A communications satellite is a spacecraft in Earth orbit that supports communication over long distances by relaying or reflecting radio signals sent from the ground to other points on the surface.
The possibility of using artificial satellites for radio communications
over global distances had been discussed before World War II, but the modern
concept dates from an October 1945 Wireless World article by Arthur
C. Clarke. Clarke envisioned three relay stations
in geostationary orbit by means
of which a message could be sent from any point on Earth and relayed from
space to any other point on the surface. As an application of such a system,
Clarke suggested direct broadcast TV – a remarkably advanced idea
given that television was still in its infancy and it was not yet known
whether radio signals could penetrate the ionosphere.
The concept of the geostationary orbit had been discussed earlier by Herman Noordung, but Clarke gave the first detailed
technical exposition of satellite communications. His vision was realized
through the pioneering efforts of such scientists as John Pierce of Bell Labs, head of the Telstar program
and co-inventor of the traveling wave tube amplifier, and Harold Rosen of
Hughes Aircraft who was the driving force behind the Syncom program.
Most early communications satellites, such as Echo, were simply big metal-coated balloons that passively reflected signals from transmitting stations back to the ground. Because the signals bounced off in all directions, they could be picked up by any receiving station within sight of the satellite. But the capacity of such systems was severely limited by the need for powerful transmitters and large ground antennas. SCORE, launched in 1958, was technically an active communications satellite. It had a tape recorder that stored messages received while passing over a transmitting ground station, then replayed them when the satellite came within line of sight of a receiving station. However, real-time active communications came of age in 1962 when Telstar 1 enabled the first direct TV transmission between the United States, Europe, and Japan.
In the 1970s, advances in electronics and more powerful launch vehicles made geostationary satellite communications practicable and led to its rapid expansion. The Syncom series demonstrated the viability of the technique and Early Bird – Intelsat 1 – became the first operational geostationary satellite. It was followed over the next three decades by Intelsat series of increasing power and capability.
In 1972, a policy change by the FCC (Federal Communications Commission) cleared the way for domestic satellite services and prompted RCA in 1975 to launch Satcom 1. This was immediately used by a group of entrepreneurs to transmit a new type of pay TV, known as Home Box Office (HBO), to cable providers throughout the United States.
A number of developing countries, especially those with large areas and diffuse populations, saw satellite communications as an attractive alternative to expensive microwave and coaxial land-based networks. Indonesia led the way with Palapa 1 in 1976 and Palapa 2 in 1977. The late 1970s also saw the beginning of a revolution with the introduction of small, affordable satellite dishes, about 3 m across, which consumers could purchase and install on their own property. These units made possible direct reception of virtually any transmission, including unedited network video feeds and commercial-free shows. A pioneer in this field was Ted Turner, owner of WTBS, an independent cable channel in Atlanta, Georgia. In 1976 Turner leased an available transponder and turned WTBS into the first superstation. This led to the rapid expansion of direct satellite TV networks, including Turner's own Cable News Network (CNN), which did not have to rely on the microwave relay network and local affiliate transmitters of the three major American commercial networks. The expansion of community access TV (cable TV) led to greater demand for high-quality satellite feeds. Today, direct broadcast satellites (DBS) deliver programs to home antennas measuring less than 50 cm in diameter.
The falling cost of satellites and launches have allowed a number of companies to buy and market satellites, and other systems to provide international service in competition with Intelsat. The growth of international systems has been paralleled by domestic and regional systems, such as Telstar, Galaxy, and Spacenet in the United States, Eutelsat and Telecom in Europe, and many single-nation indigenous systems.
The 1990s have seen the development of LEO (low Earth orbit) and MEO (medium Earth orbit), non-geostationary satellite concepts. More sensitive radio receivers allow satellites in LEO to receive and relay signals from ground stations without the need for antennas that track the satellite as it moves across the sky. The first commercial service to operate from LEO was Iridium, but other similar services are now coming online, including Globalstar, ICO, and Ellipso. Depending on the height of their orbit, they are characterized as being either LEO or MEO and are used mainly to connect cellular telephones and business facsimile machines. They may also connect computers in areas where landlines and other relays do not exist. New digital coding methods have resulted in a tenfold reduction in the transmission rate needed to carry a voice channel, enhancing the capacity of facilities already in place and reducing the size of ground stations that provide phone service.
Communications satellite systems have entered a period of transition from point-to-point high-capacity trunk communications between large, costly ground terminals to multipoint-to-multipoint communications between small, low-cost stations. The development of multiple access methods has both hastened and facilitated this transition. With time-division multiple access (TDMA), each ground station is assigned a time slot on the same channel for use in transmitting its communications; all other stations monitor these slots and select the communications directed to them. By amplifying a single carrier frequency in each satellite repeater, TDMA ensures the most efficient use of the satellite's onboard power supply.
A technique called frequency reuse allows satellites to communicate with a number of ground stations using the same frequency, by transmitting in narrow beams pointed toward each of the stations. Beam widths can be adjusted to cover areas – footprints – as large as the entire United States or as small as a state like Maryland. Two stations far enough apart can receive different messages transmitted on the same frequency. Satellite antennas have been designed to transmit several beams in different directions, using the same reflector.
A new method for interconnecting many ground stations spread over great distances was tested in 1993 following the launch of ACTS (Advanced Communications Technology Satellite). Known as the hopping spot beam technique, it combines the advantages of frequency reuse, spot beams (tightly focused radio beams, somewhat like spotlights), and TDMA. By concentrating the energy of the satellite's transmitted signal, ACTS can use ground stations with smaller antennas and reduced power requirements.
The concept of multiple spot beam communications was successfully demonstrated in 1991 with the launch of Italsat, developed by the Italian Research Council. With six spot beams operating at 30 GHz on the uplink and 20 GHz on the downlink, the satellite interconnects transmissions between ground stations in all the major economic centers of Italy. It does this by demodulating uplink signals, routing them between uplink and downlink beams, and combining and remodulating them for downlink transmission.
Large reflector antennas have become increasingly important to communications satellites. A large, precisely shaped reflector allows the transmitter to place more of the transmission in a smaller footprint on Earth. It also allows the satellite to collect more of the incoming energy from a ground station. The size of the antenna is limited by the size of the shroud, or nose cone, covering the satellite during launch. Larger rockets now permit single-piece antennas up to 4 meters in diameter.
Resistance to radiation such as cosmic rays is becoming a vital issue to satellite operators, who now rely on advanced electronics to pack more capability into a package. Geostationary orbits lie within the Van Allen belts where solar flares can easily inject charged particles into electronic components. Discharges can short-circuit a spacecraft's electronics and render the communications payload or even the spacecraft itself useless. Since the 1970s, several geostationary satellites have been damaged, and a few lost, because of space radiation and sunlight-induced charging.
The application of laser technology to satellite communications continues to be studied. Laser beams can be used to transmit signals between a satellite and Earth, but the rate of transmission is limited because of absorption and scattering by the atmosphere. Lasers operating in the blue-green wavelength, which penetrates water vapor, have been demonstrated for use in communication between satellites and submarines.
Non-commercial satellite systems include Inmarsat (a maritime telecommunications network founded in 1979), TDRSS (a NASA system set up in the 1980s to link the Space Shuttle, the Hubble Space Telescope, and other high-value satellites to ground controllers), and various military systems, including Milstar and Molniya.