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.)
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.)
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.)
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:
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.
Acknowledgments
Ed Johnson of San Francisco International Gateway Teleport and Frank Foge of CALSAT assisted with this tutorial.
Next Time
The next "Transition to Digital" tutorial will discuss expanding the number of channels a satellite can handle.
Continue reading part two of the Satellite TVRO series.
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