Your entire history has either been transmitted in writing or has been a history verified in later ages. In other words, it can be called a history based on transmission and reports. In the past, the information obtainable in the narrow life sphere of each individual was extremely limited and it was not possible for everyone to have a direct feeling of the great flow of history. But now, with the advent of satellite communications, world events can be witnessed by people everywhere, just as they are.
The tragedy of President Kennedy, mankind's first landing on the moon - we have become aware that it is possible to experience, from one's own roon, the development of history simultaneously with different peoples of the world. This is both marvelous and exciting.
The history of satellite communications is the history of mankind's technological innovations. It can also be called the history of an information revolution that is even altering mankind's sense of history.
Firsts in Satellite History
October 4, 1957 - The Russian Sputnik 1 is the first satellite in space. Russia becomes the first space power.
November 3, 1957 - A dog named 'Laika' is the first living creature in space.
February 1, 1958 - Explorer 1 is the first American satellite in space. The USA becomes the second country in space.
November 3, 1960 - NASA launches its first satellite, the Explorer 8.
April 12, 1961 - The first man in space.
June 16, 1963 - The first woman in space.
August 19, 1964 - The Syncom 3 becomes the first geostationary telecommunications satellite.
July 21, 1969 - The first man to walk on the moon.
April 12, 1981 - The first manned space shuttle launch.
April 6, 1984 - SMM was the first satellite repaired in orbit by the space shuttle.
November 16, 1984 - The first time that satellites were brought back from space by the space shuttle.
How Satellites Work
In little over a third of a century, the launching of a satellite has gone from stopping the nations' business to guaranteeing that it runs like clockwork. Today, satellites are commonplace tools of technology, like clocks, telephones, and computers. Satellites serve us for navigation, communications, environmental monitoring, and weather forecasting. Appropriately, the word satellite means an attendant.
In 1957, the launching of the Russian satellite Sputnik changed the course of our nation. The United States immediately launched massive efforts to compete against a "backward" empire in a breakneck Race to the Moon. In the space of a decade, our nation of arm-chair explorers sat glued to their television sets while Alan Shepard went up and down in a Mercury capsule in 1961, as John Glenn circled the globe 3 times in 1962, and when Neil Armstrong set foot on the moon in 1969.
That sense of discovery has become muted over time as we became accustomed to miracles of space travel. The launching of a Space Shuttle mission may not even come up in a class discussion of current events (unless those students are involved in the classroom use of satellite technology). Yet satellites bring those same students the Olympics, the weather, and news of other events from around the world that are considered "newsworthy."
What is a satellite ?
A satellite is something that orbits, or goes around a larger something, like the Earth or another planet. Some satellites are natural, like the moon, which is a natural satellite of the Earth. Other satellites are made by scientists and technologists to go around the Earth and do certain jobs.
Some satellites send and receive television signals. The signal is sent from a station on the Earth's surface. The satellite receives the signal and rebroadcasts it to other places on the Earth. With the right number of satellites in space, one television program can be seen all over the world.
Some satellites send and receive telephone, fax, and computer communications. Satellites make it possible to communicate by telephone, fax, or computer with anyone in the world.
Other satellites observe the world's weather, feeding weather information into computer programs that help scientists know what the weather will be.
Still other satellites take very accurate pictures of the Earth's surface, sending back images that tell scientists about changes that are going on around the world and about crops, water, and other resources.
The satellite HS 376, built by Hughes Space and Communications Company. The HS 376 is used mostly for broadcast television and cable television.
The satellite HS 601 which is also built by Hughes Space and Communications Company. The HS 601 is used for many purposes, including direct broadcast TV. Direct broadcast TV is a new system of receiving television using a very small satellite dish. The television signal is relayed by an HS 601 satellite. The HS 601 also relays telephone, fax, and computer communications.
What Does a Satellite Do ?
A satellite orbits Earth. It is kept in place by gravity and centrifugal force. And while it is up there going round the Earth, it helps people communicate and learn more about our planet.
A satellite can carry a camera and take pictures of the whole Earth as it travels in its orbit. These images can be used by cartographers to make more accurate maps. Satellite pictures can also be used to predict the weather, because from the satellite, the camera can actually see the weather coming. When you watch the weather forecast on TV, you are seeing pictures of the Earth taken by a camera on a satellite.
Satellites in orbit can send messages to a special receiver carried by someone on a ship on the ocean or in a tank in the desert, telling that person exactly where he or she is.
A satellite can relay your telephone call across the country or to the other side of the world. If you decide to telephone someone in Mexico City, your call will be sent up in space to a satellite, then relayed to a ground station in Mexico and sent from there to the telephone in Mexico City.
A satellite can relay your computer message or your fax message as well. With the help of satellites, we can fax or e-mail anyplace in the world. When the satellite sends a message from your computer or fax to another computer or fax, it's called data transmission: the satellite is transmitting, or sending, data.
A satellite can transmit your favorite TV program from the studio where it is made to your TV set. From the studio where it is made, a TV program is broadcast to a satellite. This is called an uplink. Then it is rebroadcast from the satellite to another place on the Earth. This is called a downlink.
When words or pictures or computer data are sent up to a satellite, they are first converted to an invisible stream of energy, a signal. The signal travels up through space to the satellite and then travels down from the satellite to its destination, where it is converted back to a voice message, a picture, or data, so that the receiver can receive it.
Satellites do many things for people. Their most important job is to help people communicate by telephone, telegraph, television, data transmission, and photography with other people, wherever they are in the world.
What is an orbit ?
When a satellite is launched, it is placed in orbit around the Earth. The Earth's gravity holds the satellite in a certain path as it goes around the Earth, and that path is called an "orbit." There are several kinds of orbits. Here are three of them.
LEO, or Low Earth Orbit
A satellite in low Earth orbit circles the Earth 100 to 300 miles above the Earth's surface. Because it is close to the Earth, it must travel very fast to avoid being pulled out of orbit by gravity and crashing into the Earth. Satellites in low Earth orbit travel about 17,500 miles per hour. These satellites can circle the Earth in about an hour and a half.
MEO, or Medium Earth Orbit
Communications satellites that cover the north pole and the south pole are placed in a medium altitude, oval orbit. Receivers on the ground must track these satellites. Because their orbits are larger than LEOs, they stay in sight of the ground receiving stations for a longer time. They orbit 6,000 to 12,000 miles above the Earth.
GEO, or Geostationary Earth Orbit
Satellites that provide continuous communications services or weather data are placed in geosynchronous orbit at a distance from the Earth of 22,282 miles. These satellites circle the Earth in 24 hours the same time it takes the Earth to rotate one time. If these satellites are positioned over the equator and travel in the same direction as the Earth rotates, they appear "fixed" with respect to a given spot on Earth that is, they hang like lanterns over the same spot on the Earth all the time. Satellites in GEO are always able to "see" the receiving stations below, and a satellite in this high orbit can cover a large part of the planet; three satellites can cover the globe, except for the parts near the north and south poles.
How Does a Satellite Get Into Space ?
A satellite is launched on a launch vehicle, which is like a taxicab for satellites. The satellite is packed carefully into the vehicle and carried into space, powered by a rocket engine.
Certain places in the world are especially good for launching satellites, because of weather conditions, local geography, and where the satellite is intended to go. Satellite destination is important, because the rotation of the Earth, prevailing winds, and other conditions can either help the satellite get where it's going or make it more difficult.
Alcāntara in Brasil, is one of the good places to launch a satellite. Others include Kourou (French Guiana), Cape Canaveral (Florida), Xichang (China) and Tanegashima (Japan).
Putting everything together for a launch is very complicated. Many people in many companies and sometimes in many countries have to work together and coordinate their work so that everything will be ready for a launch.
At launch, the launch vehicle's rockets lift the satellite off the launch pad and carry it into space, where it circles the Earth in a temporary low Earth orbit. Then the spent rockets and the launch vehicle drop away, and a small "kick" motor attached to the satellite moves it into an elliptical transfer orbit. From there, another small motor is used to push the satellite into its permanent geosynchronous orbit. A satellite takes several days after launch to reach its permanent orbit.
When the satellite reaches its orbit, a motor is used to point it in the right direction and its antennas are deployed from their traveling position so the satellite can start sending and receiving signals.
Who Owns the Satellites ?
Satellites are usually owned by countries or companies. The companies that own satellites usually want to make money by renting out part of the satellite to other companies. The countries or governments that own satellites want to improve the communication networks in their countries and build a national identity.
The government of Indonesia decided that the best way to communicate with its people and teach them about their country would be to put a satellite up in space over Indonesia and put a television set in each village. Gradually the people would learn about themselves and their country, and eventually they would use their new knowledge to increase trade and to communicate with people in other parts of the world by satellite. So the Indonesian government asked a satellite manufacturer to build a satellite for them, and that satellite is now in orbit over Indonesia.
Being able to communicate better with people all over the world helps countries develop trading opportunities, increase business, and get information they need. Many countries including Australia, Brazil, Canada, China, Japan, Luxembourg, Malaysia, Mexico, Thailand, and the United States are now increasing opportunities for their people by using satellites.
Many large companies also own and operate satellites. They rent space on the satellite to other companies and businesses. For example, a large communication company might buy a satellite and then rent space on the satellite to television companies, telephone companies, and businesses who want to do business in other parts of the world.
A satellite operator can let its satellite "see" as much as one-third of our planet at a time, or it can shape the signal to reach a smaller area. For example, if you were the Indonesian government, you might want your satellite to cover only Indonesia, and not spill over into other countries. The satellite signal can be shaped to cover the exact area that the operator wants to reach.
The area of the Earth's surface that is covered by a satellite's signal is called the satellite's footprint or beam pattern.
What's Inside a Satellite ?
Satellites have a great deal of equipment packed inside them. Most satellites have seven subsystems, and each one has special work to do.
1. The propulsion subsystem includes the rocket motor that brings the spacecraft to its permanent position, as well as small thrusters (motors) that help to keep the satellite in its assigned place in orbit. Satellites drift out of position because of solar wind or gravitational or magnetic forces. When that happens, the thrusters are fired to move the satellite back into the right position in its orbit.
2. The power subsystem generates electricity from the solar panels on the outside of the spacecraft. The solar panels also store electricity in storage batteries, which can provide power at times when the sun isn't shining on the panels. The power is used to operate the communications subsystem. The entire communications subsystem can be operated with about the same amount of power as would be used by 10 light bulbs.
3. The communications subsystem handles all the transmit and receive functions. It receives signals from the Earth, amplifies them, and transmits (sends) them to another satellite or to a ground station.
4. The structures subsystem helps provide a stable framework so that the satellite can be kept pointed at the right place on the Earth's surface. Satellites can't be allowed to jiggle or wander, because if a satellite is not exactly where it belongs, pointed at exactly the right place on the Earth, the television program or the telephone call it transmits to you will be interrupted.
5. The thermal control subsystem keeps the active parts of the satellite cool enough to work properly. It does this by directing the heat that is generated by satellite operations out into space, where it won't interfere with the satellite.
6. The attitude control subsystem points the spacecraft precisely to maintain the communications "footprints" in the correct location. When the satellite gets out of position, the attitude control system tells the propulsion system to fire a thruster that will move the satellite back where it belongs.
7. Operators at the ground station need to be able to transmit commands to the satellite and to monitor its health. The telemetry and command system provides a way for people at the ground stations to communicate with the satellite.
How Big Is a Satellite ?
Different kinds of satellites are used in different situations, for different purposes. To talk about the sizes of satellites, we'll use two examples, the HS 376, which is used mostly for network and cable TV, and the HS 601, which is used mostly for direct broadcast TV and business communication networks.
The HS 376 is a small, barrel-shaped structure with an antenna reflector that looks like a lid on the barrel. When the HS 376 is first launched, its antenna reflector and solar panels are stowed that is, put away so it can fit inside a launch vehicle. After launch, the satellite travels through space until it reaches its assigned orbital position. Then its reflector and solar panels are deployed that is, unpacked and put in the right position for doing their work.
A typical HS 376 is 2.16 meters (7 feet 1 inch) in diameter, and 2.82 meters (9 feet 3 inches) high, in its stowed position. When it is deployed, its diameter is the same, but it is much taller: 6.57 meters (21 feet 7 inches) tall. The height of the deployed satellite is more than twice its height when stowed.
The satellite body is made like a telescope; when it is deployed, an outer cylinder is driven down by tiny electric motors to reveal the inner cylinder and locks into place. All of the outside of the satellite body is made of solar cells, which take the sun's energy and convert it to electricity. That means that when the outside is in its full telescope position, more solar cells are exposed to the sun, and the satellite can generate more power. The deployed HS 376 can generate more than twice as much power as the stowed HS 376.
Satellites weigh more at the beginning of life in orbit than at the end. This is because they carry rocket fuel for the thruster engines that will keep them in place in their orbits. As the fuel is used up, the satellite gets lighter. The HS 376 weighs 634 kg (or 1395 pounds) at the beginning of its life in orbit.
The HS 601 is a larger and more powerful satellite. When it is stowed for launch, it looks like a big box, 3.8 meters (12.6 feet) high. The HS 601 carries its solar panels on long wing-like structures. When the satellite is deployed, the solar panels are extended to a width of 26 meters (86 feet), and the antenna reflectors make the middle of the satellite 7.1 meters (23.3 feet) wide. The HS 601 weighs 1727 kg (3800 pounds) at the beginning of its life in orbit.
The Anatomy of a Satellite
Satellites have only a few basic parts: a satellite housing, a power system, an antenna system, a command and control system, a station keeping system, and transponders.
The configuration of the satellite housing is determined by the system employed to stabilize the attitude of the satellite in its orbital slot. Three-axis-stabilized satellites use internal gyroscopes rotating at 4,000 to 6,000 revolutions per minute (RPM). The housing is rectangular with external features as shown below:
The materials used in the construction of satellite housings are typically very expensive. In newer satellites, lightweight and extremely durable epoxy-graphite composite materials are often used.
Satellites must have a continuous source of electrical power 24 hours a day, 365 days a year. The two most common power sources are high performance batteries and solar cells. Solar cells are an excellent power source for satellites. They are lightweight, resilient, and over the years have been steadily improving their efficiency in converting solar energy into electricity. Currently the best gallium arsenide cells have a solar to electrical energy conversion efficiency of 15-20%. There is however, one large problem with using solar energy. Twice a year a satellite in geosynchronous orbit will go into a series of eclipses where the sun is screened by the earth. If solar energy were the only source of power for the satellite, the satellite would not operate during these periods. To solve this problem, batteries are used as a supplemental on-board energy source. Initially, Nickel-Cadmium batteries were utilized, but more recently Nickel-Hydrogen batteries have proven to provide higher power, greater durability, and the important capability of being charged and discharged many times over the lifetime of a satellite mission.
A satellite's antennas have two basic missions. One is to receive and transmit the telecommunications signals to provide services to its users. The second is to provide Tracking, Telemetry, and Command (TT&C) functions to maintain the operation of the satellite in orbit. Of the two functions, TT&C must be considered the most vital. If telecommunications services are disrupted, users may experience a delay in services until the problem is repaired. However, if the TT&C function is disrupted, there is great danger that the satellite could be permanently lost drifting out of control with no means of commanding it.
Command and Control System
This control system includes tracking, telemetry & control (TT&C) systems for monitoring all the vital operating parameters of the satellite, telemetry circuits for relaying this information to the earth station, a system for receiving and interpreting commands sent to the satellite, and a command system for controlling the operation of the satellite.
Although the forces on a satellite in orbit are in balance, there are minor disturbing forces that would cause a satellite to drift out of its orbital slot if left uncompensated. For example, the gravitational effect of the sun and moon exert enough significant force on the satellite to disturb its orbit. As well, the South American land mass tends to pull satellites southward.
Station keeping is the maintenance of a satellite in its assigned orbital slot and in its proper orientation. The physical mechanism for station keeping is the controlled ejection of hydrazine gas from thruster nozzles which portrude from the satellite housing. When a satellite is first deployed, it may have several hundred pounds of compressed hydrazine stored in propellent tanks. Typically, the useful life of a satellite ends when the hydrazine supply is exhausted usually after ten years or so.
A transponder is an electronic component of a satellite that shifts the frequency of an uplink signal and amplifies it for retransmission to the earth in a downlink. Transponders have a typical output of 5 to 10 watts. Communications satellites typically have between 12 and 24 on-board transponders.
Satellite Frequency Bands
The three most commonly used satellite frequency bands are the C-band, Ku-band, and Ka-band. C-band and Ku-band are the two most common frequency spectrums used by today's satellites. To help understand the relationship between antenna diameter and transmission frequency, it is important to note that there is an inverse relationship between frequency and wavelength when frequency increases, wavelength decreases. As wavelength increases, larger antennas (satellite dishes) are necessary to gather the signal.
C-band satellite transmissions occupy the 4 to 8 GHz frequency range. These relatively low frequencies translate to larger wavelengths than Ku-band or Ka-band. These larger wavelengths of the C-band mean that a larger satellite antenna is required to gather the minimum signal strength, and therefore the minimum size of an average C-band antenna is approximately 2-3 meters in diameter.
Ku-band satellite transmissions occupy the 11 to 17 GHz frequency range. These relatively high frequency transmissions correspond to shorter wavelengths and therefore a smaller antenna can be used to receive the minimum signal strength. Ku-band antennas can be as small as 18 inches in diameter.
Ka-band satellite transmissions occupy the 20 to 30 GHz frequency range. These very high frequency transmissions mean very small wavelengths and very small diameter receiving antennas.
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