By Steven Dick, Ph.D.

Southern Illinois University - Carbondale

Floating in space like so many stars, communications satellites represent some of the most difficult platforms in our communication infrastructure. They can also represent some of the most useful. Even though there has never been a practical application, a properly positioned satellite can theoretically serve one third of the earth. The power and flexibility provided by a single satellite platform more than makes up for the difficulty.

Satellites should become even more important as technical issues are solved. In recent years communications companies have been investing millions in new satellites and their related support systems. The number of communication satellites is growing dramatically. The growth is, in part, due to lower obits that require more satellites and lower cost launch vehicles.

Satellite communication is a relatively old concept. Arthur C. Clarke, soon to become a popular science fiction author, is normally credited with envisioning satellite communication through a 1945 publication (Clarke, 1945). Clarke suggested that a radio receiver/transmitter could be positioned in a specific orbit over the equator. From this "geostationary orbit", satellites travel at a speed making them appear to hang above a specific point on the earth below.

Despite the early start, practical satellite tests did not begin until the late 1950s. Spurred, in part, by Soviet Union’s 1957 Sputnik test, satellites road the early crest of the space race. The first United States successful attempt at satellite communication was the 1960 launch of the 100-foot-diameter balloon called Echo I (Whitehouse, 1986, p. 149). Its metallic surface simply reflected signals whenever it was overhead.

Eighteen years after Clarke published his proposal, a satellite called Syncom (1963) achieved the required orbit. The following year (1964) a consortium of nations established Intelsat, an organization created to provide international satellite communication. The geostationary orbit was immediately adopted as the primary location for these satellites. The problem became one of parking places in this narrow band. Interference and lack of frequencies severely limited the number of satellites. Thirty-five years later (1998) Iridium Incorporated launched a constellation of seventy-six low Earth orbit satellites and created a second satellite location with the market potential of the geostationary orbit.

While it is difficult to establish the platform, once done the system is rather simple. An "uplink" antenna transmits the signal to the satellite. The satellite receives, amplifies and moves (transposes) the signal to a different frequency. The area on the Earth that can receive signals from the satellite is the satellite’s footprint. The signal is then transmitted down to earth and received by "downlink" antennas. The satellite creates a "bent pipe" transmission path that is fairly distance insensitive. Once the signal travels the long distance to the satellite and back, it matters little whether the receiver is hundreds or thousands of miles from the transmitter.

The successful satellite is really the combination of technologies. The most obvious division is ground versus space vehicle systems. However when analyzing the satellite industry, it is more useful to divide the systems on functional basis. For example, there is telemetry on both the satellite and the ground. It is not useful to look at these systems separately since they must interact. A more useful division would include launch vehicle, spacecraft, station keeping, transmission, and reception systems.

The launch vehicle is used to take the satellite out of the Earth’s atmosphere. In the early days, only governments supplied launch capacity. If you were not able to convince the government to do the launch, the launch did not take place. In addition, few governments had the ability to launch the spacecraft.

While it may continue to be necessary to obtain government permission to place a satellite in orbit (often several governments to put a satellite into service), more launch choices have become available. Governments have generally supported privatization of all aspect of the telecommunication industry — including launch services. The trend toward private and semi-government launch services is likely to continue. There are also more governments attempting to create their own launch industries.

The spacecraft can be divided into main parts (Miller, Vucetic & Berry, 1993). The spacecraft bus or service module consists of the structural, telemetry, power, thermal control, and location control subsystems. The bus moves the satellite into its final position and protects its valuable cargo.

The cargo or communications payload is what we normally think of with communication satellites. The uplink antenna receives the signal. A low-noise amplifier adds power to the signal. The transponder will translate the carrier frequency of the signal — so the uplink and downlink signal will not interfere. A high power amplifier will prepare the signal for the downlink antenna.

Satellites are expected to grow dramatically is size. Lebrow (1986) noted that size of a twenty-four transponder satellite was a little more than half a ton. The Economist (1995) predicted four-ton satellites by 2010. Increased power and more transponders are likely to account for the change in weight. Batteries should be lighter in the future. Fuel may more efficient with likely methods to refuel satellites that are not simply replaced.

For the satellite to remain useful, it must achieve and remain in its designated orbit. Telemetry systems help locate the satellite while the spacecraft’s propulsion is used to push the satellites into position and maintain the orbit. A satellite’s lifetime is often determined by the life of its propulsion system. Other parts of the satellite may break down, but a satellite can only carry so much fuel. When that fuel is gone, the satellite will eventually drift out of orbit.

Before widespread use of the geostationary orbit, telemetry systems helped guide ground-based antennas as they tracked the satellite across the sky. Modern Low Earth Orbit satellite system may put intelligence in receiver to track and determine which satellite provides the best reception.

Transmission includes ground-based transmitters, antennas, spacecraft antennas through the transponder. To the marketplace, maximizing satellite capacity is most important. Increasing capacity has been accomplished in three ways: increasing the number of transponders; multiplexing existing frequencies; and signal compression.

The transponder is the center of the satellite capacity. The number of available frequencies and the capacity of the spacecraft bus determine the number of transponders. More transponders require more frequencies, amplifiers, power, and backup systems. Each transponder is approximately equal to one uncompressed video signal. Early satellites had only twelve transponders. Later, twenty-four or more was likely.

Most attempts at frequency reuse amount to good signal planning. High transponder costs force strategic investment. Satellite signals can be polarized for to double or triple channel capacity (Lebow, 1986). For example, waves can be produced so that they carry different signals on the horizontal and vertical plane. Polarizing elements in the antenna and their waveguide-feeds can produce this effect. The signal must be carefully constructed to minimize mutual interference.

Satellites must compete for frequencies with many other services. In addition, given the distance signals must travel, high frequencies waves are best. They are less likely affected by atmospheric refraction. The same frequencies, however, are just as useful on the ground for point-to-point transmissions. The first satellite band, C-band, was commonly used for ground-based communication so downlink antennas had to be placed outside of major cities and power was limited to avoid interference. Other bands, Ku and Ka can be seriously affected by bad weather. Finally, L and S bands have been used in only in some emerging mobile satellite services.

Source: Lin, 1998

Satellite frequencies are under competing pressures. First, there is the desire to efficiently reuse or reallocate frequencies as much as possible. Second, for the sake of standardization, it is also desirable to dedicate a set of frequencies to a single service — despite its popularity. When you consider the vast difference in needed capacity, the problem becomes even more apparent. A single uncompressed television signal (6 MHz) requires the capacity of 1500 telephone calls (4 kHz each).

Most satellites use digital signals and digital is excellent at compression. The simplest form of digital compression applies algorithms to remove redundant information from the signal. It may be easier to imagine a compressed video image. Compression may simply take something away from the image such as the number of frames of video, lines of resolution or the amount of color information. Other systems reduce redundant information within each frame (usually the background) or redundant information between frames (everything that does not change from one frame to another).

Satellite program distribution began in 1962. The people who operated downlinks were a select group of professionals. Satellite programming was not intended for the home audience and it was logical to think so. Reception equipment was expensive and bulky. Broadcast engineers thought only about reception at broadcast quality standards rather than those desired by home consumers.

In 1978 Taylor Howard, an engineering professor at Stanford, published a how-to manual outlining his method for receiving satellite signals with relatively low cost equipment (Parone, 1994). As word spread, the age of television receive-only (TVRO) antennas began. Even as TVRO became popular, it was too large and expensive for most consumers. TVRO was primarily practical in rural areas where cable did not reach and zoning law did not prohibit the large dish antennas required. In addition, many consumers understood that there would eventually be some kind of charge or encryption on the signal — even though many were told otherwise. Finally, the cable industry worked hard to keep the TVRO market from being successful. Considering all that was against it, TVRO did very well.

In June 1994, true direct broadcast satellite (DBS) service began after eleven years of delays. DBS was a dramatic improvement over TVRO. Higher powered satellites, access to programming, compression and better home technology made the system small enough and cheap enough to compete directly with cable television. In 1998, Iridium launched a worldwide satellite telephone service based on a constellation of sixty-six low-Earth orbit (LEO) satellites. A satellite-based digital audio broadcasting (DAB) system should be launched in 1999 or 2000. In the same period, consumer use of the global positioning satellite (GPS) system became commonplace.

In the five-year period from 1994 to 1999, consumer use of satellite reception equipment became practical and marketable. It is likely this trend will only increase. As satellites systems continue to mature, other delivery systems will have to compete directly. Cable television, broadcast television, and radio industries are already preparing with digital systems of their own.

Satellite systems can be used for several purposes that do not directly qualify as media. Military, space exploration, global positioning, weather and remote sensing satellites may affect communications. They may provide content and do compete for space and frequencies. Any of these industries may have unexpected affect on the satellite industry as society develops. For example, military applications may have a catastrophic effect on communications should conflict extend into space. Space exploration and space-based manufacturing may dramatically increase traffic in useable orbits plus the desire for better disposal of waste products. These problems are beyond the scope of this chapter so we will only consider communication services. This section will concentrate on two predictable areas service classes and potential orbits.

Broadly defined, there are two divisions in future satellite applications. Broadband programming is the delivery of high capacity content to consumers and distributors. Broadband content includes television, multimedia and virtual environments. Telephony is all on-demand access to content or services including telephone.

Broadband

Satellites are the perfect platform for the mass distribution of content. Direct broadcast satellites can now deliver more than two hundred channels of programming. It is likely that further compression and satellite power would allow this number to increase. Two hundred plus channels should allow enough capacity to provide the mass program content for large areas. For the near term, it should even provide a simple pay-per-view system — with content determined by the provider.

A true Direct Broadcast Satellite (DBS) service was slow to start. Cable television interests and the technology itself conspired to keep companies from launching the service authorized by the U.S. Federal Communication Commission (FCC) in 1983 (see 27 C.F.R. Sect. 100.19b). However, in 1989, the FCC declared that the companies authorized to build DBS had not done their job. The FCC reassessed and awarded DBS construction permits to a new group of companies (Carlin, 1998).

Two issues had a major impact on the failure of DBS. First, it was difficult to get programming at a reasonable cost. Shared corporate ownership between cable companies and cable networks combined with industry pressure to keep program costs high — if available at all. The Cable Television Consumer Protection and Competition Act of 1992 forced sales to DBS.

Second, a standard digital compression standard was necessary so that DBS companies could deliver enough channels to compete with cable television. In 1993, MPEG-1 was chosen as an international standard for video compression. This compression algorithm allowed eight digital video channels in the space on one analog. In 1995 DBS companies adopted MPEG-2. MPEG-2 allowed for broadcast quality pictures and varying levels of compression.

Once equitable programming and channel capacity was assured investors could be found. The first DBS satellite was launched in 1994. By the March of 1998, DBS enjoyed 6.6 million subscribers for three companies.

However, current technology is not efficient at delivering either local content or video on demand. In an attempt to deliver broadcast network content. DBS companies have selected affiliates to redistribute nationally. Such distribution has become a marketing and legal problem for DBS. Consumers prefer to receive local stations providing the same content along with local news. Networks argue that such importation of distant signals disrupt network-affiliate contracts.

DBS has been left with few choices. One, disregard networks and local stations and try to sell consumers on distant signals. This plan continues the current legal and marketing battles — both look like a losing proposition. Second, create a satellite of only local stations to distribute along side the current DBS. This choice may created a new market for distance stations but current satellites lack the capacity and possibly the legal right for such a system. Third, use additional antennas or local cable television to deliver local content. A reasonable choice if the DBS and local cable company can partner. Otherwise, it creates additional costs and maintenance issues.

Satellites continue to be effective at providing networked content. Most broadcast and cable networks use a satellite distribution system. The problem develops as satellite attempts to deliver interactive broadband content. Satellite access to the internet is growing — especially in rural areas underserved by cable. But cable access to the internet is much faster. Plus, for most systems the return path must be done by ground-based telephony.

Telephony

The FCC and other governing bodies have effectively created a tiered system of mobile satellite (Schwartz, 1996). Mobile satellite service (MSS) has been defined as that between 1.) a mobile earth station and a space station, and 2.) mobile Earth stations and another through a space stations. In effect, all services that can be provided on the wired network will be provide on a satellite network.

There has, in the past, been a division between voice telephony and data services. As networks switch to digital distribution this division is disappearing. Telephony will be defined (if not already) as any occasional or on-demand use of the network. This would include private networks that can be created and altered on demand. Telephony will be a major consumer of satellite services due to the relative ease of installation, alteration and mobility.

For all the interest in the geostationary orbit, it is easy to overlook the options of the past and soon the future. The original concept for a communication satellite is one that resides in a fairly narrow orbit above the equator. In recent year, new choices have been explored. These choices may make a dramatic difference in the services that can be offered and the convenience of satellite communication.

Geostationary

The geostationary or geosynchronous orbit (GEO) is sometimes called the Clarke orbit after Arthur C. Clarke, the person who proposed it. The satellite is placed in a circular orbit 22,300 miles (35,800 kilometers) above the equator. The satellite will take about one day to orbit the Earth. In effect, it will seem to hang above a single point on the equator. The advantage over previous locations is that Earthbound antennas can remain fixed on one point and do not have move as the satellite tracks across the sky.

GEO became a lighting rod for international competition and fairness. There were a limited number of useful spaces to park a satellite. Satellite spacing was dependent upon the power and frequencies used (Whitehouse, 1986). Satellites were originally spaced at four degrees apart and later upgraded to two degrees. GEO being a circular orbit, 360 degrees divided by 2 meant 180 parking places worldwide. Subtract parking places that were not over a major land mass and the problem gets worse. More spaces were added as more frequencies were used but the competition for these places remained stiff.

For GEO, a second problem comes down to one of distance. The extreme distance requires highly focused intense signals to travel to our most distant communication orbit. Large antennas are often difficult to place because there must be a direct line of sight to the satellite. In northern latitudes the problem gets even worse as the satellite is lower on the horizon. Higher power DBS satellites help but only bring the antenna size down to something that can be put on a rooftop. Indoor or non-visible (inside the receiver) antennas are yet not possible.

For telephone systems, a related problem exists. The distance is so great that there is a delay for the signal to travel. The delay is only noticeable to human perception if a double hop or two satellite trips are needed to complete the call. Still, it is a limiting factor in highly interactive data exchange. As data rates and interactivity increase, the distance to any satellite will become a problem.

Elliptical

As discussed above, northern latitudes have a greater problem with receiving signals form the geostationary orbit. It is difficult to cut through all the atmosphere or find locations where the antenna can be placed. The former Soviet Union, with its large northern landmass, dealt with the problem by adopting an elliptical orbit. For almost half of the time it takes to complete the orbit, the satellite spends at the apogee — a high point over satellite’s footprint. Three satellites can serve the footprint effectively. On the ground, the antennas can still remain fixed since a new satellite will take over as one moves out of range. In addition, the orbit even at its apogee is lower than the GEO satellites. New elliptical orbits can be designed to give special coverage to other southern or northern countries.

Low and Medium Earth Orbit

Orbital paths other than GEO were not used because of the necessity to track the satellite with ground-based antenna. Telecommunications engineers have learned a lesson from cellular telephone. A fixed network must track mobile cellular users. The logic can now be extended to satellites where relatively fixed users must track a highly mobile network. It requires determining the strongest signal and moving from one point in the network to another to maintain signal strength.

The solution to the low Earth orbit (LEO) and medium Earth orbit (MEO) problem above suggests a second problem with lower orbits. The satellites keep moving so more satellites are required — dramatically more. The Iridium LEO network was started with a constellation of sixty-six satellites and six spares. Teledesic, another LEO system, is planning a constellation of 288 active satellites (Wood, 1998). A compromise between LEO and GEO is the ICO MEO system planned with only ten satellites.

The difference between LEO and MEO is a matter is a matter of height. A LEO satellite will be placed at an altitude between 100 and 1,000 miles above the Earth’s surface. The MEO satellite is placed between 5,000 and 10,000 miles. Between the two orbits is the disruptive Van Allen radiation belt.

For both LEO and MEO satellites orbit height requires a trade-off (Schwartz, 1996). For a higher the orbit, fewer satellites are required due to larger footprint of each satellite. Less fuel is required due to less effect of Earth’s gravity. The satellite’s life span is often determined but the amount of fuel so lower fuel requirements mean a longer projected life.

Satellites traveling in lower orbits can eliminate transmission delays. The satellite can be lighter and cheaper. Since more signal power reaches the ground, receive antennas can be lighter, cheaper and smaller. The FCC has mandated a class of little-LEO satellites which operate in the lowest orbits, frequencies and power. These will be data service satellites for business similar to the GEO very small aperture terminal (VSAT) services.

In the next couple decades, we should see a race for these new LEO and MEO orbits. It may be similar to the development of the GEO orbit. With the proven success of GEO satellites and the ability to build satellites faster, the adoption of LEO and MEO should be even faster.

High Altitude Long Endurance

In 1961, a group of educators from the Big Ten Conference did something crazy. They loaded a DC-6 aircraft with two UHF transmitters, brand new AMPEX videotape machines and a couple days worth of tapes. They flew the plane to an altitude of 23,000 feet above north central Indiana and started broadcasting educational television. For six years the Midwest Program for Airborne Television Instruction (MPATI) broadcast programming to parts of Indiana, Illinois, Ohio, Kentucky, Michigan, Wisconsin and Canada. As crazy as that idea sounds, it is being revisited. The project lasted for four years before losing its experimental license from the FCC.

Advances in pilotless aircraft and dirigibles make it realistic to consider the placement of a communication platform above twelve miles (twenty kilometers) above the earth. These craft would stay in place for a week or two and be replaced for refueling. High Altitude Long Endurance (HALE) platforms would create a communication relay point similar to a satellite but at a lower cost and increased flexibility. Initially, HALE would be designed for high traffic areas like major cities. HALE could rely traffic back down to the ground or to and from a variety of communication satellites.

Satellite communication has a promising yet complicated future. The incredible investment, scale and inhospitable environment is unmatched in other areas of communication. To complicate matters, satellite communication is by definition international and often global. Finally, the development of satellites in the next twenty-five years will take place within the framework of equal development on the ground.

The biggest limiting factor for satellite communication is pure competition. Satellites must compete for frequencies, orbital positions and customers. In addition to satellite communication, it is reasonable to believe that space will be exploited by other industries. The military, science, manufacturing and travel industries may move into space. All will need satellite communication but will demand some of the resources.

However satellites do have a distinct advantages. Satellites can provide service where ground based communication systems are either not established or difficult to install. Satellites can often offer quick solutions where installed. With cheaper, easier to build satellites, it is likely that new networks can be created faster as long as there is space. Scarcity may work to the satellite industry’s favor. Scarce resources may put a natural cap on the industry’s investment, while driving up costs for those that need the service.

Current satellites amount to a patchwork of technologies and orbits. Satellite power, customers, and service classes are more an accident of history than they are a conscious plan. In the next twenty-five years, usable life spans will expire and systems will be upgraded and replaced. Anticipating the future of satellite communication means understanding the real estate it uses. For the foreseeable future there are two major obit classes geostationary and non-geostationary. Given the advantages of each class, there should be three major service classes.

Mass delivery GEOs

Geostationary satellites will hold the communication high ground. Their footprint is largest while interactivity will be more difficult. Because they can operate in only one narrow orbit, the real estate they require should remain very valuable. Given their relative distance, GEO satellites are best suited for mass delivery of point to multi-point content. Each GEO satellite can serve one third of the Earth. Efficiency dictates that these satellites should increase broadband capacity to serve the widest possible audience with the fewest number of signals. The caveat is that compression and power will dramatically increase the number of channels anyway. The next generation of satellites should be able to provide multiple footprints in its range. For example, a single satellite should provide content to both North and South America — rather than one or the other.

Flexible use non-GEOs

Non-GEO satellites should become more important for mobile, and occasional use systems. Increased power, decreased delays, smaller footprints and cheaper systems should encourage the adoption of lower satellites that are adaptable and specifically designed. Smaller footprints should discourage mass distribution of non-interactive content. Too many satellites will be required to distribute a product over a large area.

Virtual satellites

With increased compression and efficiency, excess capacity will become commonplace both on satellite and on the ground. A market should develop that will sell excess capacity to users that can stand occasional delays or rollover onto higher cost networks. These virtual satellites, or larger virtual networks, will use something akin to Internet protocol to select lowest cost network for transportation of communication resources.

More than most, the satellite segment of the communication industry has adopted technological leaps first. Satellites are well on their way towards a market structure that will take them into 2025. Being there first has a certain advantage. Satellites can help secure their place as the "traditional" method of communication delivery.

However, satellite must keep their advantage in mind. They best provide distance insensitive communication to mobile communications. When possible, there will always be more space on the ground to string cable than room in the air to park satellites. A single optic fiber can dwarf the capacity of the most powerful satellite. Cable and local broadcast can provide local content and local representatives better than any international satellite company. Keeping these limitations in mind, satellites should excel if they concentrate on the type of delivery they do best.

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