What Is a Satellite?
A satellite is something that goes around and 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, Internet, or computer with anyone in the world.
Other satellites observe the world's weather, feeding weather information into giant computer programs that help scientists know what the weather will be. The weather reporters on your favorite TV news program get their information from those scientists.
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.
This is one kind of satelliteâ€a Boeing 376, built by Boeing Satellite Systems. The Boeing 376 is used mostly for broadcast television and cable television.
This is another, larger kind of satelliteâ€the Boeing 601. The Boeing 601 is used for many purposes, including direct broadcast TV. Direct broadcast TV is a system for receiving television using a very small satellite dish. The Boeing 601 also relays telephone, fax, and computer communications.
The most powerful commercial satellite in the world is the Boeing 702.
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. Satellites in low earth orbit travel about 17,500 miles per hour. These satellites can circle the whole 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. Instead of making circles around the earth, these satellites make ovals. They orbit 6,000 to 12,000 miles above the earth.
GEO, or Geostationary Earth Orbit
A satellite in geosynchronous orbit circles the earth in 24 hoursâ€the same time it takes the earth to rotate one time. Satellites in GEO orbit 22,282 miles above the earth. In this high orbit, GEO satellites are always able to "see" the receiving stations below, and their signals can cover a large part of the planet.
How Does a Satellite Get Into Space?
A satellite is launched on a launch vehicle. The satellite is packed carefully into the vehicle and carried into space, powered by a rocket engine.
Satellites are launched from only a few places in the world, primarily Cape Canaveral, Florida;French Guiana; Xichang, China, and Kazakstan. The best places to launch satellites are near the ocean, so that when the launch vehicle falls away, it lands in the water and not on people.
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 orbit. Then the spent rockets and the launch vehicle drop away, and one or more motors attached to the satellite move it into its permanent geosynchronous orbit. A motor is started up for a certain amount of time, sometimes just one or two minutes, to push the satellite into place. When one of these motors is started, it's called a "burn." It may take many burns, over a period of several days, to move the satellite into its assigned orbital position.
When the satellite reaches its orbit, a motor points it in the right direction and its antennas and solar panels deployâ€that is, they unfold from their traveling position and spread out so the satellite can start sending and receiving signals.
What Does a Satellite Do?
Satellites do many things for people. Their most important job is helping people communicate with other people, wherever they are in the world.
Â¢ A satellite can carry a camera as it travels in its orbit and take pictures of the whole earth. Mapmakers can use these pictures to make more accurate maps. Satellite pictures can also help experts predict the weather, because from the satellite, the camera can actually see the weather coming.
Â¢ Satellites in orbit can send messages to a special receiver carried by someone on a ship in the ocean or in a truck in the desert, telling that person exactly where he or she is.
Â¢ A satellite can relay your telephone call across the country or to the other side of the world. If you decide to telephone your friend in Mexico City, your call can be sent up in space to a satellite, then relayed to a ground station in Mexico and sent from there to your friend's telephone.
Â¢ A satellite can relay your computer message, your fax message, or Internet data as well. With the help of satellites, we can fax, e-mail, or download information 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.
Â¢ A satellite can transmit your favorite TV program from the studio where it is made to your TV setâ€even if the studio is in Japan and your TV set is in Inglewood. 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.
Â¢ When words or pictures or computer data are sent up to a satellite, they are first converted to an invisible stream of energy, called 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.
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 Boeing 601, which is used mostly for direct broadcast TV and business communication networks, and the Boeing 702, which is used mostly for video distribution, satellite telephone and Internet services, and digital radio.
The Boeing 601 has a box-shaped center with several antenna reflectors that look like big dinner plates. Long, wing-like structures attach on two sides. These are the solar panels. The outside of the solar panels or wings is covered with solar cells, which convert the sun's energy to electricity.
The typical Boeing 702 is a larger, more powerful satellite. With a 702, you can put two or three satellites' worth of communication electronics in orbit using one satellite and one launch. When the Boeing 702 is stowed for launch, it is 23 feet (7 meters) high..
Who Owns the Satellites?
Satellites are usually owned by companies or countries. The companies that own satellites usually want to make money by renting out part of the satellite to other companies. The countries or government agencies that own satellites want to make people's lives better by improving the communication networks in their countries.
For example, Indonesia is a country made up of 13,677 islands whose people speak more than 250 languages. Imagine how much time and money it would take to connect them all with wires and telephone poles. Using satellites, Indonesia bridged all the islands at once and helped people learn the national language.
Many large companies also own and operate satellites. They may 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, Internet service companies, and businesses that want to do business in other parts of the world
The area of the earth's surface covered by a satellite's signal is called the satellite's footprint.
What's Inside a Satellite?
A satellite has seven subsystems, and each one has its own work to do.
1. The propulsion subsystem includes the electric or chemical motor that brings the spacecraft to its permanent position, as well as small motors that help keep the satellite in its assigned place in 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 provide power when the sun isn't shining on the panels.
3. The communications subsystem handles all the transmit and receive functions. It receives signals from the earth, amplifies or strengthens them, and transmits (sends) them to another satellite or to a ground station.
4. The structures subsystem distributes the stresses of launch and acts as a strong, stable framework for attaching the other parts of the satellite.
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 maintains the communications "footprint" in the correct location. 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.
7. The telemetry and command subsystem provides a way for people at the ground stations to communicate with the satellite.
Geosynchronous Orbit (GEO) Satellites
GEO Satellite Definition:
A satellite in geosynchronous (or geostationary) are positioned a fixed point at approximately 35,786 kilometers above the earth's surface. At this fixed height, the satellite matches the Earthâ„¢s rotation speed and allows the satellites a full-disc view at a stationary positionTo stay over the same spot on earth, a geostationary satellite also has to be directly above the equator. Otherwise, from the earth the satellite would appear to move in a north-south line every day.
GEO satellites primary purpose is weather imagery to optimize forecasting. In addition to weather imagery, these satellites include instrumentation used in environmental monitoring communications via a relay system. The world network of GEO satellites used with weather imagery and environemental monitoring communications are as follows:
Purpose of GEO Satellites
GEO satellites provide the kind of continuous monitoring necessary for intensive data analysis. By orbiting the equatorial plane of the Earth at a speed matching the Earth's rotation, these satellites can continuously over one position on the surface. Because they stay above a fixed spot on the surface, they provide a constant vigil for the atmospheric "triggers" for severe weather conditions such as tornadoes, flash floods, hail storms, and hurricanes. When these conditions develop these GEO satellites are able to monitor storm development and track their movements.
The GEO satellite functions as a repeater of the data back to an earth ground stations. Stevens also designs and manufactures a GEO receive system called a Direct Readout Ground Station (DRGS) and also provides an alternative Internet Access to the GOES data.
Advantages of GEO Satellite Telemetry
Â¢ Low communications cost (Free for GOES)
Â¢ Low maintenance
Â¢ Ideal for remote locations
Â¢ Data easily shared among government users
Â¢ Very reliable data transmissions as system is supported governmental agencies
Â¢ Available for environmental or home-land security monitoring applications
Disadvantages of GEO Satellite Telemetry
Â¢ Scheduled transmission times assigned by governmental agency and based on Channel/Time availability
Â¢ Interference detection difficult
Â¢ Troubleshooting capabilities minimal
Â¢ Data is available to Government and public
Â¢ Hardware cost more expensive than other telemetry costs
Â¢ One-way transmissions
Â¢ No acknowledgement of successful data transmission.
Â¢ If a transmission fails, it cannot be repeated at a later time.
Â¢ Primarily available only to Federal, state or local governmental agencies or government sponsored environmental monitoring applications.
LARGE GEO SATELLITES
During the past five years, there has been a renewed emphasis on providing satellite-based services to consumers. The acceptance of these services is determined to a great degree by cost to the consumer, including the cost of the equipment as well as monthly service charges. Consumer electronics benefits from competition as well as cost decreases associated with volume manufacturing and distribution and this is vividly demonstrated by the rapid decrease in the cost of DBS home equipment. The power of the signal from the satellite is a critically important factor in the determination of the cost of the ground equipment or terminals. The more the power from the satellite, the less the cost of the terminal. The size of the antennas and the cost of the amplifiers decrease as the power from the satellite increases. Business customers benefit from this increased power for the same reasons.
The need for more power and bandwidth from commercial satellites is obvious to all the satellite manufacturers. Typically, you would expect that increasing the power and bandwidth from the satellite would require a larger, and thus heavier, satellite. However, increasing the weight of the satellite adds to the cost of the launch.
Thus today the increased demand for power is the dominant factor in driving the development and utilization of new GEO satellite technology, especially to meet these weight and cost constraints. Bandwidth per satellite has been increasing as combined C and Ku-band satellites become more common. The need for more bandwidth is especially evident for the new data applications, which are expected to be met with Ka-band and possibly V-band satellites. Here again, more total power is needed to meet power per channel (or Hertz) requirements.
(How the weight of the typical GEO satellite has increased over the past 30 years.)
Other factors driving the increased size and weight of the satellite are the needs for larger antennas, onboard processing electronics, and intersatellite links.
The demand for increased microwave power from the satellite is probably the most important factor in driving the insertion of new technology into modern GEO satellites. Higher power at the customers' antenna translates into lower cost equipment and the availability of new services and thus the need for the manufacture of more satellites and their associated launches. The demand for more power from satellites is driving the development of considerable new technology, with the requirement that this new technology does not add to the cost of the satellite or its weight, which translates into increased launch costs.
The power subsystem is composed of the solar array (solar cells on the supporting structure including pointing devices), batteries and the power conditioning electronics. Considerable progress has been made in the last five years.
MARKET FORCES AND FUTURE DRIVERS
Satellites are uniquely suited to certain applications. These include (1) broadcasting, (2) service to mobile users (including ships, aircraft, land mobile and emergency services), and (3) providing nearly "instant infrastructure" in underserved areas. This last feature is the basis for a large number of recent filings in the United States for Ka-band systems, many of which seek to offer global or nearly global service
This Section discusses three telecommunication trends that are fueling interest in satellite systems. These are direct-to-the-home television (DTH) broadcasting, or direct broadcast satellite (DBS); the enormous growth in wireless hand-held phone usage (cellular, personal communication services (PCS) and paging); and the growth in the number of personal computers (PC's) in the world, increasing numbers of which are multimedia ready and are being used to interconnect with the Internet and/or collect information from the World Wide Web. These three topics are treated in turn in the sections that follow.
Direct Broadcast Satellite
The distribution of TV signals via satellite began in the United States as an inexpensive means of delivering program material (e.g., CNN news) to several hundred cable head-ends spread over the country. This service began at C-band and caused satellite manufacturers (such as Hughes) to launch powerful domestic satellites carrying many transponders so that many cable systems could receive all of their program material with a single earth-station antenna. In time, a cottage industry developed, selling C-band receive-only systems (with typically 2 or 3 meter (8 or 10') diameter antennas) to consumers to eavesdrop on these broadcasts. The number of such installations is now thought to be around 2 million.
It is widely believed that a small size receiving antenna-something that can readily be mounted on the side of a house, for example-is necessary to reach a large subscriber base. Hughes has been the first to approach this market. In 1994 it launched a high-power (~120 watts/transponder) 16-transponder satellite (DBS-1) capable of beaming over 100 digitally-compressed TV channels to viewers, who receive the signals with a 45 cm (18") diameter antenna and set top box converter costing initially about $700. (Prices have since dropped because the service providers have begun to subsidize the purchase). Further capacity increases were achieved with the launches of DBS-2 and DBS-3, and this service (known as DirecTV) was expecting to have over 3 million subscribers by the end of 1997. Hughes DirecTV and Stanley Hubbard's United States Satellite Broadcasting (USSB) both use these satellites.
Primestar, which is owned by the five biggest U.S. cable companies (COMCAST, Continental Cablevision, Cox, TCI, and Time Warner) offers a competing service via a GE Americom satellite placed in service in January 1997. Primestar's subscribers must use a large 1 m (3') dish, but do not have to purchase the equipment whose cost is recovered via the rental agreement. At year-end 1997, Primestar had over 1.9 million subscribers and Echostar had close to one million subscribers. It should be noted that there are currently 2.2 million subscribers to the C-band backyard systems.
MCI and News Corp. won the rights (at a cost of $682.5 million) to occupy the last Ku-band slot from which to broadcast over 200 channels across the nation via a partnership known as American Sky Broadcasting (ASkyB), but MCI has since indicated its desire to scale back its involvement. This forced Time News to seek a merger with Echostar, which is due to receive its powerful Echostar III satellite (being built by Lockheed Martin) in 1998. In addition, TCI plans to inaugurate DBS service at the end of 1997 with a high-power satellite launched into an orbital slot it already controlled, and to use digital compression to deliver Primestar programming to smaller dishes, as well as to cable head ends for distribution on existing cable networks that have limited capacity. Current expectations are that U.S. DBS subscribers will number about 6 million by the end of 1997 and could be double this number by the end of the year 2000.
DBS has enjoyed an even more solid growth in Europe, in part from an earlier start, and in part from the poorer penetration of cable systems. A French media group based in Paris, (Canal+) launched a direct TV service in 1995 via the Luxembourg-based Astra satellites. The British Sky Broadcasting Group offers DBS to 5 million U.K. subscribers. In all, it is estimated that there are 25 million European su
In 1996, the United States had almost 40 million cellular telephone subscribers, while Western Europe had a little over 30 million, Japan perhaps 15 million and Latin America only 5 million. In the United States the expectation is that the number of subscribers to wireless phone services will double by 2000, with less than half being served by the older analog (Amps) service and the rest being served by newer digital ones (CDMA and TDMA) as well as personal communications (PCS, which is offered at higher frequencies). By 2005, the number of subscribers to these services is expected to exceed 250 million world-wide with the largest concentrations in Asia, the United States and Europe in that order.
For a small number of channels this can be raised to 11 dB.
The Iridium system is being built by Motorola, together with subcontractors (e.g., Lockheed Martin, Raytheon, COM DEV). It consists of a fleet of 66, low earth orbiting satellites at 780 kilometer altitude. Eleven satellites will be equally spaced in each of six, circular, nearly polar orbits. Subscribers access the satellites via L-band spot beams (each satellite can activate up to 48) using a TDMA scheme for transmitting voice, coded at 2.4 kbps, or data. Each satellite can handle up to 1,100 simultaneous calls. TDMA packets arriving at a satellite are demodulated and, depending on their destination, routed (at 20 GHz) to a gateway earth station (if one is in view), or (at 23 GHz) to the satellite ahead or behind in the same orbital plane, or the satellite to the east or west in the adjacent orbital plane.
Satellites for Fixed Services
Several factors are driving an explosion of interest in fixed satellite service (FSS) systems. These include:
Â¢ Strong growth in demand for telecommunications services worldwide, and especially for data service (fueled by use of the Internet)
Â¢ Liberalization of telecommunications markets through deregulation and through the WTO agreements
Â¢ The ability of satellites to provide "instant infrastructure" requiring little in the way of civil works (which can be expensive)
Â¢ A number of large U.S. aerospace companies looking for new opportunities in the commercial sector, with the end of the Cold War
Â¢ The fact that large players, such as Motorola, have chosen to enter the market
Proposed services include voice, data, video, imaging, video teleconferencing, interactive video, TV broadcast, multimedia, global Internet, messaging, and trunking. A wide range of applications is planned through these services, including distance learning, corporate training, collaborative workgroups, telecommuting, telemedicine, wireless backbone interconnection (i.e., wireless LAN/WAN), video distribution, direct-to-home video, and satellite news-gathering, as well as the distribution of software, music, scientific data, and global financial and weather information.
PORTABLE AND MOBILE TERMINALS FOR MULTIMEDIA & BUSINESS USE
Satellite-based multimedia service for the consumer is an important part of the business plans of many of the Ka-band systems now under construction. This has developed as an important activity since the 1992/1993 study. Research programs like ACTS, Japan's program in highly intelligent communications, Italy's ITALSAT, and the European DIGISAT, ISIS and MMIS projects, demonstrate that the feasibility of satellite-based interactive multimedia services, are laying the necessary groundwork. The development of portable and mobile terminals for these applications should proceed rapidly along an evolutionary path since--except for reducing terminal size and cost--few hardware innovations are involved.
The increased use of commercial satellites to meet the burgeoning worldwide market for telecommunications has placed increased demands on the launch service industry. The capacity of this industry will not be adequate to meet the needs of all the proposals for new satellites. Even though not all the proposals will get to the marketplace, there appears to be a shortage of launch capacity. In addition, this industry has new challenges to meet. In contrast to the past when most of the commercial satellites were placed into GEO, new satellites will also be placed into LEO and MEO. These latter orbits will be used by constellations of satellites requiring the launch of numerous satellites at a time and the launch of satellites to replace failed satellites, with little lead time. In addition, there is considerable pressure on the launch industry to make a considerable decrease in the price of entrance into space as well as to increase the reliability of the launches, a point that has been watched with considerable interest by the investment banking community.
Ten GEO launches per year were adequate to satisfy the satellite communications business a few years ago. It is now up to thirty and appears that it will increase to almost 100 during the next decade. Launching to LEO will soon exceed launches to GEO. Constellations composed of many satellites, in some cases over one hundred, will put pressures on the industry for timely launches. The launch of commercial satellites is no longer the sole province of the United States. Europe (Arianespace), Russia, China, Japan and the Ukraine have entered this business, with Arianespace replacing the United States as the dominant launch provider. No longer do satellite manufacturers and service providers purchase a launch at a time. They purchase blocks of launches from numerous vendors to ensure the availability of launches when needed. The launch industry has then responded positively to these assured future orders by upgrading the capability of existing rockets and by proposing new launch systems. An example of bulk ordering is the 1997 Hughes purchase of 5 launch options aboard the Chinese Long March rocket. Hughes followed this up with the purchase of 10 launches from Japan's Rocket System Corporation. Space Systems Loral followed a similar path and purchased several launches on the Proton from International Launch Services. The inaugural contracts by Hughes have been important factors in establishing the viability of the Boeing (McDonnell Douglas) Delta III and Sea Launch as well as the Japan H-IIA and the upgrading of the Proton launch facilities. The July 1997 Motorola RFP to provide launch services for its new Celestri system (since cancelled), Iridium replacements, Iridium follow-ons and other satellites totaled 516 satellites, quite an impressive number.
Satellites must cope with the high temperatures of sitting in direct sunlight, and the near absolute-zero temperatures they drop down to in the shadow of the earth. The very fact that they move rapidly between the two extremes in temperature means their lifespans are very short. As if this were not enough, they must cope with the solar wind, which creates a buildup of static electricity on the satellite.
All these factors contribute to requiring that the components of the satellite be very durable and therefore very, very expensive. This is why very few companies and governments operate satellites, and only a handful of companies build them.