned into electrical signals.
Satellite dishes range in size from 18in to 70m. The most common for broadcast use is a parabolic dish in the 2m to 4m range, which is measured across the dish’s width. Cassegrain dishes are more commonly found in very large dishes, 5m and larger, and the LNBs are located within the hub of the dish where it’s more easily accessible. Small offset dishes are used for direct-to-home satellite reception and can be seen in most neighborhoods. But larger offset dishes of up to 6ft work very well for Ku-band reception.
Satellite dishes come in one of three basic configurations: solid, mesh and petal. A solid dish is just that — solid, made of a single piece of metal — which makes for higher delivery costs but will assure a longer trouble-free lifetime. (See Figure 2.) Mesh dishes are very common in backyards for “free-to-air” satellite programming. The largest is usually 10ft to 12ft. Mesh dishes suffer from a shorter lifetime, but this can be offset by their lower cost and less wind resistance. The petal configuration is easier to ship because it comes in pieces, each shaped like a triangle or petal. These are almost always made out of fiberglass with a metal structure to support the pieces and attach them to the mounting post. While they do work very well, petal dishes do not age well, and if you need to move them, there is the definite risk of unintentional damage. The only variable for dishes is their gain, which is directly related to their size. The bigger the dish, the more gain it has, so it can pickup weaker signals with less noise.
There is one more type of dish: the very wide-angle dish that can see the entire arc of satellites, or over 100 degrees, all at once. These are massive structures that weigh several tons and require very precise installation. Their advantage is the ability to see all satellites at once; to receive a new satellite, only a low-noise block converter (LNB) must be installed at the correct location. The downside is their cost and amount of space they require, but it’s the only dish you ever need to install.
The components of a satellite dish include the mounting pole, the mounting frame, the dish itself, the feed horn and the LNBs. The mounting pole is sunk into the ground and cemented in place, attached to a stand that sits on the ground or, in the case of a roof, a “nonpenetrating” mount is used. These last two must use weights to hold the mount, and the pole, in place. How much weight depends on the size of the dish, the type of dish and where it is being mounted. Without enough weight, the dish can move in high winds, throwing off the satellite dish’s aim. The pole must be plum for a steerable dish to be able to track the satellite arc correctly; fixed dish imitations only require the pole to be close to plum because the adjustments will compensate for errors. The mounting frame is either Az-El or polar. Az-El stands for azimuth-elevation, where each parameter can be adjusted independently, most common on fixed dish mounts. A polar mount allows the dish to track the arc of the satellites in the Clarke Belt with a single movement from east to west. The polar mount causes the dish to change elevation during the east-west movement. This is almost exclusively used on motorized, steerable dishes. The feed horn is the antenna of the satellite dish. Its position above the center of the dish sets the focus point for the reflected satellite signals from the dish. This focal length adjustment is a critical part of setting up a dish. It is also where the polarity of the received signal is selected. Feed horns also come in a number of configurations. The LNBs attach to the feed horn to receive the satellite signal and downconvert it to a frequency band that the receiver will accept. For broadcasters, LNBs are either for C-band or Ku-band signals. Some feed horns will accept one C and one Ku on the same polarity or two C or two Ku each on a separate polarity, so all transponders on a satellite can be received at the same time. (See Figure 3.)
Aligning a fixed dish
The three parameters for satellite dish positioning are elevation, azimuth and polarity. Elevation is the angle of the dish above the horizon; Azimuth is the angle the dish is facing on the compass scale; and Polarity is the angle of the receive antenna within the feed horn in relation to the polarity of the signal sent by the desired satellite. Properly adjusting all three of these parameters will allow you to pick up the satellite you want. (See Figure 4.)
If your satellite dish was located on the equator, the dish would be pointed straight up at a 90-degree angle, and then swung from east to west and all the satellites would be picked up — this is not the case for most of us. That straight line (east to west) at the equator is the line of geostationary satellites; as our dish moves north of the equator, that line becomes elliptical. The further north we move (higher latitudes), the more pronounced the elliptical curve of the geostationary satellites becomes. So we not only move the satellite dish from east to west, but also raise and lower its elevation to be able to track all the satellites in the Clarke Belt.
For fixed or stationary satellite dishes, you simply raise it to a fixed elevation above the horizon and aim it at a particular point on the compass, and the satellite you are looking for should be there (or close to it). There are many Web sites that allow you to enter your satellite dish’s location and which satellite you are looking for, and it will provide you with the elevation and azimuth readings to align your dish.
To find your dish’s elevation, it’s easiest to use an inclinometer or digital level, which measures the tilt or incline of an object. There are mechanical inclinometers with a large dial and the electronic variety that look like a carpenter’s level and have a digital readout — either one should work fine. The hardest part of setting a dish’s elevation is to find a flat space on the back of the dish where you can place the inclinometer that is perpendicular to the dish’s line of sight to the satellite. The back of all dishes is rounded, so that makes it harder to use; a part of the mounting frame can be used if it is parallel to the dish. The most assured way to measure elevation on a parabolic or Cassegrain dish is to place a straight board across the face of the dish, make sure it’s resting on the edges and then measure the elevation on the board.
Offset dishes pose their own set of problems in setting elevation, because the line of sight to the satellite is not perpendicular to the face of the dish. To align an offset dish, you must contact the manufacturer for the correct placement of the inclinometer.